Origin and evolution of genes related to ABA metabolism and its signaling pathways

The efficient capture of polysaccharides in Tetradesmus obliquus of indole-3-acetic acid coupling sludge extraction

Polysaccharide is an important biomass of algae. The sludge extract is rich in organic substances, which can be used by algae for biomass growth and high-value biomass synthesis, but its organic toxicity has an inhibitory effect on algae. To overcome inhibition and improve polysaccharide enrichment, Tetradesmus obliquus was cultured with sludge extract with different indole-3-acetic acid (IAA) concentrations. Within 30 days of the culture cycle, T. obliquus showed in good condition at the IAA dosage content of 10-6 M, the maximum cell density and dry weight were respectively (106.78 ± 2.20) × 106 cell/mL and 2.941 ± 0.067 g/L while the contents of chlorophyll-a, chlorophyll-b, and carotenoid were 1.79, 1.91 and 2.80 times that of the blank group, respectively. The highest polysaccharide accumulation was obtained under this culture condition, reaching 533.15 ± 21.11 mg/L on the 30th day, which was 2.49 times that in the blank group. By FT-IR and NMR analysis, it was found that the polysaccharides of T. obliquus were sulfated polysaccharide with glucose and rhamnose as the main monosaccharides. Proteomic showed that the up-regulation of A0A383WL26 and A0A383WLM8 enhanced the light trapping ability, and A0A383WMJ2 enhanced the accumulation of NADPH. The up-regulation of A0A383WHD5 and A0A383WAY6 indicated that IAA culture could repair the damage caused by sludge toxicity, thus promoting the accumulation of biomass. The above findings provided new insights into the mechanism of sludge toxicity removal of T. obliquus and the enhancement of the polysaccharide accumulation effect under different concentrations of IAA.

Light on perenniality: Para-dormancy is based on ABA-GA antagonism and endo-dormancy on the shutdown of GA biosynthesis

Perennial para- and endo-dormancy are seasonally separate phenomena. Whereas para-dormancy is the suppression of axillary buds (AXBs) by a growing shoot, endo-dormancy is the short-day elicited arrest of terminal and AXBs. In hybrid aspen (Populus tremula x P. tremuloides) compromising the apex releases para-dormancy, whereas endo-dormancy requires chilling. ABA and GA are implicated in both phenomena. To untangle their roles, we blocked ABA biosynthesis with fluridone (FD), which significantly reduced ABA levels, downregulated GA-deactivation genes, upregulated the major GA3ox-biosynthetic genes, and initiated branching. Comprehensive GA-metabolite analyses suggested that FD treatment shifted GA production to the non-13-hydroxylation pathway, enhancing GA₄ function. Applied ABA counteracted FD effects on GA metabolism and downregulated several GA_3/4 -inducible α- and γ-clade 1,3-β-glucanases that hydrolyze callose at plasmodesmata (PD), thereby enhancing PD-callose accumulation. Remarkably, ABA-deficient plants repressed GA₄ biosynthesis and established endo-dormancy like controls but showed increased stress sensitivity. Repression of GA₄ biosynthesis involved short-day induced DNA methylation events within the GA3ox2 promoter. In conclusion, the results cast new light on the roles of ABA and GA in dormancy cycling. In para-dormancy, PD-callose turnover is antagonized by ABA, whereas in short-day conditions, lack of GA₄ biosynthesis promotes callose deposition that is structurally persistent throughout endo-dormancy.

Advances in mass spectrometry-based phosphoproteomics for elucidating abscisic acid signaling and plant responses to abiotic stress

Abiotic stresses have significant impacts on crop yield and quality. Even though significant efforts during the past decade have been devoted to uncovering the core signaling pathways associated with the phytohormone abscisic acid (ABA) and abiotic stress in plants, abiotic stress signaling mechanisms in most crops remain largely unclear. The core components of the ABA signaling pathway, including early events in the osmotic stress-induced phosphorylation network, have recently been elucidated in Arabidopsis with the aid of phosphoproteomics technologies. We now know that SNF1-related kinases 2 (SnRK2s) are not only inhibited by the clade A type 2C protein phosphatases (PP2Cs) through dephosphorylation, but also phosphorylated and activated by upstream mitogen-activated protein kinase kinase kinases (MAP3Ks). Through describing the course of studies to elucidate abiotic stress and ABA signaling, we will discuss how we can take advantage of the latest innovations in mass-spectrometry-based phosphoproteomics and structural proteomics to boost our investigation of plant regulation and responses to ABA and abiotic stress.

PYR/PYL/RCAR Receptors Play a Vital Role in the Abscisic-Acid-Dependent Responses of Plants to External or Internal Stimuli

Abscisic acid (ABA) is a phytohormone that plays a key role in regulating several developmental processes as well as in response to stressful conditions such as drought. Activation of the ABA signaling cascade allows the induction of an appropriate physiological response. The basic components of the ABA signaling pathway have been recognized and characterized in recent years. Pyrabactin resistance, pyrabactin resistance-like, and the regulatory component of ABA receptors (PYR/PYL/RCAR) are the major components responsible for the regulation of the ABA signaling pathway. Here, we review recent findings concerning the PYR/PYL/RCAR receptor structure, function, and interaction with other components of the ABA signaling pathway as well as the termination mechanism of ABA signals in plant cells. Since ABA is one of the basic elements related to abiotic stress, which is increasingly common in the era of climate changes, understanding the perception and transduction of the signal related to this phytohormone is of paramount importance in further increasing crop tolerance to various stress factors.

Genome-Wide Identification and Characterization of Polygalacturonase Gene Family in Maize (Zea mays L.)

Polygalacturonase (PG, EC 3.2.1.15) is a crucial enzyme for pectin degradation and is involved in various developmental processes such as fruit ripening, pollen development, cell expansion, and organ abscission. However, information on the PG gene family in the maize (Zea mays L.) genome and the specific members involved in maize anther development are still lacking. In this study, we identified 55 PG family genes from the maize genome and further characterized their evolutionary relationship and expression patterns. Phylogenetic analysis revealed that ZmPGs are grouped into six Clades, and gene structures of the same Clade are highly conserved, suggesting their functional conservation. The ZmPGs are randomly distributed across maize chromosomes, and collinearity analysis showed that many ZmPGs might be derived from tandem duplications and segmental duplications, and these genes are under purifying selection. Furthermore, gene expression analysis provided insights into possible functional divergence among ZmPGs. Based on the RNA-seq data analysis, we found that many ZmPGs are expressed in various tissues while 18 ZmPGs are highly expressed in maize anther, and their detailed expression profiles in different anther developmental stages were further investigated by using RT-qPCR analysis. These results provide valuable information for further functional characterization and application of the ZmPGs in maize.

Integrating stomatal physiology and morphology: evolution of stomatal control and development of future crops

Stomata are central players in the hydrological and carbon cycles, regulating the uptake of carbon dioxide (CO₂) for photosynthesis and transpirative loss of water (H₂O) between plants and the atmosphere. The necessity to balance water-loss and CO₂-uptake has played a key role in the evolution of plants, and is increasingly important in a hotter and drier world. The conductance of CO₂ and water vapour across the leaf surface is determined by epidermal and stomatal morphology (the number, size, and spacing of stomatal pores) and stomatal physiology (the regulation of stomatal pore aperture in response to environmental conditions). The proportion of the epidermis allocated to stomata and the evolution of amphistomaty are linked to the physiological function of stomata. Moreover, the relationship between stomatal density and [CO₂] is mediated by physiological stomatal behaviour; species with less responsive stomata to light and [CO₂] are most likely to adjust stomatal initiation. These differences in the sensitivity of the stomatal density—[CO₂] relationship between species influence the efficacy of the ‘stomatal method’ that is widely used to infer the palaeo-atmospheric [CO₂] in which fossil leaves developed. Many studies have investigated stomatal physiology or morphology in isolation, which may result in the loss of the ‘overall picture’ as these traits operate in a coordinated manner to produce distinct mechanisms for stomatal control. Consideration of the interaction between stomatal morphology and physiology is critical to our understanding of plant evolutionary history, plant responses to on-going climate change and the production of more efficient and climate-resilient food and bio-fuel crops.

Genome-wide identification and characterization of ABA receptor PYL gene family in rice

BACKGROUND

Abscisic acid (ABA), a key phytohormone that controls plant growth and stress responses, is sensed by the pyrabactin resistance 1(PYR1)/PYR1-like (PYL)/regulatory components of the ABA receptor (RCAR) family of proteins. Comprehensive information on evolution and function of PYL gene family in rice (Oryza sativa) needs further investigation. This study made detailed analysis on evolutionary relationship between PYL family members, collinearity, synteny, gene structure, protein motifs, cis-regulatory elements (CREs), SNP variations, miRNAs targeting PYLs and expression profiles in different tissues and stress responses.

RESULTS

Based on sequence homology with Arabidopsis PYL proteins, we identified a total of 13 PYLs in rice (BOP clade) and maize (PACCMAD clade), while other members of BOP (wheat - each diploid genome, barley and Brachypodium) and PACCMAD (sorghum and foxtail millet) have 8-9 PYLs. The phylogenetic analysis divided PYLs into three subfamilies that are structurally and functionally conserved across species. Gene structure and motif analysis of OsPYLs revealed that members of each subfamily have similar gene and motif structure. Segmental duplication appears be the driving force for the expansion of PYLs, and the majority of the PYLs underwent evolution under purifying selection in rice. 32 unique potential miRNAs that might target PYLs were identified in rice. Thus, the predicted regulation of PYLs through miRNAs in rice is more elaborate as compared with B. napus. Further, the miRNAs identified to in this study were also regulated by stresses, which adds additional layer of regulation of PYLs. The frequency of SAPs identified was higher in indica cultivars and were predominantly located in START domain that participate in ABA binding. The promoters of most of the OsPYLs have cis-regulatory elements involved in imparting abiotic stress responsive expression. In silico and q-RT-PCR expression analyses of PYL genes revealed multifaceted role of ABARs in shaping plant development as well as abiotic stress responses.

CONCLUSION

The predicted miRNA mediated regulation of OsPYLs and stress regulated expression of all OsPYLs, at least, under one stress, lays foundation for further validation and fine tuning ABA receptors for stress tolerance without yield penalty in rice.

Genetic manipulation of abscisic acid receptors enables modulation of water use efficiency

The phytohormone abscisic acid (ABA) is a key signal molecule for controlling stomatal movement and stress responses such as drought. Genetic manipulation of ABA receptors has recently been reported to improve plant water use efficiency (WUE). We therefore compared details of WUE traits between the ABA receptor quintuple mutant and wheat ABA receptor-overexpressing Arabidopsis lines. Biomass and seed productivities per liter of water were both reduced in the receptor mutant but improved in the transgenic Arabidopsis lines. This result suggests that appropriate modulation of ABA receptors can extend crop potential productivity.

Different divergence events for three pairs of PEBPs in Gossypium as implied by evolutionary analysis

INTRODUCTION

The phosphatidylethanolamine-binding protein (PEBP) gene family plays a crucial role in seed germination, reproductive transformation, and other important developmental processes in plants, but its distribution in Gossypium genomes or species, evolutionary properties, and the fates of multiple duplicated genes remain unclear.

OBJECTIVES

The primary objectives of this study were to elucidate the distribution and characteristics of PEBP genes in Gossypium, as well as the evolutionary pattern of duplication and deletion, and functional differentiation of PEBPs in plants.

METHODS

Using the PEBP protein sequences in Arabidopsis thaliana as queries, blast alignment was carried out for the identification of PEBP genes in four sequenced cotton species. Using the primers designed according to the PEBP genome sequences, PEBP genes were cloned from 15 representative genomes of Gossypium genus, and the gene structure, CDS sequence, protein sequence and properties were predicted and phylogenetic analysis was performed. Taking PEBP proteins of grape as reference, grouping of orthologous gene, analysis of phylogeny and divergence of PEBPs in nine species were conducted to reconstruct the evolutionary pattern of PEBP genes in plants.

RESULTS

We identified and cloned 160 PEBPs from 15 cotton species, and the phylogenetic analysis showed that the genes could be classified into the following three subfamilies: MFT-like, FT-like and TFL1-like. There were eight single orthologous group (OG) members in each diploid and 16 double OG members in each tetraploid. An analysis of the expression and selective pressure indicated that expression divergence and strong purification selection within the same OG presented in the PEBP gene family.

CONCLUSION

An evolutionary pattern of duplication and deletion of the PEBP family in the evolutionary history of Gossypium was suggested, and three pairs of genes resulted from different whole-genome duplication events.

Small Molecule Probes of ABA Biosynthesis and Signaling

The phytohormone ABA mediates many physiological and developmental responses, and its key role in plant water relations has fueled efforts to improve crop water productivity by manipulating ABA responses. ABA's core signaling components are encoded by large gene families, which has hampered functional studies using classical genetic approaches due to redundancy. Chemical approaches can complement genetic approaches and have the advantage of delivering both biological probes and potential agrochemical leads; these benefits have spawned the discovery and design of new chemical modulators of ABA signaling and biosynthesis, which have contributed to the identification of ABA receptors and helped to define PYR1 and related subfamily III receptors as key cellular targets for chemically manipulating water productivity. In this review, we provide an overview of small molecules that have helped dissect both ABA signaling and metabolic pathways. We further discuss how the insights gleaned using ABA probe molecules might be translated to improvements in crop water productivity and future opportunities for development of small molecules that affect ABA metabolism and signaling.

Phosphoproteomic profiling reveals ABA-responsive phosphosignaling pathways in Physcomitrella patens

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.

Desiccation tolerance in Physcomitrella patens: Rate of dehydration and the involvement of endogenous abscisic acid (ABA)

The moss Physcomitrella patens, a model system for basal land plants, tolerates several abiotic stresses, including dehydration. We previously reported that Physcomitrella patens survives equilibrium dehydration to -13 MPa in a closed system at 91% RH. Tolerance of desiccation to water potentials below -100 MPa was only achieved by pretreatment with exogenous abscisic acid (ABA). We report here that gametophores, but not protonemata, can survive desiccation below -100 MPa after a gradual drying regime in an open system, without exogenous ABA. In contrast, faster equilibrium drying at 90% RH for 3-5 days did not induce desiccation tolerance in either tissue. Endogenous ABA accumulated in protonemata and gametophores under both drying regimes, so did not correlate directly with desiccation tolerance. Gametophores of a Ppabi3a/b/c triple knock out transgenic line also survived the gradual dehydration regime, despite impaired ABA signaling. Our results suggest that the initial drying rate, and not the amount of endogenous ABA, may be critical in the acquisition of desiccation tolerance. Results from this work will provide insight into ongoing studies to uncover the role of ABA in the dehydration response and the underlying mechanisms of desiccation tolerance in this bryophyte.

SlbZIP38, a Tomato bZIP Family Gene Downregulated by Abscisic Acid, Is a Negative Regulator of Drought and Salt Stress Tolerance

The basic leucine zipper (bZIP) transcription factors have crucial roles in plant stress responses. In this study, the bZIP family gene SlbZIP38 (GenBank accession No: XM004239373) was isolated from a tomato (Solanum lycopersicum cv. Ailsa Craig) mature leaf cDNA library. The DNA sequence of SlbZIP38 encodes a protein of 484 amino acids, including a highly conserved bZIP DNA-binding domain in the C-terminal region. We found that SlbZIP38 was differentially expressed in various organs of the tomato plant and was downregulated by drought, salt stress, and abscisic acid (ABA). However, overexpression of SlbZIP38 significantly decreased drought and salt stress tolerance in tomatoes (Ailsa Craig). The findings that SlbZIP38 overexpression reduced the chlorophyll and free proline content in leaves but increased the malondialdehyde content may explain the reduced drought and salt tolerance observed in these lines. These results suggest that SlbZIP38 is a negative regulator of drought and salt resistance that acts by modulating ABA signaling.

Surviving a Dry Future: Abscisic Acid (ABA)-Mediated Plant Mechanisms for Conserving Water under Low Humidity

Angiosperms are able to respond rapidly to the first sign of dry conditions, a decrease in air humidity, more accurately described as an increase in the vapor pressure deficit between the leaf and the atmosphere (VPD), by abscisic acid (ABA)-mediated stomatal closure. The genes underlying this response offer valuable candidates for targeted selection of crop varieties with improved drought tolerance, a critical goal for current plant breeding programs, to maximize crop production in drier and increasingly marginalized environments, and meet the demands of a growing population in the face of a changing climate. Here, we review current understanding of the genetic mechanisms underpinning ABA-mediated stomatal closure, a key means for conserving water under dry conditions, examine how these mechanisms evolved, and discuss what remains to be investigated.

Xylem and stomata, coordinated through time and space

Land plants exhibit a degree of homeostasis in leaf water content to protect against damage to photosynthetic and xylem tissues, and to maintain an efficient allocation of resources. This is achieved by a strong coordination between the systems regulating water delivery (xylem) and water loss (stomata). This review discusses evolution in xylem and stomatal function, specifically focussing on the interactions between them.

The decision to germinate is regulated by divergent molecular networks in spores and seeds

Dispersal is a key step in land plant life cycles, usually via formation of spores or seeds. Regulation of spore- or seed-germination allows control over the timing of transition from one generation to the next, enabling plant dispersal. A combination of environmental and genetic factors determines when seed germination occurs. Endogenous hormones mediate this decision in response to the environment. Less is known about how spore germination is controlled in earlier-evolving nonseed plants. Here, we present an in-depth analysis of the environmental and hormonal regulation of spore germination in the model bryophyte Physcomitrella patens (Aphanoregma patens). Our data suggest that the environmental signals regulating germination are conserved, but also that downstream hormone integration pathways mediating these responses in seeds were acquired after the evolution of the bryophyte lineage. Moreover, the role of abscisic acid and diterpenes (gibberellins) in germination assumed much greater importance as land plant evolution progressed. We conclude that the endogenous hormone signalling networks mediating germination in response to the environment may have evolved independently in spores and seeds. This paves the way for future research about how the mechanisms of plant dispersal on land evolved.

An ancient and conserved function for Armadillo-related proteins in the control of spore and seed germination by abscisic acid

Armadillo-related proteins regulate development throughout eukaryotic kingdoms. In the flowering plant Arabidopsis thaliana, Armadillo-related ARABIDILLO proteins promote multicellular root branching. ARABIDILLO homologues exist throughout land plants, including early-diverging species lacking true roots, suggesting that early-evolving ARABIDILLOs had additional biological roles. Here we investigated, using molecular genetics, the conservation and diversification of ARABIDILLO protein function in plants separated by c. 450 million years of evolution. We demonstrate that ARABIDILLO homologues in the moss Physcomitrella patens regulate a previously undiscovered inhibitory effect of abscisic acid (ABA) on spore germination. Furthermore, we show that A. thaliana ARABIDILLOs function similarly during seed germination. Early-diverging ARABIDILLO homologues from both P. patens and the lycophyte Selaginella moellendorffii can substitute for ARABIDILLO function during A. thaliana root development and seed germination. We conclude that (1) ABA was co-opted early in plant evolution to regulate functionally analogous processes in spore- and seed-producing plants and (2) plant ARABIDILLO germination functions were co-opted early into both gametophyte and sporophyte, with a specific rooting function evolving later in the land plant lineage.

Linking Turgor with ABA Biosynthesis: Implications for Stomatal Responses to Vapor Pressure Deficit across Land Plants

Stomatal responses to changes in vapor pressure deficit (VPD) constitute the predominant form of daytime gas-exchange regulation in plants. Stomatal closure in response to increased VPD is driven by the rapid up-regulation of foliar abscisic acid (ABA) biosynthesis and ABA levels in angiosperms; however, very little is known about the physiological trigger for this increase in ABA biosynthesis at increased VPD Using a novel method of modifying leaf cell turgor by the application of external pressures, we test whether changes in turgor pressure can trigger increases in foliar ABA levels over 20 min, a period of time most relevant to the stomatal response to VPD We found in angiosperm species that the biosynthesis of ABA was triggered by reductions in leaf turgor, and in two species tested, that a higher sensitivity of ABA synthesis to leaf turgor corresponded with a higher stomatal sensitivity to VPD In contrast, representative species from nonflowering plant lineages did not show a rapid turgor-triggered increase in foliar ABA levels, which is consistent with previous studies demonstrating passive stomatal responses to changes in VPD in these lineages. Our method provides a new tool for characterizing the response of stomata to water availability.

Molecular characterization of a mutation affecting abscisic acid biosynthesis and consequently stomatal responses to humidity in an agriculturally important species

Mutants deficient in the phytohormone abscisic acid (ABA) have been instrumental in determining not only the biosynthetic pathway for this hormone, but also its physiological role in land plants. The wilty mutant of Pisum sativum is one of the classical, well-studied ABA-deficient mutants; however, this mutant remains uncharacterized at a molecular level. Using a candidate gene approach, we show that the wilty mutation affects the xanthoxin dehydrogenase step in ABA biosynthesis. To date, this step has only been represented by mutants in the ABA2 gene of Arabidopsis thaliana. Functional ABA biosynthesis appears to be critical for normal stomatal responses to changes in humidity in angiosperms, with wilty mutant plants having no increase in foliar ABA levels in response to a doubling in vapour pressure deficit, and no closure of stomata. Phylogenetic analysis of the ABA2 gene family from diverse land plants indicates that an ABA-biosynthesis-specific short-chain dehydrogenase (ABA2) evolved in the earliest angiosperms. The relatively recent origin of specificity in this step has important implications for both the evolution of ABA biosynthesis and action in land plants.

Hormonal dynamics contributes to divergence in seasonal stomatal behaviour in a monsoonal plant community

The plant hormone abscisic acid (ABA) is a primary regulator of plant transpiration, but its influence in determining seasonal stomatal behaviour in natural plant communities is poorly understood. We examined distantly related vascular plants growing together in a seasonally dry, monsoonal environment to determine whether ABA dynamics contributed to contrasting water use patterns in this natural setting. Regular sampling of angiosperm, cycad, conifer and fern species revealed characteristic seasonal patterns in ABA production, but these were highly distinct among species. Although no general relationship was observed between ABA levels, plant hydration or stomatal conductance among species, the seasonal dynamics in stomatal behaviour within species were predictable functions of either ABA or leaf water potential. Strong divergence in the seasonal role of ABA among species suggests that modification in ABA-stomatal interactions represents an important evolutionary pathway for adaptation in plant water use.

Phosphoproteomic analysis of the non-seed vascular plant model Selaginella moellendorffii

BACKGROUND

Selaginella (Selaginella moellendorffii) is a lycophyte which diverged from other vascular plants approximately 410 million years ago. As the first reported non-seed vascular plant genome, Selaginella genome data allow comparative analysis of genetic changes that may be associated with land plant evolution. Proteomics investigations on this lycophyte model have not been extensively reported. Phosphorylation represents the most common post-translational modifications and it is a ubiquitous regulatory mechanism controlling the functional expression of proteins inside living organisms.

RESULTS

In this study, polyethylene glycol fractionation and immobilized metal ion affinity chromatography were employed to isolate phosphopeptides from wild-growing Selaginella. Using liquid chromatography-tandem mass spectrometry analysis, 1593 unique phosphopeptides spanning 1104 non-redundant phosphosites with confirmed localization on 716 phosphoproteins were identified. Analysis of the Selaginella dataset revealed features that are consistent with other plant phosphoproteomes, such as the relative proportions of phosphorylated Ser, Thr, and Tyr residues, the highest occurrence of phosphosites in the C-terminal regions of proteins, and the localization of phosphorylation events outside protein domains. In addition, a total of 97 highly conserved phosphosites in evolutionary conserved proteins were identified, indicating the conservation of phosphorylation-dependent regulatory mechanisms in phylogenetically distinct plant species. On the other hand, close examination of proteins involved in photosynthesis revealed phosphorylation events which may be unique to Selaginella evolution. Furthermore, phosphorylation motif analyses identified Pro-directed, acidic, and basic signatures which are recognized by typical protein kinases in plants. A group of Selaginella-specific phosphoproteins were found to be enriched in the Pro-directed motif class.

CONCLUSIONS

Our work provides the first large-scale atlas of phosphoproteins in Selaginella which occupies a unique position in the evolution of terrestrial plants. Future research into the functional roles of Selaginella-specific phosphorylation events in photosynthesis and other processes may offer insight into the molecular mechanisms leading to the distinct evolution of lycophytes.

Overexpression of PYL5 in rice enhances drought tolerance, inhibits growth, and modulates gene expression

Abscisic acid (ABA) is a phytohormone that plays important roles in the regulation of seed dormancy and adaptation to abiotic stresses. Previous work identified OsPYL/RCARs as functional ABA receptors regulating ABA-dependent gene expression in Oryza sativa. OsPYL/RCARs thus are considered to be good candidate genes for improvement of abiotic stress tolerance in crops. This work demonstrates that the cytosolic ABA receptor OsPYL/RCAR5 in O. sativa functions as a positive regulator of abiotic stress-responsive gene expression. The constitutive expression of OsPYL/RCAR5 in rice driven by the Zea mays ubiquitin promoter induced the expression of many stress-responsive genes even under normal growth conditions and resulted in improved drought and salt stress tolerance in rice. However, it slightly reduced plant height under paddy field conditions and severely reduced total seed yield. This suggests that, although exogenous expression of OsPYL/RCAR5 is able to improve abiotic stress tolerance in rice, fine regulation of its expression will be required to avoid deleterious effects on agricultural traits.

Structure and function of abscisic acid receptors

The phytohormone abscisic acid (ABA) plays a crucial role in adaptive responses to environmental stresses, such as drought and high salinity, as well as in plant development, such as seed maturation and dormancy. PYR/PYL/RCAR has been identified as a bona fide ABA receptor (ABAR) that constitutes the core regulatory component of ABA signaling networks in plants. Here, we review recent structural and functional studies of the ABAR that have elucidated its activation mechanism, early signaling components, and physiological responses. A crucial event in the receptor's activation was found to be an open-to-closed conformational change in the gate loop of the receptor protein. More recent progress has provided strategies for controlling the gate's closure using chemical agonists or protein engineering approaches.

ABI4: versatile activator and repressor

The ABSCISIC ACID INSENSITIVE4 (ABI4) gene was discovered to be an abscisic acid (ABA) signaling responsive transcription factor active during seed germination. The evolutionary history of the ABI4 gene supports its role as an ABA signaling intermediate in land plants. Investigating the ABI4 protein-cis element interaction supports the proposal that ABI4 binding to its known CE1 cis-element competes with transcription factor binding to the overlapping G-Box element. Recent publications report on ABI4 as a regulatory factor in diverse processes. In developing seedlings, ABI4 mediates sugar signaling, lipid breakdown, and plastid-to-nucleus signaling. Moreover, ABI4 is a regulator of rosette growth, redox signaling, cell wall metabolism and the effect of nitrate on lateral root development.

Photosynthetic characteristics and the response of stomata to environmental determinants and ABA in Selaginella bryopteris, a resurrection spike moss species

Selaginella bryopteris is a spike-moss lycophyte species with resurrection capability. These plants have small sized stomata that occur in higher density than in other fern species. The diurnal gas-exchange studies under natural conditions showed a bell shaped net photosynthesis curve. The effective quantum yield of PSII (ΔF/F(m')) showed an inverse relationship with light and recovered to its maximum at sunset. This suggests that there was a complete recovery of PSII efficiency during the late evening hours. S. bryopteris displayed broad temperature optima for net photosynthesis from 28 °C to 37 °C. The stomatal sensitivity in response to vapor pressure deficit (VPD), was maximum at 25 °C temperature while at temperatures from 30 to 35 °C it was low. Our study demonstrates that S. bryopteris plants show a very poor mechanism for its stomatal regulation in response to high light, high temperature, high VPD, high CO₂ and to ABA treatment. At the same time they show a high stomatal conductance leading to unrestricted rates of transpiration and a lack of capacity to optimize water use efficiency (WUE).