No abstract

Reactive oxygen species and redox signaling undergo synergistic and antagonistic interactions with phytohormones to regulate protective responses of plants against biotic and abiotic stresses. However, molecular insight into the nature of this crosstalk remains scarce. We demonstrate that the hydrogen peroxide-responsive UDP-glucosyltransferase UGT74E2 of Arabidopsis thaliana is involved in the modulation of plant architecture and water stress response through its activity toward the auxin indole-3-butyric acid (IBA). Biochemical characterization of recombinant UGT74E2 demonstrated that it strongly favors IBA as a substrate. Assessment of indole-3-acetic acid (IAA), IBA, and their conjugates in transgenic plants ectopically expressing UGT74E2 indicated that the catalytic specificity was maintained in planta. In these transgenic plants, not only were IBA-Glc concentrations increased, but also free IBA levels were elevated and the conjugated IAA pattern was modified. This perturbed IBA and IAA homeostasis was associated with architectural changes, including increased shoot branching and altered rosette shape, and resulted in significantly improved survival during drought and salt stress treatments. Hence, our results reveal that IBA and IBA-Glc are important regulators of morphological and physiological stress adaptation mechanisms and provide molecular evidence for the interplay between hydrogen peroxide and auxin homeostasis through the action of an IBA UGT.

Chloroplast ferredoxin (Fd) plays a pivotal role in plant cell metabolism by delivering reducing equivalents to various essential oxidoreductive pathways. Fd levels decrease under adverse environmental conditions in many microorganisms, including cyanobacteria, which share a common ancestor with chloroplasts. Conversely, stress situations induce the synthesis of flavodoxin (Fld), an electron carrier flavoprotein not found in plants, which can efficiently replace Fd in most electron transfer processes. We report here that chloroplast Fd also declined in plants exposed to oxidants or stress conditions. A purified cyanobacterial Fld was able to mediate plant Fd-dependent reactions in vitro, including NADP 1 and thioredoxin reduction. Tobacco (Nicotiana tabacum) plants expressing Fld in chloroplasts displayed increased tolerance to multiple sources of stress, including redox-cycling herbicides, extreme temperatures, high irradiation, water deficit, and UV radiation. Oxidant buildup and oxidative inactivation of thioredoxin-dependent plastidic enzymes were decreased in stressed plants expressing plastid-targeted Fld, suggesting that development of the tolerant phenotype relied on productive interaction of this flavoprotein with Fd-dependent oxidoreductive pathways of the host, most remarkably, thioredoxin reduction. The use of Fld provides new tools to investigate the requirements of photosynthesis in planta and to increase plant stress tolerance based on the introduction of a cyanobacterial product that is free from endogenous regulation in higher plants.

No abstract

Under environmental stresses, plant development is adaptively modulated. This modulation is influenced by the steady-state balance (homeostasis) between reactive oxygen species (ROS) and phytohormones. Frequently observed symptoms in plant stress adaptation responses include growth retardation, reduced metabolism and photosynthesis, reallocation of metabolic resources and increased antioxidant activities to maximize plant survival under adverse environmental conditions. In view of stressinduced morphogenetic changes during adaptation, ROS and auxin are the main players in the regulatory networks because both are strongly affected by exposure to environmental cues. However, the mechanisms underlying the crosstalk between ROS and auxin are poorly understood. In this review, we aim at surveying how the integration of environmental stress-related signals is modulated by crosstalk between ROS and auxin regulatory networks.

SUMMARYAttempted infection of plants by pathogens elicits a complex defensive response. In many non-host and incompatible host interactions it includes the induction of defence-associated genes and a form of localized cell death (LCD), purportedly designed to restrict pathogen advance, collectively known as the hypersensitive response (HR). It is preceded by an oxidative burst, generating reactive oxygen species (ROS) that are proposed to cue subsequent deployment of the HR, although neither the origin nor the precise role played by ROS in the execution of this response are completely understood. We used tobacco plants expressing cyanobacterial flavodoxin to address these questions. Flavodoxin is an electron shuttle present in prokaryotes and algae that, when expressed in chloroplasts, specifically prevents ROS formation in plastids during abiotic stress episodes. Infiltration of tobacco wild-type leaves with high titres of Xanthomonas campestris pv. vesicatoria (Xcv), a nonhost pathogen, resulted in ROS accumulation in chloroplasts, followed by the appearance of localized lesions typical of the HR. In contrast, chloroplast ROS build-up and LCD were significantly reduced in Xcv-inoculated plants expressing plastid-targeted flavodoxin. Metabolic routes normally inhibited by pathogens were protected in the transformants, whereas other aspects of the HR, including the induction of defence-associated genes and synthesis of salicylic and jasmonic acid, proceeded as in inoculated wild-type plants. Therefore, ROS generated in chloroplasts during this non-host interaction are essential for the progress of LCD, but do not contribute to the induction of pathogenesis-related genes or other signalling components of the response.

Hydrogen peroxide (H2O2) operates as a signaling molecule in eukaryotes, but the specificity of its signaling capacities remains largely unrevealed. Here, we analyzed whether a moderate production of H2O2 from two different plant cellular compartments has divergent effects on the plant transcriptome. Arabidopsis thaliana overexpressing glycolate oxidase in the chloroplast (Fahnenstich et al., 2008; Balazadeh et al., 2012) and plants deficient in peroxisomal catalase (Queval et al., 2007; Inzé et al., 2012) were grown under non-photorespiratory conditions and then transferred to photorespiratory conditions to foster the production of H2O2 in both organelles. We show that H2O2 originating in a specific organelle induces two types of responses: one that integrates signals independently from the subcellular site of H2O2 production and another that is dependent on the H2O2 production site. H2O2 produced in peroxisomes induces transcripts involved in protein repair responses, while H2O2 produced in chloroplasts induces early signaling responses, including transcription factors and biosynthetic genes involved in production of secondary signaling messengers. There is a significant bias towards the induction of genes involved in responses to wounding and pathogen attack by chloroplastic-produced H2O2, including indolic glucosinolates-, camalexin-, and stigmasterol-biosynthetic genes. These transcriptional responses were accompanied by the accumulation of 4-methoxy-indol-3-ylmethyl glucosinolate and stigmasterol.

SummaryPeroxiredoxin Q (Prx Q) is one out of 10 peroxiredoxins encoded in the genome of Arabidopsis thaliana, and one out of four that are targeted to plastids. Peroxiredoxin Q functions as a monomeric protein and represents about 0.3% of chloroplast proteins. It attaches to the thylakoid membrane and is detected in preparations enriched in photosystem II complexes. Peroxiredoxin Q decomposes peroxides using thioredoxin as an electron donor with a substrate preference of H 2 O 2 > cumene hydroperoxide ) butyl hydroperoxide ) linoleoyl hydroperoxide and insignificant affinity towards complex phospholipid hydroperoxide. Plants with decreased levels of Prx Q did not have an apparently different phenotype from wildtype at the plant level. However, similar to antisense 2-cysteine (2-Cys) Prx plants [Baier, M. et al. (2000) Plant Physiol., 124, 823-832], Prx Q-deficient plants had a decreased sensitivity to oxidants in a leaf slice test as indicated by chlorophyll a fluorescence measurements. Increased fluorescence ratios of photosystem II to I at 77 K and modified transcript levels of plastid-and nuclear-encoded proteins show that regulatory mechanisms are at work to compensate for the lack of Prx Q. Apparently Prx Q attaches to photosystem II and has a specific function distinct from 2-Cys peroxiredoxin in protecting photosynthesis. Its absence causes metabolic changes that are sensed and trigger appropriate compensatory responses.