Constitutive expression of aldose reductase 1 from Zea mays exacerbates salt and drought sensitivity of transgenic Escherichia coli and Arabidopsis

Exogenous Sorbitol Application Confers Drought Tolerance to Maize Seedlings through Up-Regulating Antioxidant System and Endogenous Sorbitol Biosynthesis

This study aims to explore the impacts of exogenous sorbitol on maize seedlings under polyethylene glycol (PEG)-simulated drought stress. Six treatments were set: normal condition (CK), PEG (P), 10 mM sorbitol (10S), PEG plus 10 mM sorbitol (10SP), 100 mM sorbitol (100S) and PEG plus 100 mM sorbitol (100SP). Maize seedlings' growth under PEG-simulated drought stress was significantly inhibited and exogenous sorbitol largely alleviated this growth inhibition. The seedlings under 10SP treatment grew much better than those under P, 100S and 100SP treatments and no significant difference in growth parameters was observed between the control and 10S treatment. The seedlings treated with 10SP had higher contents of soluble sugar, soluble protein, proline, ascorbic acid (AsA), reduced glutathione (GSH), sorbitol and relative water content, higher activities of antioxidant enzymes and aldose reductase, but lower contents of malondialdehyde (MDA), H₂O₂ and relative electrical conductivity than those treated with P, 100S and 100SP. qRT-PCR analysis showed that the transcript levels of genes encoding putative aldose reductase (AR) under P treatment were significantly up-regulated in sorbitol-applied treatments. Taken together, the results demonstrated that exogenous sorbitol application conferred drought tolerance to maize seedlings by up-regulating the expression levels of AR-related genes to enhance the accumulation of intracellular osmotic substances such as sorbitol and improve antioxidant systems to tone down the damage caused by drought stress.

Cold stress-induced changes in metabolism of carbonyl compounds and membrane fatty acid composition in chickpea

In this study, changes in membrane fatty acid (FA) composition and damage indices contents as well as the transcript patterns of carbonyl-detoxifying genes were evaluated in two chickpea (Cicer arietinum L.) genotypes, cold-tolerant Sel96th11439 and cold-sensitive ILC533 under cold stress (CS; 4 °C). During CS, H2O2 and malondialdehyde (MDA) contents increased (by 47% and 57%, respectively) in the sensitive genotype, while these contents remained unchanged in the tolerant genotype. In tolerant plants, higher content of linoleic, linolenic, unsaturated FAs (UFAs), total FAs and double bond index (DBI) (by 23, 21, 19, 17 and 9%, respectively) was observed at 6 days after stress (DAS) compared to sensitive plants, which, along with alterations of the damage indices, indicate their enhanced tolerance to CS. Compared with the sensitive genotype, less lipoxygenase (LOX) activity (by 59%) in the tolerant genotype was accompanied by decreased MDA and increased levels of UFAs and DBI during CS, particularly at 6 DAS. Upregulation of aldehyde dehydrogenase and aldo-keto reductase genes (by 9- and 10-fold, respectively) at 1 DAS, along with the enhanced transcript levels of aldehyde reductase and 2-alkenal reductase (by 3- and 14.7-fold, respectively) at 6 DAS were accompanied by increased UFAs and reduced MDA contents in the tolerant genotype. Overall, the results suggest that cold tolerance in chickpea was partly associated with regulation of membrane FA compositions and the potential metabolic networks involved in synthesis and degradation of carbonyl compounds.

Pentatricopeptide Repeat Gene-Mediated Mitochondrial RNA Editing Impacts on Rice Drought Tolerance

Mitochondrial RNA editing plays crucial roles in the plant development and environmental adaptation. Pentatricopeptide repeat (PPR) genes, which are involved in the regulating mitochondrial RNA editing, are potential gene resources in the improvement of rice drought tolerance. In this study, we investigated genome-wide mitochondrial RNA editing in response to drought between upland and lowland rice. Responses of mitochondrial RNA editing to drought exhibit site-specific and genotype-specific patterns. We detected 22 and 57 ecotype-differentiated editing sites under well-watered and drought-treated conditions, respectively. Interestingly, the RNA editing efficiency was positively correlated with many agronomic traits, while it was negatively correlated with drought tolerance. We further selected two mitochondrial-localized PPR proteins, PPR035 and PPR406, to validate their functions in drought tolerance. PPR035 regulated RNA editing at rps4-926 and orfX-406, while PPR406 regulated RNA editing at orfX-355. The defectiveness in RNA editing at these sites had no apparent penalties in rice respiration and vegetative growth. Meanwhile, the knockout mutants of ppr035 and ppr406 show enhanced drought- and salt tolerance. PPR035 and PPR406 were under the balancing selection in upland rice and highly differentiated between upland and lowland rice ecotypes. The upland-dominant haplotypes of PPR035 and PPR406 shall contribute to the better drought tolerance in upland rice. They have great prospective in the improvement of rice drought tolerance.

Versatile roles of sorbitol in higher plants: luxury resource, effective defender or something else?

Sorbitol metabolism plays multiple roles in many plants, including energy and carbon enrichment, effective defence against various stresses and other emerging specific roles. The underlying mechanisms are, however, incompletely understood. This review provides the current state-of-the-art, highlights missing knowledge and poses several remaining questions. The basic properties of sugar alcohols are summarised and pathways of sorbitol metabolism, including biosynthesis, degradation and key enzymes are described. Sorbitol transport within the plant body is discussed and individual roles of sorbitol in different organs, specific cells or even cellular compartments, are elaborated, clarifying the critical importance of sorbitol allocation and distribution. In addition to plants that accumulate and transport significant quantities of sorbitol (usual producers), there are some that synthesize small amounts of sorbitol or only possess sorbitol metabolising enzymes (non-usual producers). Modern analytical methods have recently enabled large amounts of data to be acquired on this topic, although numerous uncertainties and questions remain. For a long time, it has been clear that enriching carbohydrate metabolism with a sorbitol branch improves plant fitness under stress. Nevertheless, this is probably valid only when appropriate growth and defence trade-offs are ensured. Information on the ectopic expression of sorbitol metabolism genes has contributed substantially to our understanding of the sorbitol roles and raises new questions regarding sorbitol signalling potential. We finally examine strategies in plants producing sorbitol compared with those producing mannitol. Providing an in-depth understanding of sugar alcohol metabolism is essential for the progress in plant physiology as well as in targeted, knowledge-based crop breeding.

Mobile forms of carbon in trees: metabolism and transport

Plants constitute 80% of the biomass on earth, and almost two-thirds of this biomass is found in wood. Wood formation is a carbon (C)-demanding process and relies on C transport from photosynthetic tissues. Thus, understanding the transport process is of major interest for understanding terrestrial biomass formation. Here, we review the molecules and mechanisms used to transport and allocate C in trees. Sucrose is the major form in which C is transported in plants, and it is found in the phloem sap of all tree species investigated so far. However, in several tree species, sucrose is accompanied by other molecules, notably polyols and the raffinose family of oligosaccharides. We describe the molecules that constitute each of these transport groups, and their distribution across different tree species. Furthermore, we detail the metabolic reactions for their synthesis, the mechanisms by which trees load and unload these compounds in and out of the vascular system, and how they are radially transported in the trunk and finally catabolized during wood formation. We also address a particular C recirculation process between phloem and xylem that occurs in trees during the annual cycle of growth and dormancy. A search of possible evolutionary drivers behind the diversity of C-carrying molecules in trees reveals no consistent differences in C transport mechanisms between angiosperm and gymnosperm trees. Furthermore, the distribution of C forms across species suggests that climate-related environmental factors will not explain the diversity of C transport forms. However, the consideration of C-transport mechanisms in relation to tree-rhizosphere coevolution deserves further attention. To conclude the review, we identify possible future lines of research in this field.