Hormone balance and abiotic stress tolerance in crop plants

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Plant hormones play central roles in the ability of plants to adapt to changing environments, by mediating growth, development, nutrient allocation, and source/sink transitions. Although ABA is the most studied stress-responsive hormone, the role of cytokinins, brassinosteroids, and auxins during environmental stress is emerging. Recent evidence indicated that plant hormones are involved in multiple processes. Cross-talk between the different plant hormones results in synergetic or antagonic interactions that play crucial roles in response of plants to abiotic stress. The characterization of the molecular mechanisms regulating hormone synthesis, signaling, and action are facilitating the modification of hormone biosynthetic pathways for the generation of transgenic crop plants with enhanced abiotic stress tolerance.

Research highlights

► Hormones play central roles in plant adaptation to changing environments. ► Hormonal cross-talk regulates plant responses. ► Large changes in hormone content can have negative effects on plant growth and development. ► The use of inducible promoters will facilitate the generation of stress tolerance crops.

Introduction

As sessile organisms, plants must regulate their growth and development in order to respond to numerous external stimuli and an ever-changing environment [1••]. These adaptations include the responses to temperature fluctuations, water and nutrients imbalance, and pathogens, etc. These responses are mediated by plant growth regulators (phytohormones), compounds derived from plant biosynthetic pathways that can act either at the site of synthesis or following their transport, elsewhere in the plant. Collectively, plant hormones regulate every aspect of plant growth and development and the responses of plants to biotic and abiotic stresses. Plant growth regulators include the five classical phytohormones: abscisic acid (ABA), ethylene, cytokinin (CK), auxin (IAA), gibberellin (GA), jasmonate (JA), as well as brassinosteroids (BR), salicylic acid (SA), nitric oxide (NO), and strigolactone (SL), and it is likely that additional growth regulators are yet to be discovered. In recent years, significant research progress contributed to the understanding of processes associated with the biosynthesis of plant hormones, their metabolism, as well as their role in signaling. Studies using plants bearing mutations in hormone-biosynthetic pathways have been instrumental in advancing our understanding of the processes associated with the plant responses to changing environments. However, hormones do not act in isolation but are interrelated by synergistic or antagonistic cross-talk so that they modulate each other's biosynthesis or responses. Reviews on hormone action and signaling of ABA [2, 3, 4], CK [5, 6, 7], ethylene [8], BR [9, 10] and JA [11], and on hormone cross-talk [12••, 13] have been published recently. Here, we highlight the latest advances in our understanding of the role of hormones and hormone cross-talk in plant responses to abiotic stresses. We then discuss the recent progress in the engineering of hormone-associated genes aimed at improving crop stress tolerance.

Section snippets

Hormones and the response to abiotic stress

Phytohormones are essential for the ability of plants to adapt to abiotic stresses by mediating a wide range of adaptive responses [13, 14, 15, 16•]. They often rapidly alter gene expression by inducing or preventing the degradation of transcriptional regulators via the ubiquitin–proteasome system [17]. One of the most studied topics in the response of plants to abiotic stress, especially water stress, is ABA signaling and ABA-responsive genes. ABA synthesis is one of the fastest responses of

Hormone cross-talk

Evidence supporting hormone cross-talk comes mainly from analysis of A. thaliana mutant phenotypes [13]. The synergistic or antagonistic hormone action and the coordinated regulation of hormone biosynthetic pathways play crucial roles in the adaptation of plants to abiotic stress. Recently, the role of auxins in drought tolerance was postulated; TLD1/OsGH3.13, encoding indole-3-acetic acid (IAA)-amido synthetase, was shown to enhance the expression of LEA (late embryogenesis abundant) genes,

Biotechnological applications

A large number of genes associated with de novo ABA biosynthesis and genes encoding ABA receptors and downstream signal relays have been characterized in Arabidopsis (reviewed by [2]). The catalytic steps of ABA biosynthesis involving the conversion of β-carotene to ABA is mediated by the action of enzymes encoded by ABA1/LOS6, ABA4, NCED, ABA2, and ABA3/LOS5 [39]. Some of these genes have been manipulated in crops. ABA3/LOS5 encodes a Mo-cofactor sulfurase (MCSU) that catalyzes the final

Conclusions

The molecular mechanisms regulating hormone synthesis, signaling, and action have been elucidated during the past few years, and the roles of plant hormones for responses to changing environments have been demonstrated. These findings will facilitate the modification of hormone biosynthetic pathways for the generation of transgenic plants with enhanced abiotic stress tolerance. Controlling the hormone dose/response ratio remains a challenge, since the hormone levels attained should be moderate

References and recommended reading

Papers of particular interest published within the period of review have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We apologize to colleagues whose work could not be cited owing to space limitations. The research in our lab was supported by grants from NSF-IOS-0802112, CGIAR GCP#3008.03, UC Discovery #bio06-10627, and the Will W. Lester Endowment of University of California. Z.P. was supported by Vaadia-BARD Postdoctoral Fellowship Award (FI-419-08) from the US-Israel Binational Agricultural Research and Development Fund.

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