Integration of Abscisic Acid Signaling with Other Signaling Pathways in Plant Stress Responses and Development

Abstract

Plants are immobile and, to overcome harsh environmental conditions such as drought, salt, and cold, they have evolved complex signaling pathways. Abscisic acid (ABA), an isoprenoid phytohormone, is a critical signaling mediator that regulates diverse biological processes in various organisms. Significant progress has been made in the determination and characterization of key ABA-mediated molecular factors involved in different stress responses, including stomatal closure and developmental processes, such as seed germination and bud dormancy. Since ABA signaling is a complex signaling network that integrates with other signaling pathways, the dissection of its intricate regulatory network is necessary to understand the function of essential regulatory genes involved in ABA signaling. In the present review, we focus on two aspects of ABA signaling. First, we examine the perception of the stress signal (abiotic and biotic) and the response network of ABA signaling components that transduce the signal to the downstream pathway to respond to stress tolerance, regulation of stomata, and ABA signaling component ubiquitination. Second, ABA signaling in plant development processes, such as lateral root growth regulation, seed germination, and flowering time regulation is investigated. Examining such diverse signal integration dynamics could enhance our understanding of the underlying genetic, biochemical, and molecular mechanisms of ABA signaling networks in plants.

Keywords: E3 ubiquitin ligase; abiotic stress signaling; abscisic acid; seed germination; stomatal regulation; ubiquitination.

Figure 1

Overview of the abscisic acid (ABA) signaling pathway. (A) Inactivation of SnRKs, CIPKs, and CDPKs under normal growth conditions (light orange box). PP2C (red oval box) plays an important role in the inactivation of SnRKs, CIPKs, and CDPKs. Inactive MAP3K17/18 (orange oval box) and AREB/ABF (Abscisic Acid Response Element/Abre-Binding Factor) (yellow oval box) undergo protein degradation. (B) Initial perception of environmental and developmental cues. ABA signaling is transduced in Ca²⁺-independent (light blue box) as well as Ca²⁺-dependent (light orange box) manners. Active SnRKs, CIPKs, and CDPKs (dark blue oval box) play important roles in downstream signal transduction. (C) Stomatal regulation via ABA signaling in response to stress and healthy conditions. Under stress conditions, stomatal regulation (purple arrow →) is carried out by active SnRK2.6/OST1 (blue oval box) through the regulation of downstream ion channel genes (green oval boxes), such as SLAH3, SLAC1, and KAT1. This regulation helps stomata remain closed to avoid loss of excessive water under adverse conditions. Under normal conditions, SnRK2.6/OST1 inactivated by PP2C cannot regulate the downstream genes; thus, stomata remain open. (D) Response to stress tolerance via the ABA signaling pathway. The stress tolerance mechanism (black arrow →) is regulated in Ca²⁺-independent as well as Ca²⁺-dependent manners. The MAP kinase cascade (orange oval box) pathway carries the signal for the response to abiotic stress tolerance. It delays ABA gene expression. Contrarily, signal transduction via only AREB/ABF (yellow oval box) shows early expression of ABA related genes, resulting in an early response to stress tolerance. (E) Involvement of ABA signaling in the plant developmental process. Downstream ABA signaling involved in different developmental processes (red arrow →) such as seed germination (light green oval and square boxes), lateral root growth (light blue oval and square boxes), and regulation of flowering time (yellow oval and square boxes). ABI5 emerges as a critical ABA signaling component in the regulation of the plant developmental process. ABA signaling integrates with light signaling (black dark oval box) to regulate plant development. The brown tack facing up (⊥) indicates the role of ubiquitination in ABA signaling. These E3 ubiquitin ligase elements in ABA signaling guide the inactive protein to undergo degradation. The question mark (?) indicates the unknown pathway.

Figure 2

A simplified schematic diagram showing synergistic and antagonistic interactions between the ABA signaling pathway and other hormonal signaling pathways during abiotic and biotic stress.

Figure 3

Integration of various signaling pathways with ABA signaling. ABA signaling plays a central role in regulating different developmental processes, including stress responses, as is evident from its interactions with calcium (Ca²⁺), jasmonic acid (JA), salicylic acid (SA), brassinosteroid (BR), ethylene (ET), and MAP kinase (MAPK) signaling pathway members.