Unlike animals, most of the plants are sessile. This may be a reason why they developed the powerful ability of organ generation throughout their lifetime, which is distinct from the animals, whose generation potential is restricted in a certain period during development. Half part of the plant body, the root system, is hidden under the ground, where there is a competition of resources, for example, water and nutrients or biotic stresses and abiotic stresses surrounding the root system. With its strong regeneration ability, the architecture of the root system is shaped by all of these environmental cues together with the internal developmental signals. In this process, phytohormones work as the regulatory molecules mediating the internal and external developmental signals, thus controlling the morphology and function of the root system architecture. This chapter introduces the development of root system regulated by various phytohormones, like auxin, cytokinin, etc.
- root architecture
- postembryonic organogenesis
1. Anatomy and development of root
1.1 Root system architecture
In different plant species, root system architecture (RSA) has diverse morphologies. There are basically two types of RSA, the taproot system (or allorhizic system) in gymnosperms and dicotyledons, like
1.2 Intrinsic developmental signals and environmental conditions modify root system architecture
It is noteworthy that not all of the pre-branch sites emerge to be LRs . The dormant pre-branch sites may present a selective mechanism for LR formation under certain growth conditions, such as water availability, nutrient levels, physical obstacles, or damage [5, 10, 11, 12, 13]. It is interesting that many of the external signals converge on phytohormones to regulate root development. Among these phytohormones, auxin functions as a central mediator.
Mechanical forces are important regulators for plant morphogenesis. LRs always emerge from the convex side of PR bending, resulting in a left-right alternation of LRs. Bending caused by gravitropic curvature led to the initiation of LRs, where a subcellular relocalization of PIN1 was observed . Release the pericycle cells from the restraints of adjacent endodermis by targeted single cell ablation of endodermal cells triggered the pericycle to reenter the cell cycle and induced auxin-dependent LR initiation . Excision of the
2. Roles of phytohormones on root formation
The phytohormone auxin which plays fundamental roles in many aspects of plant growth and development is also a well-documented key regulator of LR development [16, 17]. The natural auxin, indole-3-acetic acid (IAA), is mainly synthesized in a two-step pathway from tryptophan. First, tryptophan is converted to indole-3-pyruvate (IPA) by the TAA1/SAV3 family of aminotransferases; IPA is then converted to IAA by the YUCCA (YUC) family of flavin monooxygenases [18, 19, 20, 21, 22, 23]. Auxin biosynthesis has been shown to play an essential role on both programed and wound-induced LR and AR developments [15, 24, 25].
Polar auxin transport (PAT), mediated by auxin influx (AUX1 and LAXs) and efflux carriers (PINs and MDR/PGPs) [26, 27, 28, 29], generates auxin gradients and maintains an auxin maximum to regulate LR formation and positioning [17, 30, 31, 32, 33].
Auxin signaling is known to be an integrator of endogenous and exogenous signals for root branching [17, 30, 34, 35]. It begins with the degradation of a class of AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) through TRANSPORT INHIBITOR RESPONSE 1 (TIR1) auxin receptor [36, 37], resulting in the activation of the AUXIN RESPONSE FACTOR (ARF) [38, 39]. ARF7 and ARF19 transcription factors further induce the expression of downstream target genes like
Cytokinin is also a main player in root development. In higher plants, isopentenyladenine (iP), trans-zeatin (tZ), and dihydrozeatin (dZ) are the predominant cytokinins . Cytokinin level and patterning in plant are controlled by a fine equilibrium between cytokinin synthesis and catabolism [44, 45]. Cytokinin biosynthesis is dependent on the activity of
Although some evidences showed that cytokinins act as both local and long-distance signals [51, 63, 64, 65], and some transporter proteins have been shown to be involved in cytokine transport [66, 67, 68, 69, 70], the molecular mechanisms of cytokinin transport are still not well characterized.
Postembryonic root development is regulated by the root apical meristem (RAM), where cytokinin is known to act antagonistically with auxin to control the balance of cell division in the division zone and cell differentiation in the transition zone, which is essential for the maintenance of the RAM and affects the growth and patterning of the root [64, 71]. Application of cytokinin reduces the number of meristem cells and the size of RAM and promotes cell differentiation in the transition zone; cytokinin biosynthesis and signaling mutants as well as
On contrary to auxin, which is a positive regulator of LR development, cytokinin acts as a negative regulator of LR formation. Cytokinin suppresses LR initiation through downregulating
Through mutant analysis Chang et al.  showed that cytokinin biosynthesis genes
2.3 Other phytohormones
Other phytohormones, like abscisic acid (ABA), gibberellic acid (GA), brassinosteroid (BR), jasmonic acid (JA), ethylene, and strigolactone (SL), also participate in root growth and development.
Signora et al.  showed that ABA plays an important role in mediating the effects of nitrate on LR formation in
Hansen  reported on the GA-mediated light dependent promotion and inhibition of AR formation. Through mutant analysis, Yaxley et al.  showed that GA is important for normal root elongation in pea. Fu and Harberd  showed that auxin regulates root growth through GA-mediated DELLA protein destabilization. Steffens et al.  showed that GA is ineffective on its own but acts synergistically with ethylene to promote the number of penetrating roots and the growth rate of emerged roots in deepwater rice.
JA, a crucial plant defense hormone, also participated in the regulation of root development. Raya-González et al.  observed that low concentrations of JA inhibited PR growth through an auxin-independent manner and promoted LR formation auxin-dependently, and JA receptor COI1 is involved in JA-induced LR formation and LR positioning. Cross-talk between JA and auxin has been frequently reported. JA has been reported to be implicated in YUC9-mediated auxin biosynthesis in wounded leaves in
Ethylene is also a well-known phytohormone that participates in the plant defense signaling pathways. Strader et al.  reported that ethylene interact with auxin to control root cell expansion. Ivanchenko et al.  observed application of low level of ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) promotes LRP initiation, while higher doses of ACC strongly inhibits LRP initiation but promotes LRP emergence; this regulation of LR initiation and emergence by ethylene is through interactions with auxin. Lewis et al.  reported that ethylene suppresses LR formation through promotion of PIN3 and PIN7-mediated auxin efflux to prevent local auxin accumulation.
Jiang et al.  showed that SL analog GR24 negatively influenced LR priming and emergence, which is dependent on the intimate connection with auxins and cytokinins, with the PAT capacity as a central player.
The root system of higher plants is modified by intrinsic developmental signals and diverse environmental cues. Both the internal and the external signals converged on phytohormones to regulate the formation of a highly plastic and adaptive RSA, which sustains the growth of plants even in adverse conditions. Several lines of evidences suggest that cross-talks among different phytohormones are essential for the regulation of root development, and auxin plays a central role in these processes. Although auxin and cytokinin as the key regulators of root development have been extensively studied, the roles of other phytohormones still need to be further characterized to give us a full view of plant root development.
Conflict of interest
The authors declare no conflict of interest.
Acronyms and abbreviations
root system architecture primary root lateral root adventitious root indole-3-acetic acid polar auxin transport isopentenyladenine trans-zeatin dihydrozeatin root apical meristem lateral root primordia abscisic acid gibberellic acid brassinosteroid jasmonic acid strigolactone 1-aminocyclopropane-1-carboxylic acid
root system architecture
polar auxin transport
root apical meristem
lateral root primordia