Introductory Chapter: Integrative Technologies for Sustainable Plant Improvement

1. Introduction

Agriculture systems around the world must produce more food while producing less waste. Sustainable agricultural practices and food systems, which cover both production and consumption, must be evaluated holistically and fully to be effective. Tropical plants include fruit, climbers, flowering perennials, annuals, and bulbous plants as well as indoor and outdoor floral plants. Also included in the list of tropical plant species are ferns, palms, and other plants. To grow these plant species successfully in tropical and subtropical regions of the world, several factors are needed to be taken care of. Because they are crucial to food production and are getting harder to get in many parts of the world, healthy soils, clean water, and plant genetic resources must all be used and maintained sustainably. With the expectation that there would be 9 billion people on the planet by the year 2050, an increase in crop output and quality would be necessary to satisfy the needs. An expanded discussion on how to grow fruit and ornamental plants for esthetic, dietary, and nutraceutical benefits may be found in the introductory chapter. The impact of technological advancements on the sustainable cultivation of plants is also examined. Further, the integrative strategies including OMICS and reliable methods for gene discovery and genome sequencing, as well as the application of CRISPR/Cas and gRNA/Cas, organelles transformation innovations, and value-adding strategies such as biopharming to create superior plants for agronomic, industrial, and value-adding features are highlighted.

2. Tropical plants: economic and nutritional importance

Tropical plants are essential for providing food, fiber, and shelter, but they also have esthetic value because of their lovely hues, fragrances, and greens, which also have a cooling effect from evapotranspiration. They are crucial for people living not only in tropical areas but also in other parts of the world’s economies, cultures, and spiritual life. For example, there are over 100 different varieties of the 400 different species of bananas that are grown in Africa alone, nearly 1 million hectares of coffee plantations in Brazil, 2 million hectares of sugarcane plantations in Hawaii, and over 3 million hectares of rice paddies in tropical regions of Asia. Compared with plants found in temperate climates, tropical plants are significantly different. They thrive in a variety of environments, but particularly in rainforests, and have evolved to withstand extreme heat and heavy humidity. Additionally, as long as adequate water is available, many tropical plants may thrive on nutrient-deficient soils [1].

The economic value of tropical plants comes from their use in silviculture and timber production. Moreover, various species are used as sources of food, medicine, and raw materials (Figure 1). The esthetic value is derived from their beauty and usefulness in preserving the environment. On the other hand, they provide a vital service in maintaining soil fertility through their nutrient-rich soils and preventing erosion by providing habitats for microflora that act as decomposers of organic matter, root-dwellers, and rhizospheric antagonists [2]. Tropical plants also contribute to a variety of other services such as carbon sequestration (sequestering atmospheric carbon dioxide into biomass) by high photosynthetic rates, and water purification (through high evapotranspiration rates), and carbon capture and sequestration (through deposition of carbon on forest floors). In addition, they also provide a variety of amenities such as shade, food, and shelter for humans and animals; recreation opportunities for people; cultural heritage for indigenous civilizations; environmental education for children; protection from natural hazards such as floods and fires, and above all, biodiversity conservation.

Figure 1.

Tropical plants, their sustainable cultivation, and value-added products are presented in the graphical abstract of the introductory chapter.

Tropical plants contribute to the food supply, provide a source of nonfood raw materials, provide medicines, and are an essential part of human life in many cultures. They also have high nutritional content, making them an essential nutrition source for the tropics. Among nonfood products, tropical trees are rich in essential oils and resins used for cosmetics and medicines [3]. Medicinal plants such as the soapberry (Sapindus mukorossi) are used for treating skin ailments such as acne because it is rich in antimicrobial agents, which help to prevent bacterial or fungal infections. Other important medicinal plants include Croton zaeus, which has been used to treat diarrhea and other digestive disorders such as stomach aches; Passion vine (Passiflora quadrangularis), which contains compounds that suppress pain and Trumpet tree (Cecropia peltata), is known as a sort of cure-all in Caribbean folk medicine because of the long list of its uses.

The edible plants are used to flavor foods as well as some bitter medicines and to make dyes, perfumes, medicines, and cosmetics. The medicinal plants provide relief from pain and suffering. The flowers of these plants are used to produce dyes for dying cloths or other materials. Some flowers contain potent alkaloids that can be extracted for use in therapeutics. The leaves of some tropical plants serve as fodder for animals, while others serve as fertilizer for agriculture. Being valuable food sources, some tropical plants have been grown worldwide for thousands of years including peanuts, potatoes, cocoa beans, and rice, and now have become essential crops owing to a source of nutrition because of richness in a wide range of vitamins and minerals in addition to having high amounts of protein, dietary fiber, and low-fat levels. Further, these plants help to prevent chronic diseases such as heart diseases and cancer by reducing free radicals that cause oxidative stress due to the presence of antioxidants in them. High fiber contents help in digestion and prevent constipation that reducing the risk of colon cancer by preventing inflammations of the digestive tract. High levels of vitamin C in many tropical fruits such as papaya, mangoes, mangosteen, etc., help to boost immunity against common infections. Vitamin C also protects against oxidative stress caused by free radicals, which may lead to cell damage or death if left unchecked [4].

3. Tropical ecosystem

Tropical plants grow in a wide range of habitats, from the equator to the tropics. They thrive in warm weather and grow in rainforests, deserts, and even on mountainsides. The problems of deforestation, soil degradation, and changing climate pose serious threats to tropical ecosystems including forestry and agriculture sectors. This resulted in an average decline of 15.8 million hectares of tropical trees since 2017. If the issue remains unattended, it will erode social-ecological resilience in the tropics and will ultimately result in self-propagating feedback and regime shifts. Various factors including climate change and soil erosion pose a negative impact on the stability of natural ecosystems. Moreover, the seasonal, interannual, and decennial climatic fluctuations badly affect the vegetation dynamics in these areas. Land degradation is also more noticeable in the tropics, and it affects biodiversity and soil characteristics. The major causes of land degradation are anthropogenic [5]. Hence, sustainable management of natural ecosystems has been recommended as the only way to control factors affecting the stability and preservation of tropical ecosystems. Sustainable use encompasses the management and use of natural resources including tropical natural resources. They can sustain their natural biodiversity, yield, renewal capacity, vitality, and capacity to satisfy the relevant ecological, economic, and social functions at local, national, and global levels, and that they do not cause damage to other ecosystems. This reflects contemporary discourse in sustainable development and governance, which emphasizes the importance of public-private and civil society partnerships, with the potential to bridge multilateral norms and local action by drawing on a diverse number of actors in civil society, government, and business [6].

4. Evolution and diversification

The tropics comprise the geographic regions of the earth centered on the equator. They exhibit substantially variable landscapes ranging from deserts to rainforests and from hot lowlands to snow-capped mountains. Hence, we can find a variety of ecosystems in tropical regions with extreme climatic conditions along with a rich diversity of living organisms including plants, animals, and microbes.

Tropical plants are proposed to be originated 93–115 million years ago yet the rate of diversification increased dramatically during the last 15 million years. Hence, the tropics are considered a diverse rich region with a huge number of crop plants including cereals such as maize, rice, sorghum, and millet; tuberous crops such as potato, sweet potato, and cassava; vegetables such as tomato, peppers, many cucurbits; cash crops such as cocoa, rubber, tobacco, cotton; fruits such as banana, pineapple, mango, papaya; and other crops such as peanut, common beans, oil palm, coconut, sugarcane, and coffee. It is estimated that two-thirds of all angiosperms including crop as well as non-crop plant species are found within the tropics. Moreover, rich biodiversity of plants other than angiosperms such as Ferns [7], bryophytes, and liverworts are found highly concentrated in the tropics. The dispersal of plant species in tropical areas is not uniform. The Neotropical areas and the region of Asia Pacific are the most biodiverse while Africa and oceanic islands contain the least biodiversity. The tropical hyper-diversity is not well understood, and it has variously been attributed to be a museum of diversity, showing constant speciation and low extinction [8], or a cradle of diversity, referring to more recent and rapid speciation [9].

5. Tropical plant species—as an ornament

Tropical plants, with unique colors, shapes, and fragrances, have a great esthetic value and hence are very popular among culturists, collectors, hobbyists, and even enthusiasts. They not only lift the spirit of the room and indoor environment, uniquely and refreshingly but also make us feel alive, lively, and fresh. The craze for growing these housing plants has accelerated to multifold during the COVID pandemic particularly. Tropical flowering plants have enchanting colors and are key elements in the beautification of any landscape. The most popular tropical flowers are Amaryllis, Paper flower, Garden cosmos, Flamingo flower, Bush lily, and Ohe naupaka, etc. Though, most of the indoor plants available in nurseries are either grown by cuttings or seeds. However, these conventional techniques are slow and the resultant plants are susceptible to diseases. A valuable alternative is the use of tissue culture techniques for the mass production of disease-free plants [10]. The plants developed through tissue culture have uniform growth, hence providing rapid mass production of disease-free plants, off-season propagation, mass production in limited space, and can overcome the challenges of cultivating plants by traditional propagation techniques.

Researchers have worked out different protocols for the mass multiplication of these flowering plants. Different combinations of growth hormones and culture conditions were optimized for the establishment of in vitro regeneration and micropropagation. Different explants (leaf disc, mature embryos, and bulb scale) were tested in Amaryllis and other tropical flowers. Different varieties of hardy ornamental tropical plants have been explored, i.e., Bamboo (Goldstripe, Ghost bamboo, Slender weavers, and Chinese dwarf), Colocasia (Pink china), Hibiscus (Berry awesome, Perfect storm, and Cranberry crush), Bird of Paradise (Mexican bird of paradise), Palms (Dwarf Palmetto, Kumaon, Chusan, Miniature Chusan palm and Windmill), Canna (Peach, Stuttgart, Gigantum and Skyhawk), Hostas (Abiqua drinking gourd, Lakeside shore master and Dancing queen), and Ferns (Lady fern, Western sword fern, and Christmas fern). Over the years, numerous researchers have tried to manipulate bamboo flowering and seed sets under in vivo and in vitro conditions [11]. Two different polyploidy groups were found to be present in woody bamboo. Tropical bamboo is hexaploid, whereas temperate bamboo is tetraploid. Likewise, different molecular markers have been employed to explore genetic diversity. Using these markers, it has been elucidated that lowland bamboo is associated with distinct geographic regions. Genetic diversity among the ornamental palm accessions helped to explore their origin, genetic identification, and conservation [12]. Some other prominent indoor plants, i.e., spider plants, African violet, baby rubber plant, and weeping fig had also been worked out to overcome existing bottlenecks in propagation.

6. Tropical plant species—as a source of fruits and lumber

In addition to flowering and herbaceous plants, tropical plant species are at the forefront to fulfill the fruit and lumber demands of the rapidly increasing population. The most valuable tropical fruits are jackfruit, dragon fruit, lychee, banana, passion fruit, papaya, acai, rambutan, coconut, and mango. They are not only an important source of nutrients, bioactive compounds, and primary as well as secondary antioxidants but also bring high economic returns to the growers. So, tropical fruit plants are in high demand worldwide owing to their nutritional and nutraceutical value. Recent updates in cultivation techniques, efforts to develop climate-resilient genotypes, control of fruit diseases, exploitation of postharvest physiology, and the discovery of bioactive compounds have further promoted the adoption and usage of tropical fruits. As a result, numerous fruit species have been explored that have the potential to be transformed from minor tropical fruits to major tropical fruits and can be introduced elsewhere [13].

Tropical lumber trees are also very important for domestic as well as industrial applications resulting in the substantial economic growth of lumber-producing areas. Furthermore, such areas not only contribute to the conservation of animal and plant biodiversity but are of central importance in the global trade of sawn wood, roundwood, and plywood too. They are so important that only rosewood has a market of 26 billion USD in China per annum. More than 600 species of lumber trees have been explored so far, in different tropical regions, worldwide. Though numerous tree species are on the verge of extinction, researchers are striving hard to protect and promote them using various interventions. Advancements in clonal propagation, micrografting, nodal culture, genetic transformation, development of DNA databases for the forensic identification of lumber [14], and establishment of seed banks have helped not only to secure endangered plant species but have also helped to improve their production and quality.

7. Tropical plants—a source of medicine

Tropical plants are second major source of oxygen on the earth after oceanic phytoplankton. They are valuable indoor plants that can help to restore oxygen balance in the closed space and are an exclusive part of the top-five indoor plant species having the ability to produce maximum oxygen. These include Boston Fern (Nephrolepis exaltata), Peace Lily (Spathiphyllum spp.), Snake Plant (Sansevieria trifasciata), and Areca Palm (Dypsis lutescens), and Gerber Daisy (Gerbera jamesonii). These tropical plants play a critical role to moderate the level of carbon dioxide in the atmosphere. The modern world is highly reliant on fossil fuels to fulfill ever-increasing energy needs. This has led to an alarming increase in carbon accumulation in the atmosphere. Here comes the importance of carbon sinks for balancing carbon concentration. Grasslands, peat bogs, coastal ecosystems, coral reefs, wetlands, boreal forests, and tropical rainforests are important carbon sink ecosystems playing their role in balancing oxygen–carbon levels.

Tropical plants have also been explored as a source of valuable industrial products and drugs. Owing to the enriched biodiversity, they provide 60% of the chemical entities all over the world. They have been called the largest pharmacy in the world because more than 70% of the drugs are derived from these plants, directly or indirectly. Most synthetic drugs are also derivatives of tropical plant products. Half of the best 25 pharmaceutical agents also come from tropical forests. They have been identified as a valuable source of anticancer agents. Further, the first known antimalarial drug “quinine” was also derived from a neotropical tree [15].

8. Technological interventions

Indoor tropical plants not only act as oxygen balancing agents, but also have positive psychological effects, help in reducing indoor pollution, purifying indoor air, and absorbing volatile compounds such as formaldehyde, benzene, and trichloroethylene. Indoor air often contains volatile organic compounds such as formaldehyde, benzene, and chloroform. These toxins come from different sources including cooking, showering, furniture, and smoking. House plants can remove some toxins from the air, but they aren’t very efficient: A homeowner would need more than 20 plants to remove formaldehyde from a typical room. Developing improved plants through recent innovative approaches can be of great help in this context (Figure 2). Researchers have worked out that the detoxification ability of the plants can be improved by the expression of the mammalian gene(s). Stuart strand and colleagues introduced a rabbit gene (CYP2E1) into a common houseplant, pothos ivy (Epipremnum aureum), and resultant plants were able to remove injected chloroform and benzene from the vial containing transgenic ivy plants [16].

Figure 2.

Integrative strategies and their applications to improve plants for better nutrition, medicine, and vaccines.

Developing fancy indoor plants has gotten attention and different research groups have attempted to produce glowing plants. Mitiouchkina et al. [17] engineered tobacco plants with a fungal bioluminescence system that converts caffeic acid (present in the plants) into luciferin for the production of self-sustained luminescence, visible to the naked eye. Researchers have also engineered metabolic pathways to divert the natural supply of caffeic acid resulting in their ability to glow. Hence, these plants can glow throughout their life cycle. Likewise, transgenic papaya has been developed to stand against viral infection and is successfully grown in Hawaii. SunUp and Rainbow are the commonly grown varieties of virus-resistant transgenic papaya.

With the advent of next-generation techniques and advancements in DNA sequencing, it has become much more feasible to explore the genome of any plant for the desired traits. Evolutionary biology has got a great pace with these advancements. Phylogenetic and macroevolutionary analysis has been employed to define their genetic relatedness, thus helping out to track better plant species. Non-coding and repetitive DNA sequences play critical roles in determining the phenotype and genome evolution. The pan-genome analysis offers a valuable platform to evaluate the genetic diversity of species via investigation of their entire genome repertoire. It is now feasible to make multiple high-quality genomes that can be used to construct high-resolution pan-genomes making it possible to track all of the variations. However, high-throughput new tools would be required for the assembling, displaying, and interaction studies of such high-resolution pan-genomes.

Genome editing is one of the emerging innovations in the current millennium that has proved its potential to develop plants of desire. This innovative technology has been employed for stacking gene mutations, manipulating gene expression, and improvement of yield. Further, CRISPR-edited plants are not taken as conventional GMOs so, need not get approval from the regulatory bodies provided they are free from exogenous DNA. This has emerged as a user-friendly tool to address challenges in the production of tropical plants and improve their nutritional value. Numerous tropical plants have been targeted to improve through CRISPR/Cas9 and have achieved phenomenal successes during the last decade. Researchers at Cold Spring Harbor Laboratory precisely edited tomato genes involved in fruit size and shape, flowering time, self-pruning, and growth habitat, hence generating new alleles for valuable traits and improved plant architecture [18]. RAS-PDS1 and RAS-PDS2 (phytoene desaturase genes) were mutated in bananas to improve carotenoid biosynthesis, and it was reported to be improved by 59% [19]. Mutants of cassava plants were developed by targeting the MePDS gene and more than 95% of mutants exhibited partial albino or albino phenotype in cotyledonary-stage somatic embryos. Mutant embryos developed into plantlets indicating that 22–47% of the mutants were stable [20]. CRISPR-mediated editing of nCBP-1 and nCBP-2 (elF4E isoforms) in cassava resulted in improved resistance against cassava brown streak disease. Differential expression and genome-wide studies revealed numerous genes involved in salinity, drought, cold, and oxidative stresses. These genes can be targeted for improved tolerance against abiotic stresses in cassava. EgWRKY genes in African oil palm appeared to be upregulated in response to abiotic stresses. These findings revealed the crucial role of EgWRKY in abiotic stress tolerance, hence providing a great opportunity to edit the palm genome for enhanced abiotic stress tolerance. Likewise, S-genes mutations in papaya boosted its defense response against insect pests and pathogens by increasing the accumulation of papain (a cysteine protease). Targeted mutation of TcNPR3NPR3 resulted in upregulation of PR gene expression and increased resistance against pathogens in Theobroma cacao. Though different tropical plants have been mutated through CRISPR/Cas9 system, certain limitations are there, which need to be addressed for the widespread applications of technology.

Acknowledgments

The author acknowledges Drs. Faiz Ahmad Joyia, Ghulam Mustafa, and Rimsha Riaz for their significant contributions to the chapter.