Amelioration of the Oxidative Stress Generated by Simple or Combined Abiotic Stress through the K⁺ and Ca2+ Supplementation in Tomato Plants.

Abiotic stressors such as drought, heat, or salinity are major causes of yield loss worldwide due to the oxidative burst generated under these conditions. Recent studies have revealed that plant response to a combination of different environmental stressors is unique and cannot be deduced from the response developed to each stress when applied individually. Some studies have demonstrated that a different management of some nutrients in the irrigation solution may provide an advantage to the plants against abiotic stressors. Thus, the aim of this study was to investigate if an increase in potassium (K⁺) and calcium (Ca2+) concentration in the nutrient solution may have a positive effect on the amelioration of oxidative stress which occurs under the combination of salinity and heat in tomato plants. Our results indicated that plants irrigated with an increase in K⁺ and Ca2+ concentrations in the irrigation solution from 7mM (K⁺) to 9.8 mM and from 4 mM (Ca2+) to 5.6 mM, respectively, induced a recovery of the biomass production compared to the plants treated with salinity or salinity + heat, and subsequently irrigated with the regular Hoagland solution. This was correlated with a better performance of all the photosynthetic parameters, a reduction in the foliar concentration of H2O2 and a lower lipid peroxidation rate, and with a better performance of the antioxidant enzymes ascorbate peroxidase ascorbate peroxidase (APX), dehydroascorbate reductactase (DHAR), glutathione reductase (GR), and NADPH oxidase. Our results showed that these enzymes were differentially regulated at the transcriptional level, showing a higher reactive oxygen species (ROS) detoxification efficiency under salinity and under the combination of salinity and heat, as compared to those plants irrigated with common Hoagland. An increase in K⁺ and Ca2+ in the irrigation solution also induced a lower Na+ accumulation in leaves and a higher K⁺/Na⁺ ratio. Thus, our study highlights the importance of the right management of the plant nutritional status and fertilization in order to counteract the deleterious effects of abiotic stress in plants.

Tolerance to Stress Combination in Tomato Plants: New Insights in the Protective Role of Melatonin.

Abiotic stresses such as drought, heat or salinity are major causes of yield loss worldwide. Recent studies have revealed that the acclimation of plants to a combination of different environmental stresses is unique and therefore cannot be directly deduced from studying the response of plants to each of the different stresses applied individually. The efficient detoxification of reactive oxygen species (ROS) is thought to play a key role in enhancing the tolerance of plants to abiotic stresses. Here, we report on the role of melatonin in the protection of the photosynthetic apparatus through the increase in ROS detoxification in tomato plants grown under the combination of salinity and heat, two of the most common abiotic stresses known to act jointly. Plants treated with exogenous melatonin showed a different modulation in the expression on some antioxidant-related genes and their related enzymes. More specifically, ascorbate peroxidase, glutathione reductase, glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase (APX, GR, GPX and Ph-GPX, resepctively) showed an antagonistic regulation as compared to plants that did not receive melatonin. This translated into a better antioxidant capacity and to a lesser ROS accumulation under stress combination. The performance of the photosynthesis parameters and the photosystems was also increased in plants treated with exogenous melatonin under the combination of salinity and heat. In accordance with these findings, tomato plants treated with melatonin were found to grow better under stress combination that the non-treated ones. Our study highlights the important role that exogenous melatonin plays in the acclimation of plants to a combination of two different abiotic stresses, and how this compound can specifically regulate oxidative stress-related genes and enzymes to increase plant tolerance.

Accumulation of Flavonols over Hydroxycinnamic Acids Favors Oxidative Damage Protection under Abiotic Stress.

Efficient detoxification of reactive oxygen species (ROS) is thought to play a key role in enhancing the tolerance of plants to abiotic stresses. Although multiple pathways, enzymes, and antioxidants are present in plants, their exact roles during different stress responses remain unclear. Here, we report on the characterization of the different antioxidant mechanisms of tomato plants subjected to heat stress, salinity stress, or a combination of both stresses. All the treatments applied induced an increase of oxidative stress, with the salinity treatment being the most aggressive, resulting in plants with the lowest biomass, and the highest levels of H2O2 accumulation, lipid peroxidation, and protein oxidation. However, the results obtained from the transcript expression study and enzymatic activities related to the ascorbate-glutathione pathway did not fully explain the differences in the oxidative damage observed between salinity and the combination of salinity and heat. An exhaustive metabolomics study revealed the differential accumulation of phenolic compounds depending on the type of abiotic stress applied. An analysis at gene and enzyme levels of the phenylpropanoid metabolism concluded that under conditions where flavonols accumulated to a greater degree as compared to hydroxycinnamic acids, the oxidative damage was lower, highlighting the importance of flavonols as powerful antioxidants, and their role in abiotic stress tolerance.

A splice acceptor site mutation in TaGW2-A1 increases thousand grain weight in tetraploid and hexaploid wheat through wider and longer grains.

KEY MESSAGE: Across 13 experiments the gw2 - A1 mutant allele shifts grain size distribution consistently across all grains significantly increasing grain weight (6.6 %), width (2.8 %) and length (2.1 %) in tetraploid and hexaploid wheat. There is an urgent need to identify, understand and incorporate alleles that benefit yield in polyploid wheat. The rice OsGW2 gene functions as a negative regulator of grain weight and width and is homologous to the wheat TaGW2 gene. Previously it was shown that transcript levels of the A-genome homoeologue, TaGW2-A1, are negatively associated with grain width in hexaploid wheat. In this study we screened the tetraploid Kronos TILLING population to identify mutants in TaGW2-A1. We identified a G to A transition in the splice acceptor site of exon 5 which leads to mis-splicing in TaGW2-A1. We backcrossed the mutant allele into tetraploid and hexaploid wheat and generated a series of backcross derived isogenic lines which were evaluated in glasshouse and field conditions. Across 13 experiments the GW2-A1 mutant allele significantly increased thousand grain weight (6.6 %), grain width (2.8 %) and grain length (2.1 %) in tetraploid and hexaploid wheat compared to the wild type allele. In hexaploid wheat, this led to an increase in spike yield since no differences were detected for spikelet or grain number between isogenic lines. The increase in grain width and length was consistent across grains of different sizes, suggesting that the effect of the mutation is stable across the ear and within spikelets. Differences in carpel size and weight between alleles were identified as early as 5 days before anthesis, suggesting that TaGW2-A1 acts on maternal tissue before anthesis to restrict seed size. A single nucleotide polymorphism marker was developed to aid the deployment of the mutant allele into breeding programmes.

The combined effect of salinity and heat reveals a specific physiological, biochemical and molecular response in tomato plants.

Many studies have described the response mechanisms of plants to salinity and heat applied individually; however, under field conditions some abiotic stresses often occur simultaneously. Recent studies revealed that the response of plants to a combination of two different stresses is specific and cannot be deduced from the stresses applied individually. Here, we report on the response of tomato plants to a combination of heat and salt stress. Interestingly, and in contrast to the expected negative effect of the stress combination on plant growth, our results show that the combination of heat and salinity provides a significant level of protection to tomato plants from the effects of salinity. We observed a specific response of plants to the stress combination that included accumulation of glycine betaine and trehalose. The accumulation of these compounds under the stress combination was linked to the maintenance of a high K(+) concentration and thus a lower Na(+) /K(+) ratio, with a better performance of the cell water status and photosynthesis as compared with salinity alone. Our findings unravel new and unexpected aspects of the response of plants to stress combination and provide a proposed list of enzymatic targets for improving crop tolerance to the abiotic field environment.

Glutathione homeostasis as an important and novel factor controlling blossom-end rot development in calcium-deficient tomato fruits.

Based on previous results in which oxidative metabolism was suggested as a possible inducer of blossom-end rot (BER), the main questions addressed here were whether calcium deficiency is the main factor that induces BER or whether this physiological disorder a general stress-related phenomenon? Tomato plants were grown under optimal or deficient calcium concentrations. Only the application of 0.1mM calcium resulted in BER induction, although only half of the fruits grown under this treatment had this disorder. Having fruits showing or not showing BER in the same plant and treatment provided us with a powerful tool that we used to investigate whether calcium deficiency operates alongside another mechanism in the induction of BER. Whether or not this other mechanism was the one controlling BER incidence was also investigated. We performed a complete study of the oxidative metabolism in the pericarp of healthy fruits and in the healthy portion of BER-affected fruits. Calcium deficiency led to an induction of NADPH oxidase, superoxide dismutase, dehydro- and monodehydroascorbate reductase, and to an inhibition of catalase, ascorbate peroxidase and glutathione reductase, with a concomitant accumulation of hydrogen peroxide and an increase in lipid peroxidation. While the ascorbate redox state was not affected by calcium deficiency, the glutathione redox state was markedly reduced. We conclude that calcium deficiency fundamentally affected the activity of the ascorbate-glutathione enzymes, with special importance to the inhibition of GR, which lead to a reduction of the glutathione redox state. This could cause the breakdown of cellular homeostasis, the inhibition of other enzymes responsible for H(2)O(2) detoxification, and ultimately an increase of lipid peroxidation. Therefore, BER is defined here as the visual symptom of a massive lipid peroxidation event caused by the breakdown of cellular glutathione homeostasis.