CML24, regulated in expression by diverse stimuli, encodes a potential Ca2+ sensor that functions in responses to abscisic acid, daylength, and ion stress

A novel rice calmodulin-like gene, OsMSR2, enhances drought and salt tolerance and increases ABA sensitivity in Arabidopsis

Many abiotic stimuli, such as drought and salt stresses, elicit changes in intracellular calcium levels that serve to convey information and activate adaptive responses. Ca²⁺ signals are perceived by different Ca²⁺ sensors, and calmodulin (CaM) is one of the best-characterized Ca²⁺ sensors in eukaryotes. Calmodulin-like (CML) proteins also exist in plants, but their functions at the physiological and molecular levels are largely unknown. In this report, we present data on OsMSR2 (Oryza sativa L. Multi-Stress-Responsive gene 2), a novel calmodulin-like protein gene isolated from rice Pei'ai 64S (Oryza sativa L.). Expression of OsMSR2 was strongly up-regulated by a wide spectrum of stresses, including cold, drought, and heat in different tissues at different developmental stages of rice, as revealed by both microarray and quantitative real-time RT-PCR analyses. Analysis of the recombinant OsMSR2 protein demonstrated its potential ability to bind Ca²⁺ in vitro. Expression of OsMSR2 conferred enhanced tolerance to high salt and drought in Arabidopsis (Arabidopsis thaliana) accompanied by altered expression of stress/ABA-responsive genes. Transgenic plants also exhibited hypersensitivity to ABA during the seed germination and post-germination stages. The results suggest that expression of OsMSR2 modulated salt and drought tolerance in Arabidopsis through ABA-mediated pathways.

Early abscisic acid signal transduction mechanisms: newly discovered components and newly emerging questions

The plant hormone abscisic acid (ABA) regulates many key processes in plants, including seed germination and development and abiotic stress tolerance, particularly drought resistance. Understanding early events in ABA signal transduction has been a major goal of plant research. The recent identification of the PYRABACTIN (4-bromo-N-[pyridin-2-yl methyl]naphthalene-1-sulfonamide) RESISTANCE (PYR)/REGULATORY COMPONENT OF ABA RECEPTOR (RCAR) family of ABA receptors and their biochemical mode of action represents a major breakthrough in the field. The solving of PYR/RCAR structures provides a context for resolving mechanisms mediating ABA control of protein-protein interactions for downstream signaling. Recent studies show that a pathway based on PYR/RCAR ABA receptors, PROTEIN PHOSPHATASE 2Cs (PP2Cs), and SNF1-RELATED PROTEIN KINASE 2s (SnRK2s) forms the primary basis of an early ABA signaling module. This pathway interfaces with ion channels, transcription factors, and other targets, thus providing a mechanistic connection between the phytohormone and ABA-induced responses. This emerging PYR/RCAR-PP2C-SnRK2 model of ABA signal transduction is reviewed here, and provides an opportunity for testing novel hypotheses concerning ABA signaling. We address newly emerging questions, including the potential roles of different PYR/RCAR isoforms, and the significance of ABA-induced versus constitutive PYR/RCAR-PP2C interactions. We also consider how the PYR/RCAR-PP2C-SnRK2 pathway interfaces with ABA-dependent gene expression, ion channel regulation, and control of small molecule signaling. These exciting developments provide researchers with a framework through which early ABA signaling can be understood, and allow novel questions about the hormone response pathway and possible applications in stress resistance engineering of plants to be addressed.

Calcium signals: the lead currency of plant information processing

Ca(2+) signals are core transducers and regulators in many adaptation and developmental processes of plants. Ca(2+) signals are represented by stimulus-specific signatures that result from the concerted action of channels, pumps, and carriers that shape temporally and spatially defined Ca(2+) elevations. Cellular Ca(2+) signals are decoded and transmitted by a toolkit of Ca(2+) binding proteins that relay this information into downstream responses. Major transduction routes of Ca(2+) signaling involve Ca(2+)-regulated kinases mediating phosphorylation events that orchestrate downstream responses or comprise regulation of gene expression via Ca(2+)-regulated transcription factors and Ca(2+)-responsive promoter elements. Here, we review some of the remarkable progress that has been made in recent years, especially in identifying critical components functioning in Ca(2+) signal transduction, both at the single-cell and multicellular level. Despite impressive progress in our understanding of the processing of Ca(2+) signals during the past years, the elucidation of the exact mechanistic principles that underlie the specific recognition and conversion of the cellular Ca(2+) currency into defined changes in protein-protein interaction, protein phosphorylation, and gene expression and thereby establish the specificity in stimulus response coupling remain to be explored.

Breaking the code: Ca2+ sensors in plant signalling

Ca2+ ions play a vital role as second messengers in plant cells during various developmental processes and in response to environmental stimuli. Plants have evolved a diversity of unique proteins that bind Ca2+ using the evolutionarily conserved EF-hand motif. The currently held hypothesis is that these proteins function as Ca2+ sensors by undergoing conformational changes in response to Ca2+-binding that facilitate their regulation of target proteins and thereby co-ordinate various signalling pathways. The three main classes of these EF-hand Ca2+sensors in plants are CaMs [calmodulins; including CMLs (CaM-like proteins)], CDPKs (calcium-dependent protein kinases) and CBLs (calcineurin B-like proteins). In the plant species examined to date, each of these classes is represented by a large family of proteins, most of which have not been characterized biochemically and whose physiological roles remain unclear. In the present review, we discuss recent advances in research on CaMs and CMLs, CDPKs and CBLs, and we attempt to integrate the current knowledge on the different sensor classes into common physiological themes.

Arabidopsis proline-rich extensin-like receptor kinase 4 modulates the early event toward abscisic acid response in root tip growth

Exogenous application of plant hormone abscisic acid (ABA) can inhibit root growth. We recently reported that the proline-rich extensin-like receptor kinase 4 (PERK4) functions at an early stage of ABA signaling to inhibit primary root cell elongation by perturbing Ca2+ homeostasis.(1) Transcription analysis indicated that PERK4 modulates the expression of the genes related to cell elongation and ABA signaling in root growth, such as polygalacturonases, AtExt1, AtMYC2 and ABR1. Under ABA treatment, the transcript level of ZAT10, a Ca(2+)-responsive gene, increased in perk4 plants compared to that of wild-type. Based on both present data and the previous evidence, we propose a probable model for PERK4-mediated ABA-regulated primary root cell growth.

Thigmomorphogenesis: a complex plant response to mechano-stimulation

In nature, plants are challenged with hurricane winds, monsoon rains, and herbivory attacks, in addition to many other harsh mechanical perturbations that can threaten plant survival. As a result, over many years of evolution, plants have developed very sensitive mechanisms through which they can perceive and respond to even subtle stimuli, like touch. Some plants respond behaviourally to the touch stimulus within seconds, while others show morphogenetic alterations over long periods of time, ranging from days to weeks. Various signalling molecules and phytohormones, including intracellular calcium, jasmonates, ethylene, abscisic acid, auxin, brassinosteroids, nitric oxide, and reactive oxygen species, have been implicated in touch responses. Many genes are induced following touch. These genes encode proteins involved in various cellular processes including calcium sensing, cell wall modifications, and defence. Twenty-three per cent of these up-regulated genes contain a recently identified promoter element involved in the rapid induction in transcript levels following mechanical perturbations. The employment of various genetic, biochemical, and molecular tools may enable elucidation of the mechanisms through which plants perceive mechano-stimuli and transduce the signals intracellularly to induce appropriate responses.

Mutations in AtCML9, a calmodulin-like protein from Arabidopsis thaliana, alter plant responses to abiotic stress and abscisic acid

Many stimuli, such as hormones and abiotic stress factors, elicit changes in intracellular calcium levels that serve to convey information and activate appropriate responses. The Ca2+ signals are perceived by different Ca2+ receptors, and calmodulin (CaM) is one of the best-characterized Ca2+ sensors in eukaryotes. Calmodulin-like (CML) proteins also exist in plants; they share sequence similarity with the ubiquitous and highly conserved CaM, but their roles at the physiological and molecular levels are largely unknown. We present data on Arabidopsis thaliana CML9 (AtCML9) that exhibits 46% amino acid sequence identity with CaM. AtCML9 transcripts are found in all major organs, and a putative AtCML9 regulatory region confers reporter gene expression at various sites, including root apex, stomata, hydathodes and trichomes. AtCML9 expression is rapidly induced by abiotic stress and abscisic acid (ABA) in young seedlings, and by using cml9 knock-out mutants we present evidence that AtCML9 plays essential roles in modulating responses to salt stress and ABA. Seed germination and seedling growth for the mutant lines present a hypersensitive response to ABA that could be correlated with enhanced tolerance to salt stress and water deficit. Mutations of the AtCML9 gene also alter the expression of several stress-regulated genes, suggesting that AtCML9 is involved in salt stress tolerance through its effects on the ABA-mediated pathways.

Arabidopsis potential calcium sensors regulate nitric oxide levels and the transition to flowering

In plants, flowering is a critical developmental transition orchestrated by four regulatory pathways. Distinct alleles encoding mutant forms of the Arabidopsis potential calcium sensor CML24 cause alterations in flowering time. CML24 can act as a switch in the response to day length perception; loss-of-function cml24 mutants are late flowering under long days, whereas apparent gain of CML24 function results in early flowering. CML24 function is required for proper CONSTANS (CO) expression; components upstream of CO in the photoperiod pathway are largely unaffected in the cml24 mutants. In conjunction with CML23, a related calmodulin-like protein, CML24 also inhibits FLOWERING LOCUS C (FLC) expression and therefore impacts the autonomous regulatory pathway of the transition to flowering. Nitric oxide (NO) levels are elevated in cml23/cml24 double mutants and are largely responsible for FLC transcript accumulation. Therefore, CML23 and CML24 are potential calcium sensors that have partially overlapping function that may act to transduce calcium signals to regulate NO accumulation. In turn, NO levels influence the transition to flowering through both the photoperiod and autonomous regulatory pathways.

Developmental and stimulus-induced expression patterns of Arabidopsis calmodulin-like genes CML37, CML38 and CML39

Various aspects of plant development and stress physiology are mediated by Ca(2+) signaling. Ca(2+) sensors, such as calmodulin, detect these signals and direct downstream signaling pathways by binding and activating diverse targets. Plants possess many unique, putative Ca(2+) sensors, including a large family (50 in Arabidopsis) of calmodulin-like proteins termed CMLs. Some of these CMLs have been implicated in Ca(2+)-based stress response but most remain unstudied. We generated transgenic plants expressing CML::GUS reporter genes for members of a subfamily of CMLs (CML37, CML38 and CML39) which allowed us to investigate their expression patterns in detail. We found that CML::GUS genes displayed unique tissue, cell-type, and temporal patterns of expression throughout normal development, particularly in the flower, and in response to a variety of stimuli, including biotic and abiotic stress, hormone and chemical treatments. Our findings are supported by semiquantitative reverse-transcription PCR as well as analyses of microarray databases. Analysis of purified, recombinant CMLs demonstrated their ability to bind Ca(2+) in vitro. Collectively, our data suggest that these CMLs likely play important roles as sensors in Ca(2+)-mediated developmental and stress response pathways and provide a framework of spatial and temporal expression to direct future studies aimed at elucidating their physiological roles.

Plant calmodulins and calmodulin-related proteins: multifaceted relays to decode calcium signals

The calmodulin (CaM) family is a major class of calcium sensor proteins which collectively play a crucial role in cellular signaling cascades through the regulation of numerous target proteins. Although CaM is one of the most conserved proteins in all eukaryotes, several features of CaM and its downstream effector proteins are unique to plants. The continuously growing repertoire of CaM-binding proteins includes several plant-specific proteins. Plants also possess a particular set of CaM isoforms and CaM-like proteins (CMLs) whose functions have just begun to be elucidated. This review summarizes recent insights that help to understand the role of this multigene family in plant development and adaptation to environmental stimuli.