The STT3a subunit isoform of the Arabidopsis oligosaccharyltransferase controls adaptive responses to salt/osmotic stress.

Arabidopsis stt3a-1 and stt3a-2 mutations cause NaCl/osmotic sensitivity that is characterized by reduced cell division in the root meristem. Sequence comparison of the STT3a gene identified a yeast ortholog, STT3, which encodes an essential subunit of the oligosaccharyltransferase complex that is involved in protein N-glycosylation. NaCl induces the unfolded protein response in the endoplasmic reticulum (ER) and cell cycle arrest in root tip cells of stt3a seedlings, as determined by expression profiling of ER stress-responsive chaperone (BiP-GUS) and cell division (CycB1;1-GUS) genes, respectively. Together, these results indicate that plant salt stress adaptation involves ER stress signal regulation of cell cycle progression. Interestingly, a mutation (stt3b-1) in another Arabidopsis STT3 isogene (STT3b) does not cause NaCl sensitivity. However, the stt3a-1 stt3b-1 double mutation is gametophytic lethal. Apparently, STT3a and STT3b have overlapping and essential functions in plant growth and developmental processes, but the pivotal and specific protein glycosylation that is a necessary for recovery from the unfolded protein response and for cell cycle progression during salt/osmotic stress recovery is associated uniquely with the function of the STT3a isoform.

C-terminal domain phosphatase-like family members (AtCPLs) differentially regulate Arabidopsis thaliana abiotic stress signaling, growth, and development.

Cold, hyperosmolarity, and abscisic acid (ABA) signaling induce RD29A expression, which is an indicator of the plant stress adaptation response. Two nonallelic Arabidopsis thaliana (ecotype C24) T-DNA insertional mutations, cpl1 and cpl3, were identified based on hyperinduction of RD29A expression that was monitored by using the luciferase (LUC) reporter gene (RD29ALUC) imaging system. Genetic linkage analysis and complementation data established that the recessive cpl1 and cpl3 mutations are caused by T-DNA insertions in AtCPL1 (Arabidopsis C-terminal domain phosphatase-like) and AtCPL3, respectively. Gel assays using recombinant AtCPL1 and AtCPL3 detected innate phosphatase activity like other members of the phylogenetically conserved family that dephosphorylate the C-terminal domain of RNA polymerase II (RNAP II). cpl1 mutation causes RD29ALUC hyperexpression and transcript accumulation in response to cold, ABA, and NaCl treatments, whereas the cpl3 mutation mediates hyperresponsiveness only to ABA. Northern analysis confirmed that LUC transcript accumulation also occurs in response to these stimuli. cpl1 plants accumulate biomass more rapidly and exhibit delayed flowering relative to wild type whereas cpl3 plants grow more slowly and flower earlier than wild-type plants. Hence AtCPL1 and AtCPL3 are negative regulators of stress responsive gene transcription and modulators of growth and development. These results suggest that C-terminal domain phosphatase regulation of RNAP II phosphorylation status is a focal control point of complex processes like plant stress responses and development. AtCPL family members apparently have both unique and overlapping transcriptional regulatory functions that differentiate the signal output that determines the plant response.

The ascorbic acid cycle mediates signal transduction leading to stress-induced stomatal closure.

Using a combination of pharmacological approaches, mutation analysis and a gene silencing strategy, we present evidence that treatment of tomato (Lycopersicon esculentum Mill.) plants with exogenous ascorbate (AsA) subsequently increases the level of cellular AsA and causes stomatal closure. Using the ABA-deficient mutants flacca and sitiens, we show that the AsA-mediated induction of stomatal closure requires the participation of ABA. In addition, ABA acts independently of its role in mediating another stress response, proline accumulation. Because cellular AsA level was not elevated during stomatal closure, we hypothesized that stomatal closure relies on the activation of the AsA cycle and possible accumulation of intermediate components, such as monodehydroascorbate, that have been reported to be involved in mediating stress-induced responses. To establish a link between H2O2 production, the AsA cycle and stomatal closure, we also evaluated the effect of AsA treatment on catalase-deficient transgenic plants, which have a constitutively high level of H2O2. Interestingly, stomata of catalase-deficient plants were much more responsive to AsA treatment, compared with wild-type control plants. Because an increase in cellular H2O2 upon stress has been widely documented in many organisms and has been interpreted as a signal that initiates a cascade of stress-induced responses, we suggest that stress-induced stomatal closure is mediated by H2O2 and activation of the AsA cycle.