Jasmonoyl isoleucine accumulation is needed for abscisic acid build‐up in roots of Arabidopsis under water stress conditions

Phytohormones are central players in sensing and signalling numerous environmental conditions like drought. In this work, hormone profiling together with gene expression of key enzymes involved in abscisic acid (ABA) and jasmonate biosynthesis were studied in desiccating Arabidopsis roots. Jasmonic acid (JA) content transiently increased after stress imposition whereas progressive and concomitant ABA and Jasmonoyl Isoleucine (JA‐Ile) accumulations were detected. Molecular data suggest that, at least, part of the hormonal regulation takes place at the biosynthetic level. These observations also point to a possible involvement of jasmonates on ABA biosynthesis under stress. To test this hypothesis, mutants impaired in jasmonate biosynthesis (opr3, lox6 and jar1‐1) and in JA‐dependent signalling (coi1) were employed. Results showed that the early JA accumulation leading to JA‐Ile build up was necessary for an ABA increase in roots under two different water stress conditions. Signal transduction between water stress‐induced JA‐Ile accumulation and COI1 is necessary for a full induction of the ABA biosynthesis pathway and subsequent hormone accumulation in roots of Arabidopsis plants. The present work adds a level of interaction between jasmonates and ABA at the biosynthetic level.     

Sucrose metabolism in spring and winter wheat in response to high irradiance, cold stress and cold acclimation

Effects of low‐temperature stress, cold acclimation and growth at high irradiance in a spring (Triticum aestivum L. cv. Katepwa) and a winter wheat (Triticum aestivum L. cv. Monopol) were examined in leaves and crowns with respect to the sucrose utilisation and carbon allocation. Light‐saturated and carbon dioxide (CO₂)‐saturated rates of CO₂ assimilation were decreased by 50% in cold‐stressed spring and winter wheat cultivars. Cold‐ or high light‐acclimated Katepwa spring wheat maintained light‐saturated rates of CO₂ assimilation comparable to those of control spring wheat. In contrast, cold‐ or high light‐acclimated winter wheat maintained higher light and CO₂‐saturated rates of CO₂ assimilation than non‐acclimated controls. In leaves, during either cold stress, cold acclimation or acclimation to high irradiance, the sucrose/starch ratio increased by 5‐ to 10‐fold and neutral invertase activity increased by 2‐ to 2.5‐fold in both the spring and the winter wheat. In contrast, Monopol winter wheat, but not Katepwa spring wheat, exhibited a 3‐fold increase in leaf sucrose phosphate synthase (SPS) activity, a 4‐fold increase in sucrose:sucrose fructosyl transferase activity and a 6.6‐fold increase in acid invertase upon cold acclimation. Although leaves of cold‐stressed and high light‐grown spring and winter wheat showed 2.3‐ to 7‐fold higher sucrose levels than controls, these plants exhibited a limited capacity to adjust either sucrose phosphate synthase or sucrose synthase activity (SS[s]). In addition, the acclimation to high light resulted in a 23–31% lower starch abundance and no changes at the level of fructan accumulation in leaves of either winter or spring wheat when compared with controls. However, high light‐acclimated winter wheat exhibited a 1.8‐fold higher neutral invertase activity and high light‐acclimated spring wheat exhibited an induction of SS(d) activity when compared with controls. Crowns of Monopol showed higher fructan accumulation than Katepwa upon cold and high light acclimation. We suggest that the differential adjustment of CO₂‐saturated rates of CO₂ assimilation upon cold acclimation in Monopol winter wheat, as compared with Katepwa spring wheat, is associated with the increased capacity of Monopol for sucrose utilisation through the biosynthesis of fructans in the leaves and subsequent export to the crowns. In contrast, the differential adjustment of CO₂‐saturated rates of CO₂ assimilation upon high light acclimation of Monopol appears to be associated with both increased fructan and starch accumulation in the crowns.     

Jasmonoyl‐l‐isoleucine hydrolase 1 (JIH1) regulates jasmonoyl‐l‐isoleucine levels and attenuates plant defenses against herbivores

For most plant hormones, biological activity is suppressed by reversible conjugation to sugars, amino acids and other small molecules. In contrast, the conjugation of jasmonic acid (JA) to isoleucine (Ile) is known to enhance the activity of JA. Whereas hydroxylation and carboxylation of JA‐Ile permanently inactivates JA‐Ile‐mediated signaling in plants, the alternative deactivation pathway of JA‐Ile by its direct hydrolysis to JA remains unstudied. We show that Nicotiana attenuata jasmonoyl‐l ‐isoleucine hydrolase 1 (JIH1), a close homologue of previously characterized indoleacetic acid alanine resistant 3 (IAR3) gene in Arabidopsis, hydrolyzes both JA‐Ile and IAA‐Ala in vitro. When the herbivory‐inducible NaJIH1 gene was silenced by RNA interference, JA‐Ile levels increased dramatically after simulated herbivory in irJIH1, compared with wild‐type (WT) plants. When specialist (Manduca sexta) or generalist (Spodoptera littoralis) herbivores fed on irJIH1 plants they gained significantly less mass compared with those feeding on wild‐type (WT) plants. The poor larval performance was strongly correlated with the higher accumulation of several JA‐Ile‐dependent direct defense metabolites in irJIH1 plants. In the field, irJIH1 plants attracted substantially more Geocoris predators to the experimentally attached M. sexta eggs on their leaves, compared with empty vector plants, which correlated with higher herbivory‐elicited emissions of volatiles known to function as indirect defenses. We conclude that NaJIH1 encodes a new homeostatic step in JA metabolism that, together with JA and JA‐Ile‐hydroxylation and carboxylation of JA‐Ile, rapidly attenuates the JA‐Ile burst, allowing plants to tailor the expression of direct and indirect defenses against herbivore attack in nature.     

OsGLP3-7 positively regulates rice immune response by activating hydrogen peroxide,jasmonic acid,and phytoalexin metabolic pathways

Although germin-like proteins (GLPs) have been demonstrated to participate in plant biotic stress responses, their specific functions in rice disease resistance are still largely unknown. Here, we report the identification and characterization of OsGLP3-7, a member of the GLP family in rice. Expression of OsGLP3-7 was significantly induced by pathogen infection, jasmonic acid (JA) treatment, and hydrogen peroxide (H₂O₂) treatment. OsGLP3-7 was highly expressed in leaves and sublocalized in the cytoplasm. Overexpression of OsGLP3-7 increased plant resistance to leaf blast, panicle blast, and bacterial blight, whereas disease resistance in OsGLP3-7 RNAi silenced plants was remarkably compromised, suggesting this gene is a positive regulator of disease resistance in rice. Further analysis showed that OsGLP3-7 has superoxide dismutase (SOD) activity and can influence the accumulation of H₂O₂ in transgenic plants. Many genes involved in JA and phytoalexin biosynthesis were strongly induced, accompanied with elevated levels of JA and phytoalexins in OsGLP3-7-overexpressing plants, while expression of these genes was significantly suppressed and the levels of JA and phytoalexins were reduced in OsGLP3-7 RNAi plants compared with control plants, both before and after pathogen inoculation. Moreover, we showed that OsGLP3-7-dependent phytoalexin accumulation may, at least partially, be attributed to the elevated JA levels observed after pathogen infection. Taken together, our results indicate that OsGLP3-7 positively regulates rice disease resistance by activating JA and phytoalexin metabolic pathways, thus providing novel insights into the disease resistance mechanisms conferred by GLPs in rice.     

Interplay Between Abscisic Acid and Jasmonic Acid and its Role in Water-oxidative Stress in Wild-type, ABA-deficient, JA-deficient, and Ascorbate-deficient Arabidopsis Plants

The interplay between jasmonic acid (JA) and abscisic acid (ABA) in plant responses to water stress and in water-stress-enhanced oxidative stress was investigated in Arabidopsis thaliana plants subjected to water stress by water deprivation. For this purpose a drought assay was conducted using Arabidopsis mutants impaired in ABA (aba2), JA (aos), and ascorbate (vtc1) biosynthesis. Our results show an interaction between ABA and JA during their biosynthesis. Moreover, the coordinated action of ABA and JA protected wild-type, aba2, and aos plants from the effects of stress. However, this effect was not observed in the vtc1 mutant, which showed a distinct decrease in the F _v/F _m ratio, concomitant with a marked fall in relative water content (RWC), despite high endogenous concentrations of JA and ABA. This finding indicates the relevance of ascorbate metabolism in plant acclimation to stress. Despite the interaction between the two phytohormones, drought-associated stomatal closure is regulated mainly by ABA and weakly by JA, whereas JA plays a role in the formation of antioxidants regulating ascorbate and glutathione metabolism. A time course analysis revealed the relevance of plant age and stress duration in the responses of the mutants compared to wild-type plants. Here we discuss the relationship between ABA, JA, ascorbate, and glutathione in plants under water stress.     

Molecular changes of Arabidopsis thaliana plastoglobules facilitate thylakoid membrane remodeling under high light stress

Plants require rapid responses to adapt to environmental stresses. This includes dramatic changes in the size and number of plastoglobule lipid droplets within chloroplasts. Although the morphological changes of plastoglobules are well documented, little is known about the corresponding molecular changes. To address this gap, we have compared the quantitative proteome, oligomeric state, prenyl-lipid content and kinase activities of Arabidopsis thaliana plastoglobules under unstressed and 5-day light-stressed conditions. Our results show a specific recruitment of proteins related to leaf senescence and jasmonic acid biosynthesis under light stress, and identify nearly half of the plastoglobule proteins in high native molecular weight masses. Additionally, a specific increase in plastoglobule carotenoid abundance under the light stress was consistent with enhanced thylakoid disassembly and leaf senescence, supporting a specific role for plastoglobules in senescence and thylakoid remodeling as an intermediate storage site for photosynthetic pigments. In vitro kinase assays of isolated plastoglobules demonstrated kinase activity towards multiple target proteins, which was more pronounced in the plastoglobules of unstressed than light-stressed leaf tissue, and which was diminished in plastoglobules of the abc1k1/abc1k3 double-mutant. These results strongly suggest that plastoglobule-localized ABC1 kinases hold endogenous kinase activity, as these were the only known or putative kinases identified in the isolated plastoglobules by deep bottom-up proteomics. Collectively, our study reveals targeted changes to the protein and prenyl-lipid composition of plastoglobules under light stress that present strategies by which plastoglobules appear to facilitate stress adaptation within chloroplasts.     

The Arabidopsis THADA homologue modulates TOR activity and cold acclimation

• Low temperature is one of the most important environmental factors that affect global survival of humans and animals and equally importantly the distribution of plants and crop productivity. Survival of metazoan cells under cold stress requires regulation of the sensor‐kinase Target Of Rapamycin (TOR). TOR controls growth of eukaryotic cells by adjusting anabolic and catabolic metabolism. Previous studies identified the Thyroid Adenoma Associated (THADA) gene as the major effect locus by positive selection in the evolution of modern human adapted to cold. Here we investigate the role of THADA in TOR signaling and cold acclimation of plants.
• We applied BLAST searches and homology modeling to identify the AtTHADA (AT3G55160) in Arabidopsis thaliana as the highly probable orthologue protein. Reverse genetics approaches were combined with immunological detection of TOR activity and metabolite profiling to address the role of the TOR and THADA for growth regulation and cold acclimation.
• Depletion of the AtTHADA gene caused complete or partial loss of full‐length mRNA, respectively, and significant retardation of growth under non‐stressed conditions. Furthermore, depletion of AtTHADA caused hypersensitivity towards low‐temperatures. Atthada displayed a lowered energy charge. This went along with decreased TOR activity, which offers a molecular explanation for the slow growth phenotype of Atthada. Finally, we used TOR RNAi lines to identify the de‐regulation of TOR activity as one determinant for sensitivity towards low‐temperatures.
• Taken together our results provide evidence for a conserved function of THADA in cold acclimation of eukaryotes and suggest that cold acclimation in plants requires regulation of TOR.

     

Cold resistance in Antarctic angiosperms

Deschampsia antarctica Desv. (Poaceae) and Colobanthus quitensis (Kunth) Bartl. (Cariophyllaceae) are the only two vascular plants that have colonized the Maritime Antarctic. The primary purpose of the present work was to determine cold resistance mechanisms in these two Antarctic plants. This was achieved by comparing thermal properties of leaves and the lethal freezing temperature to 50% of the tissue (LT50). The grass D. antarctica was able to tolerate freezing to a lower temperature than C. quitensis. The main freezing resistance mechanism for C. quitensis is supercooling. Thus, the grass is mainly a freezing‐tolerant species, while C. quitensis avoids freezing. D. antarctica cold acclimated; thus, reducing its LT50. C. quitensis showed little cold‐acclimation capacity. Because day length is highly variable in the Antarctic, the effect of day length on freezing tolerance, growth, various soluble carbohydrates, starch, and proline contents in leaves of D. antarctica growing in the laboratory under cold‐acclimation conditions was studied. During the cold‐acclimation treatment, the LT50 was lowered more effectively under long day (21/3 h light/dark) and medium day (16/8) light periods than under a short day period (8/16). The longer the day length treatment, the faster the growth rate for both acclimated and non‐acclimated plants. Similarly, the longer the day treatment during cold acclimation, the higher the sucrose content (up to 7‐fold with respect to non‐acclimated control values). Oligo and polyfructans accumulated significantly during cold acclimation only with the medium day length treatment. Oligofructans accounted for more than 80% of total fructans. The degrees of polymerization were mostly between 3 and 10. C. quitensis under cold acclimation accumulated a similar amount of sucrose than D. antarctica, but no fructans were detected. The suggestion that survival of Antarctic plants in the Antarctic could be at least partially explained by accumulation of these substances is discussed.     

Chill-responsive dehydrins in blueberry: Are they associated with cold hardiness or dormancy transitions?

To survive winters, woody perennials of temperate zones must enter into endodormancy. Resumption of spring growth requires sufficient exposure to low temperature (chill units, CUs) in winter (chilling requirement), which also plays a role in the development of cold hardiness (cold acclimation). Physiological studies on dormancy breaking have focused on identifying markers, such as appearance or disappearance of proteins in response to varying degrees of chill unit accumulation. However, whether these changes are associated with dormancy transitions or cold acclimation is not clear. In the present study, greenhouse-grown blueberry (Vaccinium section Cyanococcus) plants were used to address this question. Three blueberry cultivars, Bluecrop, Tifblue, and Gulfcoast having chilling requirement of approximately 1 200, 900 and 600 CUs, respectively, were first exposed to 4°C for long enough to provide chill units equivalent to one-half of their respective chilling requirement. This treatment was expected to result in cold acclimation. A fraction of plants was then subjected to a 15/12°C (light/dark) regime for 2 weeks, a treatment expected to be “dormancy-neutral” but cause deacclimation. Before and after each treatment, cold hardiness and dormancy status of floral buds were determined; proteins were extracted from the buds collected on the same sampling date, and separated by one-dimensional SDS-PAGE. Dehydrin-like proteins were identified by immunoblotting, using anti-dehydrin antiserum. Results indicate that the chilling treatment resulted in cold acclimation as indicated by increased bud hardiness in all three cultivars. Data also indicate a distinct accumulation of three dehydrin-like proteins of 65, 60, and 14 kDa during cold acclimation. The cold hardiness and levels of dehydrin proteins decreased during the exposure to 15/12°C for 2 weeks. Results also confirmed that this treatment had no negative effect on chill unit accumulation. Densitometric scans of protein gels indicated a close association between the abundance of dehydrins and degree of cold hardiness in these cultivars. In addition, levels of the dehydrin proteins and cold hardiness remained about the same between 100% and >100% satisfaction of chilling requirement. These results suggest that changes in dehydrin expression are more closely associated with cold hardiness than with dormancy transitions.