Decoding ABA and osmostress signalling in plants from an evolutionary point of view

Abstract

The plant hormone abscisic acid (ABA) is fundamental for land plant adaptation to water-limited conditions. Osmostress, such as drought, induces ABA accumulation in angiosperms, triggering physiological responses such as stomata closure. The core components of angiosperm ABA signalling are soluble ABA receptors, group A protein phosphatase type 2C and SNF1-related protein kinase2 (SnRK2). ABA also has various functions in non-angiosperms, however, suggesting that its role in adaptation to land may not have been angiosperm-specific. Indeed, among land plants, the core ABA signalling components are evolutionarily conserved, implying their presence in a common ancestor. Results of ongoing functional genomics studies of ABA signalling components in bryophytes and algae have expanded our understanding of the evolutionary role of ABA signalling, with genome sequencing uncovering the ABA core module even in algae. In this review, we describe recent discoveries involving the ABA core module in non-angiosperms, tracing the footprints of how ABA evolved as a phytohormone. We also cover the latest findings on Raf-like kinases as upstream regulators of the core ABA module component SnRK2. Finally, we discuss the origin of ABA signalling from an evolutionary perspective.

The Copper Responsive Transcription Factor SPL7 Represses Key Abscisic Acid Biosynthetic Genes to Balance Growth and Drought Tolerance

“…Likewise, genomic and genetic evidence also strongly suggests that the core ABA signaling pathway was already present in the common ancestor of land plants (Khandelwal et al, 2010; Bowman et al, 2017; Eklund et al, 2018; Sun et al, 2019; Komatsu et al, 2020). It was proposed that ABA functions are a major adaptation in early land plants, enabling them to balance protection against osmostress and growth for successful colonization of terrestrial environments (Khandelwal et al, 2010; Takezawa et al, 2011; Eklund et al, 2018; Sun et al, 2019; Komatsu et al, 2020).…”

“…Genetic and genomic analyses have shown that de novo ABA biosynthesis via the carotenoid pathway is likely functionally conserved in land plants, which are considered to have descended from an ancestral form of aquatic algae (Hauser et al, 2011; Takezawa et al, 2011; Takezawa et al, 2015; Bowman et al, 2017; Komatsu et al, 2020). Likewise, genomic and genetic evidence also strongly suggests that the core ABA signaling pathway was already present in the common ancestor of land plants (Khandelwal et al, 2010; Bowman et al, 2017; Eklund et al, 2018; Sun et al, 2019; Komatsu et al, 2020). It was proposed that ABA functions are a major adaptation in early land plants, enabling them to balance protection against osmostress and growth for successful colonization of terrestrial environments (Khandelwal et al, 2010; Takezawa et al, 2011; Eklund et al, 2018; Sun et al, 2019; Komatsu et al, 2020).…”

“…Based on the above considerations, we hypothesize that concurrent increases in cytosolic Cu concentration and ABA biosynthesis upon dehydration may have facilitated molecular evolution in early land plants to couple the two homeostatic events via the SPL7-GTAC circuit, enabling them to better balance desiccation tolerance under water deficiency and rapid growth under water sufficiency. This feature may have given these plants an advantage in terrestrial environments that allowed them to establish a firm foothold on land (Takezawa et al, 2011; Komatsu et al, 2020). Future studies of osmostress responses and Cu homeostasis in basal streptophytes and bryophytes would add to our understanding of land plant evolution.…”

Cellular Phosphorylation Signaling and Gene Expression in Drought Stress Responses: ABA-Dependent and ABA-Independent Regulatory Systems

“…Subclass III SnRK2s are activated in ABA-deficient and ABA-insensitive mutants, suggesting that SnRK2s can be activated by upstream kinases in an ABA-independent manner under drought stress [ 17 , 27 , 41 , 42 ]. B-type MPK KINASE KINASE (MAPKKK) Raf-like kinases have been shown to directly regulate the activity of SnRK2s under osmotic stress [ 12 , 43 , 44 , 45 ]. An ABA-insensitive Physcomitrella patens mutant was isolated by a forward genetic approach and was shown to have a mutation in a gene for ABA AND ABIOTIC STRESS-RESPONSIVE RAF-LIKE KINASE/ABA NON-RESPONSIVE (ARK/ANR), one of the B3-MAPKKK Raf-like kinases [ 46 , 47 ].…”

Emerging Molecular Strategies for Improving Rice Drought Tolerance

“…Rice ABA signaling is now understood well and it starts with ABA binding with its receptor (OsPYR1/PYL/RCAR) complex leading to the inhibition of protein phosphatase 2C (PP2C) mediated inactivation of sucrose nonfermenting 1-related protein kinase 2s (SnRK2s). This, in turn, activates ABA-responsive transcription factors like bZIPs to execute a downstream response to stress adaptation, including drought [ 11 , 12 ]. ABA levels are tightly regulated in response to development and stresses [ 13 ].…”