The heat shock factor A4A confers salt tolerance and is regulated by oxidative stress and the mitogen-activated protein kinases MPK3 and MPK6.

Heat shock factors (HSFs) are principal regulators of plant responses to several abiotic stresses. Here, we show that estradiol-dependent induction of HSFA4A confers enhanced tolerance to salt and oxidative agents, whereas inactivation of HSFA4A results in hypersensitivity to salt stress in Arabidopsis (Arabidopsis thaliana). Estradiol induction of HSFA4A in transgenic plants decreases, while the knockout hsfa4a mutation elevates hydrogen peroxide accumulation and lipid peroxidation. Overexpression of HSFA4A alters the transcription of a large set of genes regulated by oxidative stress. In yeast (Saccharomyces cerevisiae) two-hybrid and bimolecular fluorescence complementation assays, HSFA4A shows homomeric interaction, which is reduced by alanine replacement of three conserved cysteine residues. HSFA4A interacts with mitogen-activated protein kinases MPK3 and MPK6 in yeast and plant cells. MPK3 and MPK6 phosphorylate HSFA4A in vitro on three distinct sites, serine-309 being the major phosphorylation site. Activation of the MPK3 and MPK6 mitogen-activated protein kinase pathway led to the transcriptional activation of the HEAT SHOCK PROTEIN17.6A gene. In agreement that mutation of serine-309 to alanine strongly diminished phosphorylation of HSFA4A, it also strongly reduced the transcriptional activation of HEAT SHOCK PROTEIN17.6A. These data suggest that HSFA4A is a substrate of the MPK3/MPK6 signaling and that it regulates stress responses in Arabidopsis.

MAPK phosphatase MKP2 mediates disease responses in Arabidopsis and functionally interacts with MPK3 and MPK6.

Mitogen-activated protein kinase (MAPK) cascades have important functions in plant stress responses and development and are key players in reactive oxygen species (ROS) signalling and in innate immunity. In Arabidopsis, the transmission of ROS and pathogen signalling by MAPKs involves the coordinated activation of MPK6 and MPK3; however, the specificity of their negative regulation by phosphatases is not fully known. Here, we present genetic analyses showing that MAPK phosphatase 2 (MKP2) regulates oxidative stress and pathogen defence responses and functionally interacts with MPK3 and MPK6. We show that plants lacking a functional MKP2 gene exhibit delayed wilting symptoms in response to Ralstonia solanacearum and, by contrast, acceleration of disease progression during Botrytis cinerea infection, suggesting that this phosphatase plays differential functions in biotrophic versus necrotrophic pathogen-induced responses. MKP2 function appears to be linked to MPK3 and MPK6 regulation, as indicated by BiFC experiments showing that MKP2 associates with MPK3 and MPK6 in vivo and that in response to fungal elicitors MKP2 exerts differential affinity versus both kinases. We also found that MKP2 interacts with MPK6 in HR-like responses triggered by fungal elicitors, suggesting that MPK3 and MPK6 are subject to differential regulation by MKP2 in this process. We propose that MKP2 is a key regulator of MPK3 and MPK6 networks controlling both abiotic and specific pathogen responses in plants.

Molecular characterization of maize bHLH transcription factor (ZmKS), a new ZmOST1 kinase substrate.

In order to identify potential substrates of the maize kinase in the ABA signalling network, ZmOST1 was used as bait against a library of cDNAs from dehydrated young leaves. A ZmOST1-interactive polypeptide ZmKS (gene locus tag: GRMZM2G114873), showing homology with the Arabidopsis thaliana basic helix-loop-helix (bHLH) DNA-binding transcription factor was identified. Using a comparative genomic approach, the ZmKS corresponding protein was identified as conceptual translated bHLH transcription factor ABA-responsive kinase substrate. ZmKS is localized in the nucleus, shows a potential binding specificity preferentially detectable on cis-acting E-box like heptameric motifs CCACTTG and CAAGTTG, and is phosphorylated by maize protein kinase ZmOST1. ZmKS is expressed in embryo, leaf and root, expression being affected by ABA and osmotic stress. Transgenic Arabidopsis plants, with gain of ZmKS function, show a delay in germination and a transcriptional stomatal opening-facilitator activity, switchover upon ZmKS phosphorylation, suggesting that ZmKS is an ABA-repressed trans-acting activator.

Emerging roles of protein kinase CK2 in abscisic acid signaling.

The phytohormone abscisic acid (ABA) regulates many aspects of plant growth and development as well as responses to multiple stresses. Post-translational modifications such as phosphorylation or ubiquitination have pivotal roles in the regulation of ABA signaling. In addition to the positive regulator sucrose non-fermenting-1 related protein kinase 2 (SnRK2), the relevance of the role of other protein kinases, such as CK2, has been recently highlighted. We have recently established that CK2 phosphorylates the maize ortholog of open stomata 1 OST1, ZmOST1, suggesting a role of CK2 phosphorylation in the control of ZmOST1 protein degradation (Vilela et al., 2015). CK2 is a pleiotropic enzyme involved in multiple developmental and stress-responsive pathways. This review summarizes recent advances that taken together suggest a prominent role of protein kinase CK2 in ABA signaling and related processes.

Casein Kinase 2 Negatively Regulates Abscisic Acid-Activated SnRK2s in the Core Abscisic Acid-Signaling Module.

SnRK2 kinases, PP2C phosphatases and the PYR/PYL/RCAR receptors constitute the core abscisic acid (ABA) signaling module that is thought to contain all of the intrinsic properties to self-regulate the hormone signal output. Here we identify Casein Kinase (CK)2 as a novel negative regulator of SnRK2. CK2 phosphorylates a cluster of conserved serines at the ABA box of SnRK2, increasing its binding to PP2C and triggering protein degradation. Consequently, CK2 action has implications on SnRK2 protein levels, as well as kinase activity and its response to abiotic stimuli.

The maize OST1 kinase homolog phosphorylates and regulates the maize SNAC1-type transcription factor.

The Arabidopsis kinase OPEN STOMATA 1 (OST1) plays a key role in regulating drought stress signalling, particularly stomatal closure. We have identified and investigated the functions of the OST1 ortholog in Z. mays (ZmOST1). Ectopic expression of ZmOST1 in the Arabidopsis ost1 mutant restores the stomatal closure phenotype in response to drought. Furthermore, we have identified the transcription factor, ZmSNAC1, which is directly phosphorylated by ZmOST1 with implications on its localization and protein stability. Interestingly, ZmSNAC1 binds to the ABA-box of ZmOST1, which is conserved in SnRK2s activated by ABA and is part of the contact site for the negative-regulating clade A PP2C phosphatases. Taken together, our results indicate that ZmSNAC1 is a substrate of ZmOST1 and delineate a novel osmotic stress transcriptional pathway in maize.

Comparative transcriptomic profiling of Vitis vinifera under high light using a custom-made array and the Affymetrix GeneChip.

Understanding abiotic stress responses is one of the most important issues in plant research nowadays. Abiotic stress, including excess light, can promote the onset of oxidative stress through the accumulation of reactive oxygen species. Oxidative stress also arises when in vitro propagated plants are exposed to high light upon transfer to ex vitro. To determine whether the underlying pathways activated at the transfer of in vitro grapevine to ex vitro conditions reflect the processes occurring upon light stress, we used Vitis vinifera Affymetrix GeneChip (VvGA) and a custom array of genes responsive to light stress (LSCA) detected by real-time reverse transcriptase PCR (qRT-PCR). When gene-expression profiles were compared, 'protein metabolism and modification', 'signaling', and 'anti-oxidative' genes were more represented in LSCA, while, in VvGA, 'cell wall metabolism' and 'secondary metabolism' were the categories in which gene expression varied more significantly. The above functional categories confirm previous studies involving other types of abiotic stresses, enhancing the common attributes of abiotic stress defense pathways. The LSCA analysis of our experimental system detected strong response of heat shock genes, particularly the protein rescuing mechanism involving the cooperation of two ATP-dependent chaperone systems, Hsp100 and Hsp70, which showed an unusually late response during the recovery period, of extreme relevance to remove non-functional, potentially harmful polypeptides arising from misfolding, denaturation, or aggregation brought about by stress. The success of LSCA also proves the feasibility of a custom-made qRT-PCR approach, particularly for species for which no GeneChip is available and for researchers dealing with a specific and focused problem.

Regulation of MAPK signaling and cell death by MAPK phosphatase MKP2.

Mitogen-activated protein kinase (MAPK) pathways play crucial roles in developmental and adaptive responses. Depending on the stimulus, MAPK activation regulates a wide variety of plant cell responses, such as proliferation, differentiation and cell death, which normally require precise spatial and temporal control. In this context, protein phosphatases play important roles by regulating the duration and magnitude of MAPK activities. During infection by non-host and incompatible host microorganisms, MAPK activity can promote a local cell death mechanism called hypersensitivity response (HR), which is part of the plant defence response. HR-like responses require sustained MAPK activity and correlate with oxidative burst. We recently showed that MAPK phosphatase MKP2 positively controls biotic and abiotic stress responses in Arabidopsis. MKP2 interacts with MPK6 in HR-like responses triggered by fungal elicitors, suggesting that MKP2 protein is part of the mechanism involved in MAPK regulation during HR. Here we discuss the interplay of MAPK and MKP2 phosphatase signaling during cell death responses elicited by host-pathogen interactions.

Maize AKINbetagamma dimerizes through the KIS/CBM domain and assembles into SnRK1 complexes.

The SNF1/AMPK/SnRK1 complex is an intracellular energy sensor composed of three types of subunits: the SnRK1 kinase and two regulatory, non-catalytic subunits (designated beta and gamma). We have previously described an atypical plant gamma-subunit, AKINbetagamma, which contains an N-terminal tail similar to the so-called KIS domain normally present in beta-subunits. However, it is not known whether AKINbetagamma normally associates with endogenous SnRK1 complexes in vivo, nor how its unique domain structure might contribute to SnRK1 function. Here, we present evidence that maize AKINbetagamma is an integral component of active SnRK1 complexes in plant cells. Using complementary methodological approaches, we also show that AKINbetagamma associates through homomeric interactions mediated by both, the gamma- and, unexpectedly, the KIS/CBM domain.