SUMMARY
Given their sessile nature, land plants must use various mechanisms to manage dehydration under water‐deficit conditions. Osmostress‐induced activation of the SNF1‐related protein kinase 2 ...(SnRK2) family elicits physiological responses such as stomatal closure to protect plants during drought conditions. With the plant hormone ABA receptors PYR (pyrabactin resistance)/PYL (pyrabactin resistance‐like)/RCAR (regulatory component of ABA receptors) proteins and group A protein phosphatases, subclass III SnRK2 also constitutes a core signaling module for ABA, and osmostress triggers ABA accumulation. How SnRK2 is activated through ABA has been clarified, although its activation through osmostress remains unclear. Here, we show that Arabidopsis ABA and abiotic stress‐responsive Raf‐like kinases (AtARKs) of the B3 clade of the mitogen‐activated kinase kinase kinase (MAPKKK) family are crucial in SnRK2‐mediated osmostress responses. Disruption of AtARKs in Arabidopsis results in increased water loss from detached leaves because of impaired stomatal closure in response to osmostress. Our findings obtained in vitro and in planta have shown that AtARKs interact physically with SRK2E, a core factor for stomatal closure in response to drought. Furthermore, we show that AtARK phosphorylates S171 and S175 in the activation loop of SRK2E in vitro and that Atark mutants have defects in osmostress‐induced subclass III SnRK2 activity. Our findings identify a specific type of B3‐MAPKKKs as upstream kinases of subclass III SnRK2 in Arabidopsis. Taken together with earlier reports that ARK is an upstream kinase of SnRK2 in moss, an existing member of a basal land plant lineage, we propose that ARK/SnRK2 module is evolutionarily conserved across 400 million years of land plant evolution for conferring protection against drought.
Significance Statement
Here, we show that Arabidopsis ABA and abiotic stress‐responsive Raf‐like kinases (AtARKs) belonging to the B3 clade of the mitogen‐activated kinase kinase kinase family play a crucial role in Arabidopsis drought responses, such as stomatal closure, through phosphorylation‐mediated activation of subclass III SNF1‐related protein kinase 2 (SnRK2) partly in an abscisic acid‐independent manner. Our findings indicate that ARK is a novel component of osmostress signaling that connects osmosensing with SnRK2 activation.
Plant response to drought and hyperosmosis is mediated by the phytohormone abscisic acid (ABA), a sesquiterpene compound widely distributed in various embryophyte groups. Exogenous ABA as well as ...hyperosmosis activates the sucrose nonfermenting 1 (SNF1)-related protein kinase2 (SnRK2), which plays a central role in cellular responses against drought and dehydration, although the details of the activation mechanism are not understood. Analysis of a mutant of the mossPhyscomitrella patenswith reduced ABA sensitivity and reduced hyperosmosis tolerance revealed that a protein kinase designated “ARK” (for “ABA and abiotic stress-responsive Raf-like kinase”) plays an essential role in the activation of SnRK2. ARK encoded by a single gene inP. patensbelongs to the family of group B3 Raf-like MAP kinase kinase kinases (B3-MAPKKKs) mediating ethylene, disease resistance, and salt and sugar responses in angiosperms. Our findings indicate that ARK, as a novel regulatory component integrating ABA and hyperosmosis signals, represents the ancestral B3-MAPKKKs, which multiplied, diversified, and came to have specific functions in angiosperms.
Abscisic acid (ABA) controls seed dormancy and stomatal closure through binding to the intracellular receptor Pyrabactin resistance1 (Pyr1)/Pyr1-like/regulatory components of ABA receptors ...(PYR/PYL/RCAR) in angiosperms. Genes encoding PYR/PYL/RCAR are thought to have arisen in the ancestor of embryophytes, but the roles of the genes in nonvascular plants have not been determined. In the liverwort
, ABA reduces growth and enhances desiccation tolerance through increasing accumulation of intracellular sugars and various transcripts such as those of
(
)-like genes. In this study, we analyzed a gene designated Mp
which is closely related to
/
/
of angiosperms, in transgenic liverworts. Transgenic lines overexpressing Mp
-
showed ABA-hypersensitive growth with enhanced desiccation tolerance, whereas Mp
generated by CRISPR-Cas9-mediated genome editing showed ABA-insensitive growth with reduced desiccation tolerance. Transcriptome analysis indicated that Mp
is a major regulator of abiotic stress-associated genes, including all 35 ABA-induced
-like genes. Furthermore, these transgenic plants showed altered responses to extracellular Suc, suggesting that ABA and PYR/PYL/RCAR function in sugar responses. The results presented here reveal an important role of PYR/PYL/RCAR in the ABA response, which was likely acquired in the common ancestor of land plants. The results also indicate the archetypal role of ABA and its receptor in sugar response and accumulation processes for vegetative desiccation tolerance in bryophytes.
Abscisic acid (ABA) is not a plant-specific compound but one found in organisms across kingdoms from bacteria to animals, suggesting that it is a ubiquitous and versatile substance that can modulate ...physiological functions of various organisms. Recent studies have shown that plants developed an elegant system for ABA sensing and early signal transduction mechanisms to modulate responses to environmental stresses for survival in terrestrial conditions. ABA-induced increase in stress tolerance has been reported not only in vascular plants but also in non-vascular bryophytes. Since bryophytes are the key group of organisms in the context of plant evolution, clarification of their ABA-dependent processes is important for understanding evolutionary adaptation of land plants. Molecular approaches using
Physcomitrella patens
have revealed that ABA plays a role in dehydration stress tolerance in mosses, which comprise a major group of bryophytes. Furthermore, we recently reported that signaling machinery for ABA responses is also conserved in liverworts, representing the most basal members of extant land plant lineage. Conservation of the mechanism for ABA sensing and responses in angiosperms and basal land plants suggests that acquisition of this mechanism for stress tolerance in vegetative tissues was one of the critical evolutionary events for adaptation to the land. This review describes the role of ABA in basal land plants as well as non-land plant organisms and further elaborates on recent progress in molecular studies of model bryophytes by comparative and functional genomic approaches.
The SNF1-related protein kinase 2 (SnRK2) family includes key regulators of osmostress and abscisic acid (ABA) responses in angiosperms and can be classified into three subclasses. Subclass III ...SnRK2s act in the ABA response while ABA-nonresponsive subclass I SnRK2s are regulated through osmostress. Here we report that an ancient subclass III SnRK2-based signalling module including ABA and an upstream Raf-like kinase (ARK) exclusively protects the moss
from drought. Subclass III SnRK2s from both Arabidopsis and from the semiterrestrial alga
, which contains all the components of ABA signalling except ABA receptors, complement
mutants, whereas Arabidopsis subclass I SnRK2 cannot. We propose that the earliest land plants developed the ABA/ARK/subclass III SnRK2 signalling module by recruiting ABA to regulate a pre-existing dehydration response and that subsequently a novel subclass I SnRK2 system evolved in vascular plants conferring osmostress protection independently from the ancient system.
Vegetative desiccation tolerance is common in bryophytes, although this character has been lost in most vascular plants. The moss Physcomitrella patens survives complete desiccation if treated with ...abscisic acid (ABA). Group A protein phosphatases type 2C (PP2C) are negative regulators of abscisic acid signalling. Here we show that the elimination of Group A PP2C is sufficient to ensure P. patens survival to full desiccation, without ABA treatment, although its growth is severely hindered. Microarray analysis shows that the Group A PP2C-regulated genes exclusively overlap with genes exhibiting a high level of ABA induction. Group A PP2C disruption weakly affects ABA-activated kinase activity, indicating Group A PP2C action downstream of these kinases in the moss. We propose that Group A PP2C emerged in land plants to repress desiccation tolerance mechanisms, possibly facilitating plants propagation on land, whereas ABA releases the intrinsic desiccation tolerance from Group A PP2C regulation.
Summary
Abscisic acid (ABA) and its signaling system are important for land plants to survive in terrestrial conditions. Here, we took a phosphoproteomic approach to elucidate the ABA signaling ...network in Physcomitrella patens, a model species of basal land plants. Our phosphoproteomic analysis detected 4630 phosphopeptides from wild‐type P. patens and two ABA‐responsive mutants, a disruptant of group‐A type‐2C protein phosphatase (PP2C; ppabi1a/b) and AR7, a defective mutant in ARK, identified as an upstream regulator of SnRK2. Quantitative analysis detected 143 ABA‐responsive phosphopeptides in P. patens. The analysis indicated that SnRK2‐mediated phosphorylation and target motifs were partially conserved in bryophytes. Our data demonstrate that the PpSnRK2B and AREB/ABF‐type transcription factors are phosphorylated in vivo in response to ABA under the control of ARK. On the other hand, our data also revealed the following: (i) the entire ABA‐responsive phosphoproteome in P. patens is quite diverse; (ii) P. patens PP2C affects additional pathways other than the known ABA signaling pathway; and (iii) ARK is mainly involved in ABA signaling. Taken together, we propose that the core ABA signaling pathway is essential in all land plants; however, some ABA‐responsive phosphosignaling uniquely developed in bryophytes during the evolutionary process.
Significance Statement
Phosphoproteomic profiling in Physcomitrella patens revealed ABA‐dependent phosphosignaling, leading us to compare ABA responses between bryophytes and higher plants at the signaling level.
Low‐molecular‐weight sugars serve as protectants for cellular membranes and macromolecules under the condition of dehydration caused by environmental stress such as desiccation and freezing. These ...sugars also affect plant growth and development by provoking internal signaling pathways. We previously showed that both sugars and the stress hormone abscisic acid (ABA) enhance desiccation tolerance of gemma, a dormant propagule of the liverwort Marchantia polymorpha. To determine the role of ABA in sugar responses in liverworts, we generated genome‐editing lines of M. polymorpha ABA DEFICIENT 1 (MpABA1) encoding zeaxanthin epoxidase, which catalyzes the initial reaction toward ABA biosynthesis. The generated Mpaba1 lines that accumulated only a trace amount of endogenous ABA showed reduced desiccation tolerance and reduced sugar responses. RNA‐seq analysis of sucrose‐treated gemmalings of M. polymorpha revealed that expression of a large part of sucrose‐induced genes was reduced in Mpaba1 compared to the wild‐type. Furthermore, Mpaba1 accumulated smaller amounts of low‐molecular‐weight sugars in tissues upon sucrose treatment than the wild‐type, with reduced expression of genes for sucrose synthesis, sugar transporters, and starch‐catabolizing enzymes. These results indicate that endogenous ABA plays a role in the regulation of the positive feedback loop for sugar‐induced sugar accumulation in liverworts, enabling the tissue to have desiccation tolerance.
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