Exploring the Role of Microbial Live Factories in Post-Harvest Management of Potatoes-Possible Solution to the Optimization of Supply Chain

Abstract

Potato (Solanum tuberosum L.) is the fourth most important food crop in the world with annual production of nearly 300 million tonnes. However, significant amount of the product (20–25%) is compromised to postharvest losses. Significant amount of the product (20–25%) is compromised to postharvest losses, therefore, alleviation of food security problems can be achieved through reduction in postharvest losses. Role of plant growth-promoting (PGP) microbes for the enhancement of potato production has been subject of extensive research. However, their impact on postharvest quality of horticultural crops has largely been unexplored, with limited research conducted on plant–microbe interactions in postharvest crops and their impact on storage stability. Although, microbial control has emerged as one of the most promising alternatives to chemical fungicides in several studies, however, significant research and development are required in development of sustainable microbial bio formulations for effective management of the crops under storage, in keeping with the quality of the produce. Therefore, manipulation of the bacterial microbiome, specially during crop storage, might provide microbial solutions as cleaner and sustainable alternatives to chemicals for plant production along the whole food chain. This chapter would elucidate functional analysis of the dynamics and potential of microbial live formulations for reducing the crop losses due to various diseases and status of the crop

1. Introduction

Food loss worldwide is estimated to be around 13% of the food production. Food loss is distinguished from food waste as the former occurring between harvest and supply to retail and the latter as losses in retail and consumer consumption [1]. Overall, postharvest food loss (PHL) is defined as measurable qualitative and quantitative food loss along the supply chain, starting at the time of harvest till its consumption or other end uses that varies largely between regions and also between food chains [2]. Potatoes are the world’s third most important food crop and are regularly consumed by billions of people. Potatoes are cultivated on more than 20 million hectares in 150 countries with a total global output of 359 million tonnes in 2020 [3]. The total potato production in Asia, Africa, and Latin America increased from 32 million tons in 1961 to 226 million tons in 2020, displaying six times increase, with an average annual growth rate of 3.37% [2]. According to FAO, about 30% of cereals, 20% of dairy products, 35% of fish and seafood, 45% of fruits and vegetables, 20% of meat, 20% of oilseed and pulses, and 45% of roots and tubers are lost or wasted [1]. Postharvest loss (20–25%) is one of the major problems in the potato production incurring significant constraints in food security and income generation. Therefore, application of good agricultural practices viz. good quality and disease resistance seeds, organic and inorganic soil amendments, and advanced irrigation farming techniques to meet the market requirements is only worth the investment once postharvest management is effective. In order to realize additional income by various stakeholders across the supply chain, complete food supply system comprising the postharvest chain and the production system needs to be optimized for uniform product quality. Another considerable aspect is that postharvest interventions often culminate into overemphasis on technical aspects at the expense of social, cultural, environmental, and political issues, and economic feasibility. Interventions are often not adapted to the level of development of food systems, leading to project investments that are not economically feasible for the market that is part of the food system and thus are not being used after the project ends [4].

Several factors affect food losses during storage viz. pathogens (insects, bacteria, and molds), environmental conditions (e.g. rain, humidity, heat, and frost), sprouting and quality loss (rancidity, water loss, and saccharification), or animals (rodents and birds) [5]. In potatoes, about 53–55% of the initial fresh potato production and 41–46% of the initial processing potato production are lost mainly due to pathogen infection, saccharification, water loss of tubers, and early sprouting during storage, leading to poor quality and appearance leading to rejection from consumers [6]. However, diseases due to pathogens account for the leading cause of losses during storage apart from cultivation, production, handling, transportation, and storage [7]. Some of the most important diseases worldwide are late blight (Phytophthora infestans), early blight (Alternaria solani), stem canker (Rhizoctonia solani), potato wart (Synchytrium endobioticum), powdery scab (Spongospora subterranea), bacterial wilt (Ralstonia solanacearum), black leg (Pectobacterium spp.), potato virus Y (PVY), potato leaf roll virus (PLRV), and yellow potato cyst nematode (Globodera rostochiensis) [8, 9]. To control these diseases, agricultural crops are effectively treated with synthetic pesticides, which is the most common control management strategy due to dominance of varieties with low or moderate resistance to late blight as well as low marketability and acceptance of resistant cultivars [10]. As a direct consequence of late blight, potato is one of the most fungicide-dependent crops [11]. These chemical alternatives incur significant economic and environmental costs as they leave harmful residues in the soil, water, and atmosphere, and also induce resistance in phytopathogenic strains [12, 13]. Thus, in view of the long-term health and environmental impact, there is need for development of efficient and environmental friendly technologies eventually leading to elimination or reduction of the application of synthetic pesticides in agriculture [14, 15]. Also from the perspective of increased demand for organic produce over the global markets, new technological alternatives to the use of chemicals might be an effective area of research for enhanced acceptability among the consumers. Toward this goal, microbial agents can emerge as promising alternatives.

Plant microbiomes are composed of several different types of organisms, including bacteria, fungi, protozoa, archaea, and viruses, which can play a beneficial role by protecting the plant from potential pathogens, simultaneously, improving growth, health, and production, along with conferring an adaptive advantage to the plants [16, 17]. Extensive research has been conducted on the plant-beneficial microbiome for sustainable agriculture. Species, such as Pseudomonas fluorescens, Pseudomonas putida, Pseudomonas aeruginosa, Bacillus subtilis, and other Bacillus sp., Azotobacter spp., Rhizobium spp. Arbuscular mychorrhizal (AM) fungi, etc. are widely used for the commercial production of PGPR [18]. However, limited research has been carried out on plant–microbe interactions in postharvest crops and their impact on storage stability. Therefore, manipulation of the bacterial microbiome, which might be especially important during crop storage, might provide microbial solutions as cleaner and sustainable alternatives to chemicals for plant production along the whole food chain from farm to fork.

2. Preharvest factors and storage conditions affecting quality of tuber

Abiotic factors in the field are very crucial for establishing final tuber quality, which can be only preserved post-storage by sustainable agricultural practices such as balanced fertilizer regimes that improved tuber yield and size [19]. One of the major factors is the variability of cultivar and season as it has reported improvement in the processing quality (fry color) of younger tubers (late planting) than that of the tubers planted earlier [20]. For good tuber quality, soil demands high nutrient content, which is ultimately related to the high organic matter and nitrogen input [21, 22]. Nitrogen (N) is an important nutrient for increased potato tuber yield. However, excess nitrogen fertilization levels remain one of the adverse environmental factors for tuber quality, as they enhance the proneness to loss of nutritional value during storage of ready-to-fry potato sticks [23]. Zhang et al. observed negative effect of excessive N treatment on tuber quality, with increased invertase activity along with accumulation of reducing sugars after cold storage [24]. Also, plant growth conditions affect basal metabolism in fresh potatoes, thereby inducing differences in cold adaptation and tolerance. It has been observed that heat stress during plant growth results in physiological disorders of potato tubers such as shortening of dormancy period and higher sugar accumulation after storage [25]. Vine desiccation/killing by physical or chemical means is another factor that strongly impacts quality. This aide in controlling the spread of viral or bacterial infections from vines to the tubers and hence, limit further occurrence during storage and in seed potato production, it can also control tuber size. Also, it triggers both maturation of the tuber periderm and stolon release, thereby reducing bruising that also reduces the danger of diseases in storage as many pathogens enter tubers through breaks in the skin [26].

3. Factors affecting postharvest storage

Free amino acids and reducing sugars are the important components of tubers. Irrational nutrient management during cultivation and cold stress during postharvest storage are two abiotic stresses that potatoes may often encounter [24, 27]. After harvest, mature potato tubers enter a stage of deep dormancy for a certain period of time, which is defined as a developmental stage in which bud growth will not occur even under favorable growth conditions. The length of dormancy varies among cultivars and is affected by pre- and postharvest factors, but mainly by the temperature conditions during growth and storage [28]. Cultivated potatoes are mainly consumed by the fresh market, however, an increased demand by the food processing industry has prompted the need for long-term storage of tubers for maintaining a constant flow of demand and supply across the food chain. Postharvest cold storage is a key solution to extend the year-round supply of potatoes, and it can effectively prevent the loss caused by decay and sprouting. Temperature management depends on the intended market: tubers for the fresh market can be stored at temperatures below 7°C while tubers destined for the processing market need higher temperature (8–13°C) to preserve frying quality [29]. Control of sprouting is critical to potato storage because it leads to alterations in weight, increases in respiration, changes in texture, and nutritional value, softening, shrinkage, and the formation of toxic glycoalkaloids [30]. Also, during sprouting, there is a fast buildup of soluble sugars and increased activity of oxidative enzymes [31]. These processes result in lower quality and intense browning of French fries and potato chips. A major quality requirement for these tubers is the ability to produce light-colored, flavorful fried products desired for consumption, which depends mainly on the amount of reducing sugars, primarily glucose, and fructose, in raw tubers [32, 33]. During frying, these reducing sugars react with free amino acids in a nonenzymatic Maillard reaction that results in dark-colored, bitter-tasting products [34]. Alternately, there is production of acrylamide, a neurotoxin, and suspected carcinogen [35]. It has been suggested that decreasing the reducing sugar content in tubers is a highly effective way to minimize acrylamide production in potato products [36].

3.1 Storage at low temperature

Postharvest storage of crops at low temperature (4°C) results in “cold-induced sweetening” (CIS) due to saccharification of starch, when starch is converted to the reducing sugars glucose and fructose compromising the quality of stored potatoes. As averse to senescent sweetening (natural irreversible process occurring as a result of cellular breakdown due to tuber aging); cold-induced sweetening can be partially reversed by temperature reconditioning [37]. Since cold is also an abiotic stress; as an adaptive behavior of tubers to cold stress, it can induce enhanced starch–sugar metabolism and cause sugar accumulation [38]. The contents and basal metabolism of small–molecule metabolites such as sugars and free amino acids directly affect tuber adaptability to cold. This is attributed to their crucial roles in maintaining cell osmotic pressure, preventing cell damage, and restoring homeostasis in plant cells [39]. The problem arises during processing of starchy crops, such as frying, roasting, or baking, when the reducing sugars react with free amino acids (asparagine) to acrylamide, a potential carcinogen [40]. One of the effective interventions to reduce the accumulation of reducing sugars in tubers would be preventing sucrose accumulation as well as sucrose conversion into reducing sugars during cold storage through manipulation of the expression of genes encoding the key regulatory enzymes, transcriptional factors, and other regulatory molecules that are directly or indirectly involved in CIS [41, 42]. In a recent study, cold storage significantly affected certain metabolites like sugars, sugar alcohols, amino acids, and organic acids [43]. Based on this, it has been indicated that metabolites such as glucose, fructose, sucrose, asparagine, glutamine, citrate, malate, proline, and 4-aminobutyrate can be potentially utilized for the selection and development of potato cultivars for long-term storage, nutritional, and processing attributes [37, 38, 39, 40, 43].

3.2 Sprout control

Among major challenges, premature sprouting accounts for the leading cause of loss during postharvest storage of potatoes. It not only reduces the number of marketable tubers but also fresh weight due to water loss from sprout surfaces, and the remobilization of starch [44]. Dormancy break in potato tubers is a physiological phenomenon that is regulated by both exogenous (environmental factors) and endogenous signals [45]. The relative concentration of several biochemical compounds such as plant growth regulators (viz. abscisic acid (ABA), auxins, cytokinins (CKs), gibberellins (GAs), ethylene, and strigolactones (SLs)) and other compounds (viz. carbohydrates and organic acids) are believed to coordinate the onset and further development of dormancy break [46]. Endogenous ethylene is required at the earliest stage of dormancy initiation (endodormancy induction) [47]; however, its role during dormancy and sprouting is still unclear. Exogenous ethylene has been reported to break endodormancy when applied for short intervals and inhibit sprout growth and promote ecodormancy when supplied continuously—either starting immediately after harvest or at first instance of sprouting [48]. However, work carried out on mini tubers showed that ethylene was not involved in hormone-induced dormancy break [47]. Apparently, the effect of ethylene depends partly on the physiological state of potato tubers [44]. Moreover, there is cross-talk between ABA and other phytohormones, as well as with sugar metabolic pathways, which facilitates the onset of dormancy break and further sprouting [49, 50, 51]. Auxins are essential for their role in vascular development as they favor the symplastic reconnection of the apical bud region—a discrete cell domain that remains symplastically isolated throughout tuberisation that is essential for sucrose to reach the meristematic apical bud [52, 53]. High sucrose levels promote trehalose-6-phosphate accumulation (T6P), which supports sprouting probably decreasing sensitivity to ABA [54]. Nevertheless, the increase in ABA as a result of exogenous ethylene application has been postulated to delay dormancy break [55, 56]. Concomitant to the ABA decline, there is an increase in sucrose contents, which is considered a prerequisite for bud outgrowth [48]. It has also been demonstrated that CKs and GAs are required for the reactivation of meristematic activity and sprout growth [57]. Just prior to dormancy break, an increase in both cytokinin concentration and sensitivity has been reported as key factors for meristematic reactivation [58]. Furthermore, CKs coordinated with auxins stimulate sprout elongation [59]. Sensitivity to GAs, which is negatively affected by SLs, increases throughout postharvest storage and is possibly responsible for sprout vigor [60]. SLs act as key regulators of lateral bud development and are related to paradormancy establishment instead of eco- and endodormancy [46]. To address this issue, the cold storage industry has relied on the application of commercial sprout suppressant isopropyl-N-(3-chlorophenyl) carbamate (chlorpropham or CIPC), as thermal hotfog (single or multiple treatments) during prolonged potato storage [61]. However, there has been an alternative trend in use of chemical sprout suppressants viz. hydrogen peroxide plus (HPP) [62], 1,4-dimethylnaphthalene (1,4-DMN) [63], UV-C [64], gamma radiation [65], ethylene [66], and essential oils such as mint and monoterpene (carvone) derived from caraway seed [67]. These alternative chemical sprout suppressants during postharvest aim at damaging the meristematic tissue to cease or disrupt cell proliferation; for example, local necrosis of the bud meristem was found after the application of mint essential oils [67]. However, such alternative solutions present a number of other challenges in their applications for effective sprout control like the number of applications required that render them ineffective as compared to traditional applicants like CIPC [61].

3.3 Role of microbes in postharvest stored potatoes

Worldwide, roughly 70 diseases and physiological disorders are known to severely damage potato crops, particularly the tubers [68]. Some of the main symptoms of the diseases that affect tubers during storage like spots, and rotting caused by fungi and bacteria have been listed in Table 1. Surface spots only affect the epidermis of the tubers, and do not alter their taste or nutritional properties [69]. However, some pathogenic species such as Colletotrichum coccodes, Helminthosporium solani, Rhizoctonia solani, Spongospora f. sp. subterranea, and Streptomyces spp. may affect the peridermis of the tubers providing means of entry for opportunist microorganisms that lead to the rotting of tubers [70]. Fungi species such as Fusarium sp. frequently damage relevant damages to tubers on fields and in storage worldwide, which may affect up to 60% of the production [71]. Other species of fungi such as Fusarium, Verticillium, and bacteria such as Pectobacterium carotovorum subsp. carotovorum, P. carotovorum subsp. atrosepticum, and R. solanacearum cause rotting or more severe damage to the peridermis of tubers as a result of wide range of enzymes such as pectinases, cellulases, xylanases, and proteases, responsible for the maceration of tissues and cell death [72, 73]. On the contrary, several bacteria and fungi possess enzymes such as a-amylases, which hydrolyze starch molecules into polymers composed of glucose units [74] and so the activity of plant-associated microorganisms could play a role in the accumulation of reducing sugars in plants.

	Potato storage disease	Pathogenic microorganism	Symptom in tuber
Bacterial diseases
1.	Soft rot	Erwinia carotovora	Tan- to brown-colored water-soaked areas of granular, mushy tissue often outlined by brown to black margins. Soft-rotted tubers tend to spread bacteria to surrounding tubers invading mainly through the lenticels, resulting in localized pockets of rot or “hot spots.” The process is favored by high temperatures and anaerobic conditions. Under these conditions.
2.	Ring rot	Cornybaeterium sepedonieum	Yellowish color of the vascular ring just beneath the skin Secondary infections It is primarily detected by cutting open the tuber in advanced stage.
Fungal diseases
3.	Dry rot	Fusarium solani,; F. roseum, F. sambucinum, F. coeruleum	Characteristic symptoms are sunken areas of brown, firm rot often involving a large portion of the tuber. The surface of the affected area is sunken, wrinkled, and frequently with blue or white protuberances. Infected tuber tissues initially have firm, wet texture with dark brown-black appearance, eventually shrivel and mummify. The papery rot stage is dry, crumbly, and brown in color with collapsed tissue often laced with secondary white- or other-colored fungal growth.
4.	Pink rot (water rot) and leak	Phytophthora erythroseptica and Pythium species	Normal tuber shape, outer skin turns dark, and internal flesh when cut has a rubbery, “boiled potato” consistency that turns pink after about 15 to 20 minutes of exposure to warm air, distinctive ammonia odor.
5.	Leak	Pythium species	Outer cortex, or “shell” of the tuber is intact, with gray/brown/black rot in the interior of the tuber, with internal tissues wet and rotted, clear liquid to stream out on squeezing, typically needs a wound to infect.
6.	Vascular discoloration	Verticillium albo-atrum and Fusarium oxysporum	Vascular ring discoloration, particularly at the stem end. The disease does not spread from one tuber to another in storage, but secondary infection may occur in affected tubers.
7.	Late blight	Phytophthora infestans	Reddish or tanbrown, dry, and granular rot that extends from the skin of the tuber inward an inch or more, internal tissues are firm to the touch, unlike wet or mushy, and peeling the skin over the affected areas presents characteristic reddish color.
8.	Silver scurf	Helminthosporium solani	Gray to silvery blotches on the surface of the tuber with unaffected internal tuber tissues. Diseased skin has a silvery sheen, appears thicker, and can be unappealing to consumers. Pathogens disrupt the periderm and lose water at a higher rate of water loss than healthy tubers.
9.	Black dot	Colletotrichum coccodes	Lesions are usually more prominent and thicker than those associated with silver scurf. Within the lesions, small black sclerotia may be visible with the aid of a magnifying lens.
10.	Early blight	Alternaria solani, A. alternata	Characterized by shallow, gray to black, dry lesions, often associated with wounding often when tubers with immature skins are harvested from sandy soils. Lesion margins may appear watersoaked and have a yellow color.

Table 1.

Common potato storage diseases and symptoms.

Phytohormones play a key role in the regulation of sprouting—cytokinins and indole-3-acetic acid signaling induce the onset of sprouts, and gibberellin stimulates sprout growth [75]. Many plant-associated bacteria are able to produce plant hormone-like metabolites, and the role of auxins, cytokinins, and gibberellins produced by bacteria in plant growth regulation is well documented [76, 77]. In contrast with this, Slininger et al. described an inhibitory effect of six bacterial strains on potato sprouting [78]. Further trials on bacterial strains of P. fluorescens and Enterobacter cloacae for disease suppressiveness in tubers stored for a week at 15°C and 90% relative humidity had been shown to control late blight in addition to dry rot and sprouting [79]. Another aspect was isolation and identification of polysachharide called marginalan, which could be effective in biocontrol formulations for stored potatoes [78].

4. Role of microbial bioinoculants in mitigating postharvest challenges

4.1 Nutrient management and water use efficiency

It is generally reported that the adequate application of fertilizers enhances water use efficiency by increasing the transpiration efficiency of the crop plants [80, 81]. This in turn helps in optimizing crop yields with higher drought tolerance [82]. In several studies, soil nitrogen (N) level was positively related to water use efficiency [83, 84, 85]. Potato cultivation is quite demanding in inorganic nutrients and adequate fertilization is a key factor for maximizing yield and producing tubers of high quality [86]. Most farmers apply a nitrogen-phosphorus-potassium 15–15-15 at the time of planting. Potassium and nitrogen fertilization is required for maximum potato production. As a rule of thumb, potato plants have greater needs in nitrogen (N-P-K 34–0-0) during the first two months (when the foliar part of the plant develops rapidly). From the second month until two weeks before harvest, the plants need more potassium (12–12-17 or 14–7-21) in order to create well-shaped potatoes. Similarly, potash (K) fertilizers are directly involved in the water management of the plant since it reduces water loss through transpiration. In sandy soils, water use efficiency for total dry matter production is increased by K application [87]. However, there has been indiscriminate use of fertilizers in the past. In an attempt to maximize the yield, farmers increase the doses of N fertilizer, leading to high application rates [88]. Moreover, negative effects of excessive N application on potato growth and tuber yield have been demonstrated [81, 88]. Low phosphorus (P) availability is a major limiting factor for potatoes while the residual and unavailable P in some soils is estimated to sustain crop yield for the next century [89]. In a balanced soil fertility program, P essentially increases the water use efficiency of the crop and helps in attaining optimal performance under limited moisture conditions [90]. However, it may become toxic when plants accumulate at high concentrations. It has already been demonstrated that scientific application of chemical fertilizer may contribute to the obvious, although not full, recovery of the root growth by reversing the central metabolite changes [91]. Hence, there is widespread interest in finding alternate ways to utilize accumulated soil nutrient resources for sustainable crop production. Biofertilizer application improves the resilience of the cropping system by maintaining proper nutrient balance and with increased use efficiencies [92]. Diazotropic bacteria are able to fix atmospheric nitrogen (N) to plant available forms and include the endophytes Azoarcus sp., Burkholderia sp., Gluconacetobacter diazotrophicus and Herbaspirillum sp., and the rhizospheric bacteria Azotobacter sp. and Paenibacillus (Bacillus) polymyxa, Klebsiella spp., and Enterobacter spp. [93, 94]. Phosphorus solubilizing bacteria (Pseudomonas spp. and Bacillus spp.) and fungi (Aspergillus spp. and Penicillium spp.), etc. help in faster release of soluble P from fixed P through the release of organic acids, phosphatase enzymes, process of chelation and exchange reaction., etc. for improving [95]. Further, use of Bacillus altitudinis and Tridens flavus var. flavus has been shown to improve plant growth and enrich zinc in tubers [96]. Plethora of microorganisms has been reported in plants for improved mineral solubilization and uptake of nutrients with reduced dependence on chemical fertilizers [97]. In potatoes, several reports have confirmed that application of PGPR significantly enhances the available nutrients such as nitrogen, transferable phosphorus, magnesium, and dissolved K [98, 99, 100, 101]. Alternately, it has also been reported by Zhang et al. that sustainable agricultural practices such as balanced fertilizer regimes improved not only tuber yield but also marketing tuber size and quality of potatoes [24]. Additionally, these PGPR release certain chemical substances like indole acetic acid, which increase the plant development and enhance the acquisition of other nutrients and water for plants [102].

4.2 Alleviating the effect of drought stress

Water availability, water use, and nutrient supply to plants are closely interacting factors influencing plant growth and yield production [103]. Drought stress during plant growth results in transpiration reduction and this continues postharvest [104]. As compared to other crops, potato is more sensitive to drought stress; especially during early tuberization, yield is affected by decreasing both number and weight of tubers [105]. Water deficiency, especially during the tuber bulking period, decreases yield to larger extents than water limitation during other growth stages of potatoes [106]. Similarly during storage moisture loss and consequently weight loss through transpiration is one of the main reasons for optical quality constraints with potatoes. A symbiotic relationship exists between plants and beneficial soil microorganisms wherein the microbes help the plants in nitrogen acquisition, water uptake, and survival during stress [81]. In view of this, different strains of PGPR have been reported to be involved in drought stress resistance through various physiological and biochemical changes [107, 108]. The major mechanisms adopted by PGPR to overcome drought stress include alteration in root morphology and production of osmolytes like sorbitol, polyols, mannitol, sucrose, fructan, proline, and ectoine, antioxidants, phytohormones, extracellular polymeric substance (EPS), and volatile organic compounds (VOCs), siderophores, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase [109]. It is well known that inoculation with certain plant-beneficial bacteria can minimize symptoms of drought stress in plants and one of the effects is that the water content in plant tissue remains higher during drought stress periods than in non-inoculated control plants [110]. The application of microbial inoculants that reduce drought stress on the field could, therefore, also have positive effects on water content of crops during storage. Several PGPR (Bacillus, Rhizobium, Azospirillum, Pseudomonas, Flavobacterium, and Arthrobacter) are reported to display improved osmoregulation, oligotrophic, endogenous metabolism, resistance to starvation, and thus showed their efficient metabolic processes to adapt dry and saline environments [111]. Recent studies demonstrated that inoculation with ACC deaminase-producing rhizobacteria increased root–shoot length, root–shoot mass, and the lateral number of roots of wheat plants compared with that of the control resulting in improved growth and yield under drought stress water and nutrient uptake [112]. Similar positive effects of ACC deaminase-producing bacteria against drought-induced oxidative damage have been reported in tomato plants with B. Subtilis [113] and in maize with E. cloacae and A. xylosoxidans [114]. In potatoes, PGPR strain of B. subtilis was observed to enhance the growth and yield under drought stress by maintaining physiological functions and antioxidant enzyme activities of the plant [115] with increase in stress-related enzymatic activity of catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD), contents of total soluble sugars, soluble proteins, and proline.

4.3 Abiotic and biotic stress tolerance

The abiotic stress tolerance in a plant is a network of complex signaling pathways where numerous cell molecules, enzymes, transcription factors, hormones, and metabolites are involved. Under different abiotic and abiotic stress conditions, PGPR like Bacillus spp. has been shown to synthesize small molecules such as carotenoids, ascorbic acid, tocopherols, anthocyanins, and antioxidant enzymes that protect potato plants from oxidative injury through elimination of stress-induced reactive oxygen species (ROS) [116]. PGPRs are also involved in the initiation of defense mechanisms like phenylpropanoid pathways and lignin biosynthesis by inducing the production of certain plant molecules such phytoalexins, salicylic acid, jasmonic acid, methyl salicylate, and methyl jasmonate that are formed under stress [115, 117]. Such molecules act as signaling molecules that trigger a cascade of the stress signaling pathways.

It has been discussed that rhizobacteria confer stress tolerance through biocontrol of phytopathogens in the rhizosphere and by the production of phytohormones and ACC deaminase, favoring osmolyte accumulation in plants [118]. Plants produce ethylene during ripening from 1-aminocyclopropane-1-carboxylate (ACC) via ACC oxidase activity [119, 120]. Many bacteria in soil and plants possess ACC deaminase activity, which cleaves ACC to 2-oxobutyrate and ammonia and can contribute to ethylene balancing in plants [111, 118]. An alternative role of ethylene in microbial-driven modulation of ripening was introduced by a recent study exploring the potential of a Rhodococcus rhodochrous strain to delay ripening in different species of climacteric fruits [121]. In this case, the proposed mechanism is ethylene-induced nitrile hydratase and/or nitrilase activity in the bacterium, which resulted in maintenance of fruit firmness and reduced spotting and spoilage [121].

Stress-tolerant and competitive species are physically adapted to limited environmental resources or carrying-capacity environments and are more stable and permanent members of the community. Pathogens that tolerate environmental stress often have few competitors, since few species can exist under such conditions [122]. Some postharvest rots result from preharvest latent infections, especially in tropical and subtropical regions where environmental conditions in the field are particularly conducive to infection. Latent contamination involves fungal spores on the surface, which fail to germinate until the host reaches maturity or senescence [123]. Nevertheless, successful control of latent infections by postharvest applications has been reported. For example, Grossi et al. has recently reported Methylobacterium as potential PGPR to alleviate salt stress and restrict P.infestans infection in potato plants [124]. Therefore, selection of stress-tolerant microbial strains that have competitive advantage over the quiescent pathogens becomes very useful for preparation of different microbial formulations for various purposes including biocontrol against the spoilage microorganism.

4.4 Biofilm production in disease management

Biofilms consist of cells and matrices where complex exopolysaccharides and proteins are major components, and they can provide an important bacterial survival strategy in natural systems [125]. It is generally recognized that surface-associated bacteria colonize as biofilms, which are microniches entirely different from their surroundings. This allows the bacteria to work as a functional unit, accomplishing tasks not possible in their planktonic state. The biocontrol ability of Paenibacillus polymyxa has been attributed to the large bacterial pool of bioactive compounds such as nonribosomal peptides/polyketides (NRPs/PKs) [126]. It has been indicated that along with the lipopeptides, the bacterial biofilm EPS compounds viz. uronates are capable of antagonizing Fusarium graminearum causing Fusarium Head Blight (FHB) in cereals [126]. Bacterial EPS is an important compound that is dependent upon the perception of numerous environmental signals from the host and the ecosystem. Such categories of sustainable biopolymers of microbial origin could be further explored in addressing various industrial and agricultural necessities.

4.5 Other compounds

Among various research efforts, application of elicitors or resistance inducers (RIs) has been of much interest as alternative control methods. Many of these compounds are nontoxic to environment and with no reported negative effects on fruit quality [71]. Resistance inducers can be divided into two groups: natural and synthetic compounds such as benzothiadiazole, β-aminobutyric acid, salicylic acid, chitosan, and chitin [127]. Among the most widely used natural compounds, chitosan is explored as an elicitor and an antifungal agent [128]. Chitosan is a natural carbohydrate polymer, nontoxic, and biodegradable, which can be applied in solution and as powder [127]. This β-(1,4) N-acetyl-glucosamine polymer is collected from the chitin that has been deacetylated to provide 70% free amino groups [129]. In the past, the antifungal activity of chitosan has been successfully used for the control of several plant diseases such as fusarium wilt of tomato [130], gray mold of table grapes [131], and orange-green molds [132]. Also, chitosan was first used at 400 μg/ml of acetic acid-distilled water solution for the control of FDR, where a significant reduction in disease severity was noted on potato tubers inoculated with F. sambucinum [133]). On the basis of inhibitory activity of chitosan against Fusarium spp. involved in potato tuber dry rot and wilt, and the capacity to activate defense-related enzymes and to increase the total phenolic contents and oxidative enzymes in potato tubers, suggestions have been made for potential use of plant RIs, as alternatives to synthetic fungicides. This eco-friendly approach could be easily applied and generally recognized safe and cheap method for controlling some soil-borne plant pathogens.

4.6 Biocontrol agents in storage disease management

Fresh potatoes are exposed to an intensive postharvest microbial colonization leading to storage rots. This is attributed to the latent contamination involving fungal spores on the surface, which fail to germinate until the host reaches maturity or senescence. However, there is intense interplay of individual members of the diverse microbial community of the crop for effective colonization of pathogens consequently variation in the degree of spoilage [16]. As evidence of this, Liebe et al. observed an unspecific resistance mechanism that has been suggested for slowing down the spread of pathogens in more resistant genotypes [134]. Several workers have studied antagonistic properties of different biocontrol agents against postharvest pathogens of potatoes some of which have been listed in Table 2 [137, 138, 139, 140, 141, 142, 143]. Studies on evolutionary genomics have shown that species are characterized by the paleome (core genes, which allow basic metabolism), whereas strains are defined by their cenome (genes, which allow cells to live in and explore a particular niche); that is responsible for strain-specific properties [144]. In this context, each strain of one species has specific antagonistic properties that can show different biocontrol effects in the field [145]. Therefore, a selection strategy considering these strain-specific effects becomes necessary for an effective outcome. So far several workers have deployed different approaches in manipulation of microbial agents for devising successful biocontrol strategies. In yet, another attempt to develop efficient biocontrol strategy, Furkranz et al. studied the diversity of endophytic microbial communities isolated from diverse microhabitats viz. from seeds, roots, flowers, and fruits of three different pumpkin cultivars [146]. On the basis of broad-spectrum antagonism, Pseudomonas chlororaphis treatment, and combined treatment of strains (Lysobacter gummosus, P. chlororaphis, P. polymyxa, and Serratia plymuthica) significantly reduced the disease severity on pumpkins. In concordance with this, it has been observed that the seed microbiome of oilseed rape is cultivar-specific and cultivars hosting a higher indigenous microbial diversity showed better resistance toward colonization by pathogenic microorganisms [147]. In a recent study, it has been concluded that PGPR strain of P. chlororaphis can be used as probiotics in order to increase plant tolerance/resistance to abiotic/biotic stresses [145]. Additionally, many species of endophytic Bacillus are known to exhibit antagonistic properties against the tested plant pathogens in melons, which have been hinted at protecting plants not only in the field but also for postharvest [148, 149]. Further, the efficacy of biocontrol agents can be enhanced by isolation of the antagonists and can be improved by using undisturbed habitats, where natural populations have not been disturbed by conventional management with agrochemicals, with rich pool of potential antagonists. A variety of enrichment procedures have been used that favor isolation of microorganisms; one such rapid and cost-effective method of antagonist isolation was adopted by Wilson and colleagues in 1993 with application of rinsing waters from tomatoes and apples directly on wounds inoculated with the pathogen [7]; subsequently isolating antagonists from asymptomatic wounds. One of the drawbacks of this strategy is that it favors the selection of antagonists that are generally fast growers with the ability to colonize a specific niche rich in nutrients that mainly exhibit protective rather than curative activity, with seemingly poor effect on latent infections [135].

Microbial antagonists	Pathogens/Diseases controlled
Pseudomonas putida	Erwinia carotovora [135]
Muscodor albus	B. cinerea, Colletotrichum coccodes, Cerastium acutatum, Fusarium sambucinum, G. candidum, Helminthosporium solani, Monilinia fructicola, Pectobacterium atrosepticum, P. expansum, Rhizopus spp. [135]
Bacillus subtilis and Trichoderma harzianum	Common scab disease caused by Streptomyces spp. [136]
Pseudomonas aeruginosa, Alcaligenes feacalis and Serratia marcescens	Fusarium wilt caused by Fusarium sp. [137]
Enterobacter amnigenus, Serratia plymuthica, Serratia rubidaea and Rahnella aquatilis	Soft rot caused by E. carotovora [138]
commercial arbuscular mycorrhiza fungi (AMF)	Bacterial wilt [139]
Bacillus cereus, B.subtilis, B.weihenstephanensis, Pseudomonas sp.	Pectobacterium spp. and Dickeya spp. [140, 141, 142, 143]
Bacillus thuringiensis, Rhodococcus erythropolis, S.plymuthica, Delftia acidovorans, Ochrobactrum sp.	Piliocolobus parmentieri and Pectobacterium carotovorum subsp. carotovorum [140, 141, 142, 143]

Table 2.

Biocontrol agents with antagonistic properties against postharvest pathogens of potatoes.

5. Conclusions

Postharvest loss is one of the major problems in the potato production incurring significant constraints in food security and income generation. Despite the fact, integrated soil and water management can potentially optimize yields and marketable quality of potatoes; additional income can be realized by various stakeholders through uniform product quality during storage and transfer across the supply chain. In the previous studies, wide range of plant growth-promoting (PGP) microbes with potential biocontrol abilities have been identified for improvement of plant growth and yield in potatoes. Also, much progress has been made in the development of various biocontrol agents (BCAs) for control of diseases in potatoes, especially during storage. However, additional research is required to validate their stability for their wide use at the commercial level. Moreover, limited research has been carried out on plant–microbe interactions in postharvest crops and their impact on storage stability. Manipulation of the bacterial microbiome, which might be especially important during crop storage, might provide microbial solutions as cleaner and sustainable alternatives to chemicals for plant production along the whole food chain from farm to fork. Therefore, concerted research efforts are required to validate their stability for wide use at the commercial level.

Conflict of interest

The authors declare no conflict of interest.