Water deficit and salinity, especially under high light intensity or in combination with other stresses, disrupt photosynthesis and increase photorespiration, altering the normal homeostasis of cells and cause an increased production of reactive oxygen species (ROS). ROS play a dual role in the response of plants to abiotic stresses functioning as toxic by-products of stress metabolism, as well as important signal transduction molecules. In this review, we provide an overview of ROS homeostasis and signalling in response to drought and salt stresses and discuss the current understanding of ROS involvement in stress sensing, stress signalling and regulation of acclimation responses.

C 2 H 2 -zinc finger proteins that contain the EAR repressor domain are thought to play a key role in modulating the defense response of plants to abiotic stress. Constitutive expression of the C 2 H 2 -EAR zinc finger protein Zat10 in Arabidopsis was found to elevate the expression of reactive oxygen-defense transcripts and to enhance the tolerance of plants to salinity, heat and osmotic stress. Surprisingly, knockout and RNAi mutants of Zat10 were also more tolerant to osmotic and salinity stress. Our results suggest that Zat10 plays a key role as both a positive and a negative regulator of plant defenses.

The zinc finger domain enables different proteins to interact with or bind DNA, RNA, or other proteins, and is present in the proteomes of many different organisms. Proteins containing zinc finger domain(s) were found to play important roles in eukaryotic cells regulating different signal transduction pathways and controlling processes, such as development and programmed cell death. There are many types of zinc finger proteins, classified according to the number and order of the Cys and His residues that bind the Zinc ion. Among these, the C2H2-type zinc finger proteins, with 176 members in Arabidopsis thaliana, constitute one of the largest families of transcriptional regulators in plants. They are mostly plant-specific and contain a conserved QALGGH sequence within their zinc finger domain. Recent studies revealed that C2H2 zinc finger proteins could function as key transcriptional repressors involved in the defense and acclimation response of plants to different environmental stress conditions. Here we highlight recent functional characterization studies of different C2H2 proteins in Arabidopsis, and suggest that many of these proteins function as part of a large regulatory network that senses and responds to different environmental stimuli.

Cys2/His2-type zinc finger proteins, which contain the EAR transcriptional repressor domain, are thought to play a key role in regulating the defense response of plants to biotic and abiotic stress conditions. Although constitutive expression of several of these proteins was shown to enhance the tolerance of transgenic plants to abiotic stress, it is not clear whether the EAR-motif of these proteins is involved in this function. In addition, it is not clear whether suppression of plant growth, induced in transgenic plants by different Cys2/His2 EAR-containing proteins, is mediated by the EAR-domain. Here we report that transgenic Arabidopsis plants constitutively expressing the Cys2/His2 zinc finger protein Zat7 have suppressed growth and are more tolerant to salinity stress. A deletion or a mutation of the EAR-motif of Zat7 abolishes salinity tolerance without affecting growth suppression. These results demonstrate that the EAR-motif of Zat7 is directly involved in enhancing the tolerance of transgenic plants to salinity stress. In contrast, the EAR-motif appears not to be involved in suppressing the growth of transgenic plants. Further analysis of Zat7 using RNAi lines suggests that Zat7 functions in Arabidopsis to suppress a repressor of defense responses. A yeast two-hybrid analysis identified putative interactors of Zat7 and the EAR-domain, including WRKY70 and HASTY, a protein involved in miRNA transport. Our findings demonstrate that the EAR-domain of Cys2/His2-type zinc finger proteins plays a key role in the defense response of Arabidopsis to abiotic stresses.

Over one-quarter of all plant genes encode proteins of unknown function that can be further classified as proteins with obscure features (POFs), which lack currently defined motifs or domains, or proteins with defined features, which contain at least one previously defined domain or motif. Although empirical data in the form of transcriptome and proteome profiling suggest that many of these proteins play important roles in plants, their functional characterization remains one of the main challenges in modern biology. To begin the functional annotation of proteins with unknown function, which are involved in the oxidative stress response of Arabidopsis (Arabidopsis thaliana), we generated transgenic Arabidopsis plants that constitutively expressed 23 different POFs (four of which were specific to Arabidopsis) and 18 different proteins with defined features. All were previously found to be expressed in response to oxidative stress in Arabidopsis. Transgenic plants were tested for their tolerance to oxidative stress imposed by paraquat or t-butyl hydroperoxide, or were subjected to osmotic, salinity, cold, and heat stresses. More than 70% of all expressed proteins conferred tolerance to oxidative stress. In contrast, .90% of the expressed proteins did not confer enhanced tolerance to the other abiotic stresses tested, and approximately 50% rendered plants more susceptible to osmotic or salinity stress. Two Arabidopsis-specific POFs, and an Arabidopsis and Brassica-specific protein of unknown function, conferred enhanced tolerance to oxidative stress. Our findings suggest that tolerance to oxidative stress involves mechanisms and pathways that are unknown at present, including some that are specific to Arabidopsis or the Brassicaceae.