ABA Perception Research Articles

Stomatal patterning is differently regulated in adaxial and abaxial epidermis in Arabidopsis.

Stomatal pores in leaves mediate CO2 uptake into the plant and water loss via transpiration. Most plants are hypostomatous with stomata present only in the lower leaf surface (abaxial epidermis). Many herbs, including the model plant Arabidopsis, have substantial numbers of stomata also on the upper (adaxial) leaf surface. Studies of stomatal development have mostly focused on abaxial stomata and very little is known of adaxial stomatal formation. We analysed the role of leaf number in determining stomatal density and stomatal ratio, and studied adaxial and abaxial stomatal patterns in Arabidopsis mutants deficient in known abaxial stomatal development regulators. We found that stomatal density in some genetic backgrounds varies between different fully expanded leaves, and thus we recommend using defined leaves for analyses of stomatal patterning. Our results indicate that stomatal development is at least partly independently regulated in adaxial and abaxial epidermis, as (i) plants deficient in ABA biosynthesis and perception have increased stomatal ratios, (ii) the epf1epf2, tmm, and sdd1 mutants have reduced stomatal ratios, (iii) erl2 mutants have increased adaxial but not abaxial stomatal index, and (iv) stomatal precursors preferentially occur in abaxial epidermis. Further studies of adaxial stomata can reveal new insights into stomatal form and function.

Insights into the Genes Involved in ABA Biosynthesis and Perception during Development and Ripening of the Chilean Strawberry Fruit.

Hormones act as master ripening regulators. In non-climacteric fruit, ABA plays a key role in ripening. Recently, we confirmed in Fragaria chiloensis fruit that in response to ABA treatment the fruit induces ripening-associated changes such as softening and color development. In consequence of these phenotypic changes, transcriptional variations associated with cell wall disassembly and anthocyanins biosynthesis were reported. As ABA stimulates the ripening of F. chiloensis fruit, the molecular network involved in ABA metabolism was analyzed. Therefore, the expression level of genes involved in ABA biosynthesis and ABA perception was quantified during the development of the fruit. Four NCED/CCDs and six PYR/PYLs family members were identified in F. chiloensis. Bioinformatics analyses confirmed the existence of key domains related to functional properties. Through RT-qPCR analyses, the level of transcripts was quantified. FcNCED1 codifies a protein that displays crucial functional domains, and the level of transcripts increases as the fruit develops and ripens, in parallel with the increment in ABA. In addition, FcPYL4 codifies for a functional ABA receptor, and its expression follows an incremental pattern during ripening. The study concludes that FcNCED1 is involved in ABA biosynthesis; meanwhile, FcPYL4 participates in ABA perception during the ripening of F. chiloensis fruit.

Abscisic Acid Perception and Signaling in Chenopodium quinoa

Food production and global economic stability are being threatened by climate change. The increment of drought episodes and the increase of soil salinization are major problems for agriculture worldwide. Chenopodium quinoa (quinoa), as a resilient crop, is capable of growth in harsh environments due to its versatility and adaptive capacity. Quinoa is classified as an extremophile crop, tolerant to salinity, drought and low temperature. Furthermore, quinoa is recognized as a pseudo-cereal with outstanding nutritional properties. The phytohormone ABA is a key regulator of physiological responses to salinity and drought, among others stressful conditions. In this article we want to revise recent discoveries regarding ABA perception and signaling in quinoa, and evaluate its implications on stress-tolerance breeding of this pseudocereal and other crops.

GmLCLs negatively regulate ABA perception and signalling genes in soybean leaf dehydration response.

The circadian clock allows plants to actively adapt to daily environmental changes through temporal regulation of physiological traits. In response to drought stress, circadian oscillators gate ABA signalling, but the molecular mechanisms remain unknown, especially in crops. Here, we investigated the role of soybean circadian oscillators GmLCLa1, GmLCLa2, GmLCLb1 and GmLCLb2 in leaf water stress response. Under dehydration stress, the GmLCL quadruple mutant had decreased leaf water loss. We found that the dehydration treatment delayed the peak expression of GmLCL genes by 4 hr. In addition, the circadian clock in hairy roots also responded to ABA, which led to a free-running rhythm with shortened period. Importantly, in the gmlclq quadruple mutant, diurnal expression phases of several circadian-regulated ABA receptor, ABA catabolism and ABA signalling-related genes were shifted significantly to daytime. Moreover, in the gmlclq mutant leaf, expression of GmPYL17, GmCYP707A, GmABI2 and GmSnRK2s was increased under water dehydration stress. In summary, our results show that GmLCLs act as negative regulators of ABA signalling in leaves during dehydration response.

ABA perception is modulated by membrane receptor-like kinases.

This article comments on: Shang Y, Yang D, Ha Y, Shin H-Y, Nam KH. 2020. RPK1 and BAK1 sequentially form complexes with OST1 to regulate ABA-induced stomatal closure. Journal of Experimental Botany 71, 1491–1502.

Novel Pathway Regulates ABA Perception: How ESCRTs Regulate Stability of ABA Receptors

Novel Pathway Regulates ABA Perception: How ESCRTs Regulate Stability of ABA Receptors

Reduced ABA Accumulation in the Root System is Caused by ABA Exudation in Upland Rice (Oryza sativa L. var. Gaoshan1) and this Enhanced Drought Adaptation.

Lowland rice (Nipponbare) and upland rice (Gaoshan 1) that are comparable under normal and moderate drought conditions showed dramatic differences in severe drought conditions, both naturally occurring long-term drought and simulated rapid water deficits. We focused on their root response and found that enhanced tolerance of upland rice to severe drought conditions was mainly due to the lower level of ABA in its roots than in those of the lowland rice. We first excluded the effect of ABA biosynthesis and catabolism on root-accumulated ABA levels in both types of rice by monitoring the expression of four OsNCED genes and two OsABA8ox genes. Next, we excluded the impact of the aerial parts on roots by suppressing leaf-biosynthesized ABA with fluridone and NDGA (nordihydroguaiaretic acid), and measuring the ABA level in detached roots. Instead, we proved that upland rice had the ability to export considerably more root-sourced ABA than lowland rice under severe drought, which improved ABA-dependent drought adaptation. The investigation of apoplastic pH in root cells and root anatomy showed that ABA leakage in the root system of upland rice was related to high apoplastic pH and the absence of Casparian bands in the sclerenchyma layer. Finally, taking some genes as examples, we predicted that different ABA levels in rice roots stimulated distinct ABA perception and signaling cascades, which influenced its response to water stress.

The unique mode of action of a divergent member of the ABA-receptor protein family in ABA and stress signaling

Proteins in the PYR/PYL/RCAR family (PYLs) are known as receptors for the phytohormone ABA. Upon ABA binding, PYL adopts a conformation that allows it to interact with and inhibit clade A protein phosphatase 2Cs (PP2Cs), which are known as the co-receptors for ABA. Inhibition of the PP2Cs then leads to the activation of the SnRK2 family protein kinases that phosphorylate and activate downstream effectors in ABA response pathways. The PYL family has 14 members in Arabidopsis, 13 of which have been demonstrated to function as ABA receptors. The function of PYL13, a divergent member of the family, has been enigmatic. We report here that PYL13 differs from the other PYLs in three key residues that affect ABA perception, and mutations in these three residues can convert PYL13 into a partially functional ABA receptor. Transgenic plants overexpressing PYL13 show increased ABA sensitivity in seed germination and postgermination seedling establishment as well as decreased stomatal conductance, increased water-use efficiency, accelerated stress-responsive gene expression, and enhanced drought resistance. pyl13 mutant plants are less sensitive to ABA inhibition of postgermination seedling establishment. PYL13 interacts with and inhibits some members of clade A PP2Cs (PP2CA in particular) in an ABA-independent manner. PYL13 also interacts with the other PYLs and antagonizes their function as ABA receptors. Our results show that PYL13 is not an ABA receptor but can modulate the ABA pathway by interacting with and inhibiting both the PYL receptors and the PP2C co-receptors.

アブシジン酸受容体PYR/PYL/RCARファミリーを介したアブシジン酸情報伝達機構の解析

アブシジン酸受容体PYR/PYL/RCARファミリーを介したアブシジン酸情報伝達機構の解析

Opening a new era of ABA research

IntroductionAbscisic acid (ABA) is a sesquiterpene plant hormone thatregulates a variety of plant processes including seed dor-mancy, seed maturation, closure of stomata, and adaptiveresponses to abiotic and biotic stresses. It is 50 years since‘‘abscisin’’ was originally reported (Liu and Carns 1961),but it has taken so many years of research to understand thebasic mechanism of ABA perception and early signalingevents (Cutler et al. 2010; Lumba et al. 2010).The completion of the Arabidopsis genome sequencingproject (Arabidopsis Genome Initiative 2000) and sub-sequent functional genomics approaches have had greatimpacts on elucidating the molecular mechanisms of manyplant processes including ABA signaling. The discovery ofthe PYR1/PYL/RCAR receptors was reported indepen-dently by two groups and simplified our model of ABAaction to a core signaling pathway (Ma et al. 2009; Parket al. 2009). The protein function of this family was solvedby structural analysis immediately after these reports(Melcher et al. 2009, 2010; Miyazono et al. 2009;Nishimura et al. 2009; Peterson et al. 2010; Santiago et al.2009; Yin et al. 2009). Furthermore, the ABA core sig-naling pathway has been functionally validated by in vitrore-constitution of ABA-induced gene expression (Umeza-wa et al. 2009; Fujii et al. 2009). ABA research has finallyreached an exciting goal of dissecting a minimal pathwayfrom hormone perception to transcriptional output.Breakthroughs always produce new questions. As ajournal proud of 124 years of history publishing a broadspectrum of research articles on basic plant biology, we areexcited to invite internationally recognized scientists tocomment on the ‘‘state of ABA’’ for this special issue withthe aim of summarizing recent progress in ABA researchand expand the core knowledge from Arabidopsis to a widevariety of plant species with special reference to phylog-eny, evolution and ecology. We will discuss variousaspects of ABA to foreshadow the future directions of thisimportant field of research.ABA metabolism and its signaling are responsive toboth developmental and environmental cues, which makesthis compound a key integrator of growth and developmentwith the environment in plants. Higher plants synthesizeABA via oxidative cleavage of 9-cis-epoxycarotenoids(Zeevaart and Creelman 1988). ABA is catabolizedthrough several pathways and ABA 8

Mg-chelatase H subunit affects ABA signaling in stomatal guard cells, but is not an ABA receptor in Arabidopsis thaliana

Mg-chelatase H subunit (CHLH) is a multifunctional protein involved in chlorophyll synthesis, plastid-to-nucleus retrograde signaling, and ABA perception. However, whether CHLH acts as an actual ABA receptor remains controversial. Here we present evidence that CHLH affects ABA signaling in stomatal guard cells but is not itself an ABA receptor. We screened ethyl methanesulfonate-treated Arabidopsis thaliana plants with a focus on stomatal aperture-dependent water loss in detached leaves and isolated a rapid transpiration in detached leaves 1 (rtl1) mutant that we identified as a novel missense mutant of CHLH. The rtl1 and CHLH RNAi plants showed phenotypes in which stomatal movements were insensitive to ABA, while the rtl1 phenotype showed normal sensitivity to ABA with respect to seed germination and root growth. ABA-binding analyses using 3H-labeled ABA revealed that recombinant CHLH did not bind ABA, but recombinant pyrabactin resistance 1, a reliable ABA receptor used as a control, showed specific binding. Moreover, we found that the rtl1 mutant showed ABA-induced stomatal closure when a high concentration of extracellular Ca2+ was present and that a knockout mutant of Mg-chelatase I subunit (chli1) showed the same ABA-insensitive phenotype as rtl1. These results suggest that the Mg-chelatase complex as a whole affects the ABA-signaling pathway for stomatal movements.Electronic supplementary materialThe online version of this article (doi:10.1007/s10265-011-0426-x) contains supplementary material, which is available to authorized users.

Comparative genetic analysis of a wheat seed dormancy QTL with rice and Brachypodium identifies candidate genes for ABA perception and calcium signaling

Wheat preharvest sprouting (PHS) occurs when seed germinates on the plant before harvest resulting in reduced grain quality. In wheat, PHS susceptibility is correlated with low levels of seed dormancy. A previous mapping of quantitative trait loci (QTL) revealed a major PHS/seed dormancy QTL, QPhs.cnl-2B.1, located on wheat chromosome 2B. A comparative genetic study with the related grass species rice (Oryza sativa L.) and Brachypodium distachyon at the homologous region to the QPhs.cnl-2B.1 interval was used to identify the candidate genes for marker development and subsequent fine mapping. Expressed sequence tags and a comparative mapping were used to design 278 primer pairs, of which 22 produced polymorphic amplicons that mapped to the group 2 chromosomes. Fourteen mapped to chromosome 2B, and ten were located in the QTL interval. A comparative analysis revealed good macrocollinearity between the PHS interval and 3 million base pair (mb) region on rice chromosomes 7 and 3, and a 2.7-mb region on Brachypodium Bd1. The comparative intervals in rice were found to contain three previously identified rice seed dormancy QTL. Further analyses of the interval in rice identified genes that are known to play a role in seed dormancy, including a homologue for the putative Arabidopsis ABA receptor ABAR/GUN5. Additional candidate genes involved in calcium signaling were identified and were placed in a functional protein association network that includes additional proteins critical for ABA signaling and germination. This study provides promising candidate genes for seed dormancy in both wheat and rice as well as excellent molecular markers for further comparative and fine mapping.

Abscisic acid and the control of seed dormancy and germination

AbstractAbscisic acid (ABA) is a plant hormone that regulates seed dormancy and germination. Seeds undergo changes in both ABA content and sensitivity during seed development and germination in response to internal and external cues. Recent advances in functional genomics have revealed the integral components involved in ABA metabolism (biosynthesis and catabolism) and perception, the core signalling pathway, as well as the factors that trigger ABA-mediated transcription. These allow for comparative studies to be conducted on seeds under different environmental conditions and from different genetic backgrounds. This review summarizes our understanding of the control of ABA content and the responsiveness of seeds to afterripening, light, high temperature and nitrate, with a focus on which tissues are involved in its metabolism and signalling. Also described are the regulators of ABA metabolism and signalling, which potentially act as the node for hormone crosstalk. Integration of such knowledge into the complex and diverse events occurring during seed germination will be the next challenge, which will allow for a clearer understanding of the role of ABA.

46. Mechanisms of ABA perception and early ABA signaling cascade

46. Mechanisms of ABA perception and early ABA signaling cascade

At the beginning of the route: ABA perception and signal transduction in plants

Nowadays, fundamentally important data toward the molecular mechanisms of phytohormone action, starting from hormonal signal perception and up to changes in hormone-regulated gene expression, became available. Signaling mechanisms for plant cell responses to ABA remained the least known. Although a substantial progress was achieved in our understanding of the role of protein kinases and protein phosphatases functioning downstream receptors, identification of ABA receptors has induced very hot debates. This review summarizes information obtained during the last decade and concerned the molecular mechanisms of perception and signal transduction of ABA signal in plants.

Abscisic acid plasmalemma perception triggers a calcium influx essential for RAB18 gene expression in Arabidopsis thaliana suspension cells

Pretreatment of Arabidopsis thaliana suspension cells with impermeant calcium chelator EGTA inhibited the ABA-induced RAB18 gene expression. However, extracellular calcium alone, up to 10 mM, did not trigger RAB18 expression. Spectrofluorimetric extracellular Ca 2+ measurement with Fluo-3 showed a fast, within 1 min, Ca 2+ influx associated with outer plasmalemma ABA perception. In the presence of the Ca 2+ blockers Cd 2+ and Ni 2+, RAB18 expression was suppressed. Pimozide and fluspirilene inhibited Ca 2+ influx and ABA-induced RAB18 expression. Thus we demonstrated the involvement of specific Ca 2+ influx in the ABA signaling sequence leading to RAB18 expression.

Induction of RAB18 gene expression and activation of K+ outward rectifying channels depend on an extracellular perception of ABA in Arabidopsis thaliana suspension cells.

Important progress has been made regarding the characterization of the ABA signalling components using genetic and molecular approaches (Leung and Giraudat, 1998). However, we do not yet know the mechanism of ABA perception. Conflicting results concerning the site of ABA perception have been published. The prevailing view is that since ABA controls many responses, different sites of perception for ABA might exist. In order to establish the cellular localisation of the ABA receptors in Arabidopsis thaliana suspension cells, we developed two physiological tests based upon the capacity of impermeant ABA-BSA conjugate to mimic permeant free ABA effects. We show that purified ABA-BSA conjugate is able to trigger RAB18 gene expression and that this response is strictly due to the natural (+)-ABA enantiomer. The rate of RAB18 gene expression was independent of the level of ABA uptake by the cells. Using the voltage-clamp technique we show that ABA-BSA, similarly to ABA, evokes a membrane depolarization and activates time- and voltage-dependent outward rectifying currents (ORC). We demonstrate that these ORC are due to a K+ efflux as assessed by tail currents and specific inhibition by both tetraethylammonium (TEA) and Ba2+. These observations provide evidence in favour of an extracellular site for ABA perception.

Cyclic ADP-ribose and ABA signal transduction

A new assay system utilizing ABA-inducible gene expression in tomato hypocotyl cells has placed cADPR firmly in the signal transduction cascade of this hormone[1xAbscisic acid signaling through cyclic ADP-ribose in plants. Wu, Y. et al. Science. 1997; 278: 2126–2130Crossref | PubMed | Scopus (273)See all References][1]. Furthermore, similar transduction pathways utilizing cADPR appear to be activated regardless of whether the cellular response to ABA is gene expression or stomatal closure (Fig. 2Fig. 2). Striking similarities are evident between cADPR action in plant and animal cells.Fig. 2Schematic representation of ABA signal-transduction pathways that involve cyclic ADP-ribose (cADPR) and result in gene expression in tomato hypocotyl cells or loss of guard cell turgor. The upper part shows ABA perception and transduction in tomato hypocotyls, resulting in gene expression mediated by cADPR and elevation of cytosolic Ca2+ concentration[1xAbscisic acid signaling through cyclic ADP-ribose in plants. Wu, Y. et al. Science. 1997; 278: 2126–2130Crossref | PubMed | Scopus (273)See all References][1]. The lower part shows ABA perception and transduction in guard cells, resulting in loss of cell turgor mediated by cADPR and elevation of cytosolic Ca2+ concentration.View Large Image | Download PowerPoint Slide

The effect of ABA analogs on callus viability and gene expression in Craterostigma plantagineum

We have used callus tissue of the desiccation‐tolerant plant, Craterostigma plantagineum Hochst to probe the stereochemical requirements of ABA in the desiccation tolerance response, using ABA analogs. Callus was treated with (S)‐(+)‐ABA and (R)‐(‐)‐ABA and two pure isomer derivatives of each, and viability after a drying treatment and the expression of four desiccation‐induced late embryogenesis abundant (LEA)‐type transcripts and sucrose synthase from Craterostigma were measured. Both stereoisomers alone caused transcript expression and highest viability. The two derivatives of (R)‐(‐)‐ABA gave poor or no transcript expression and no viability, suggesting that alteration of a ring double bond to a single bond in this series of compounds is significant in ABA perception for both responses. One (S)‐(+)‐ABA derivative was effective in inducing all transcripts and causing callus viability. The other analog derivative induced all protein transcripts without concomitant tissue viability after drying. This suggests that some other ABA‐inducible factor(s) are vital for desiccation tolerance of callus.