Pivotal Role of Phytohormones and Their Responsive Genes in Plant Growth and Their Signaling and Transduction Pathway under Salt Stress in Cotton

Abstract

The presence of phyto-hormones in plants at relatively low concentrations plays an indispensable role in regulating crop growth and yield. Salt stress is one of the major abiotic stresses limiting cotton production. It has been reported that exogenous phyto-hormones are involved in various plant defense systems against salt stress. Recently, different studies revealed the pivotal performance of hormones in regulating cotton growth and yield. However, a comprehensive understanding of these exogenous hormones, which regulate cotton growth and yield under salt stress, is lacking. In this review, we focused on new advances in elucidating the roles of exogenous hormones (gibberellin (GA) and salicylic acid (SA)) and their signaling and transduction pathways and the cross-talk between GA and SA in regulating crop growth and development under salt stress. In this review, we not only focused on the role of phyto-hormones but also identified the roles of GA and SA responsive genes to salt stress. Our aim is to provide a comprehensive review of the performance of GA and SA and their responsive genes under salt stress, assisting in the further elucidation of the mechanism that plant hormones use to regulate growth and yield under salt stress.

Keywords: abiotic stress; crop improvement; genes; phytohormones.

Figure 1

The regulatory role of an exogenous supply of GA and its biosynthetic pathway in mitigating salt stress. The GA 20-oxidase catalyzes GA₁₂, GA₁₅, GA₂₄, and GA_9. Similarly, GA₉ is further converted to GA₄ by GA-3 oxidase. The GA signaling activation pathway occurs due to the interaction between DeLLA proteins and GID1, which leads to the degradation of DELLA proteins. The degradation of DELLA proteins reduces the accumulation of reactive oxygen species (ROS) and improves plant growth under salt stress. The over-expression of GID1 increases plant sensitivity. GID1 with GA_S carries the GA signal to the DELLA proteins to modulate the expression of GAs synthesis genes, XERICO. XERICO genes reduce salt stress and improve crop growth and yield. GA exogenous application enhances the contents of indole-3-acetic acid (IAA) and abscisic acid (ABA) and, as a result, improves fiber strength, micornaire reading, reproductive organ development, and maturation in plant color.

Figure 2

The regulatory roles of seed priming with GA in mitigating salt stress. Salt enters the plants through the roots, which creates toxic effects on the plants due to the accumulation of sodium (Na⁺). The higher availability of Na⁺ in soils makes the plants more susceptible to osmotic pressure, ion toxicity, and nutrient imbalances. Seed priming with GA before sowing can protect the seeds from germination through to maturity and enhance the crop yield.

Figure 3

Illustration of the vital performance of an exogenous supply of SA and its biosynthetic pathway in mitigating salt stress. SA accumulation mutant snc1 triggers salt-induced injury, meanwhile SA deficit mutant nahG boosts SA signaling, blocking npr1-1 and snc1 and as a result, reduces salt stress. Due to the application of SA, salt-induced injury decreases by the variation in the expression pattern of the GST family of genes, such as SIGSTT2, SIGSTT3, and SIGSTT4 and antioxidant genes including GPX1, GPX2, DHAR, GR, GST1, HST2, MDHAR, and GS. During salt stress, SA application regulates stomata closure via secondary messengers (adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP)) and these secondary messengers further regulate a variety of physiological activities and gene expressions, maintaining cell cycles and metabolic functions of the crops.

Figure 4

The role of gibberellin and salicylic acid cross-talk in mitigating salt stress in cotton. Salt stress induces oxidative stress. Oxidative stress triggers the production of reactive oxygen species (ROS) and causes cell death. ROS production is reduced by the supply of exogenously applied GA and SA. The overexpression of GASA genes and the exogenous supply of GA₃ at the same time reverse salt stress which is further regulated by SA biosynthesis in plants due to the presence of GASA genes.