Study of responses to heat stress in durum wheat, and their relationship with nutraceutical quality

Abstract

Durum wheat [Triticum turgidum (L.) subsp. turgidum (L.) convar. durum (Desf.)] is an important crop for human nutrition, especially in the Mediterranean area, where it is the main source of semolina for the production of pasta, traditional/typical bread, couscous, and burghul. Environmental stress can strongly limit the yield potential and affect qualitative characteristics of grain. The stress conditions that are commonly experienced by crops are extreme lack or excess of water (i.e., drought, flooding), presence of salt or contaminants (e.g., heavy metals), and temperature (i.e., cold, heat). In particular, environmental temperatures have increased since the beginning of the last century and they are predicted to further increase under the present conditions of climate change. Wheat is very sensitive to high temperatures during the reproductive phase, due to direct effects of temperature on grain numbers and dry weight. These phenotypic effects are due to molecular modifications at the different levels of gene expression, and to changes in metabolite accumulation levels. The aim of this study was to investigate the transcriptomic and metabolomic variations in response to heat stress in grains of two durum wheat genotypes, and their relationship with variation in grain composition that can affect the nutraceutical quality of the final product. The study was carried out on the durum wheat cv Primadur and T1303 (PI 352395 USDA code); the first is an elite cultivar with high grain yield and yellow index and the second one an anthocyanin-rich purple cultivar. Heat shock was imposed at 37°C from 5 up to 10 days after flowering (DAF). Immature seeds (14 DAF) and mature seeds (seeds at physiological maturity stage), collected from heat shocked and control plants (20°C), were analyzed. The response to stress was investigated with different approaches. Genome–wide gene expression analysis, carried out using Illumina HiSeq2000, showed that 1202 genes were differentially expressed in response to stress in the two genotypes. The number of up-regulated genes belonging to stress-related categories was higher in Primadur with respect to T1303, whereas the number of genes with nutrient reservoir activity was larger in T1303 compared to Primadur. These results were according to the higher sensitivity to stress of Primadur with respect to T1303, measured as grain weight loss. The profile of polar metabolites was investigated by gas chromatography-mass spectrometry (GC-MS). The effects of heat stress were genotype dependent. Although some metabolites (e.g., sucrose, glycerol) increased in response to heat stress in both genotypes, clear differences were observed. Following the heat stress, there was a general increase in most of the analyzed metabolites in Primadur, with a general decrease in T1303. Heat shock applied early during seed development produced changes that were observed in immature seeds, and also long-term effects 5 that changed the qualitative and quantitative parameters of the mature grain. The level of anthocyanins increased significantly in response to the heat stress whereas the levels of carotenoids were not affected. Only for T1303, the Trolox equivalent antioxidant capacity (TEAC) method assessed an increased antioxidant capacity of ground grains, in response to heat stress. Protein content increased only in T1303 that showed lower grain weight loss with respect to Primadur, in response to stress. Therefore, short heat-stress treatments can affect the nutritional and nutraceutical value of grain of different genotypes of durum-wheat in different ways. The present study increased the knowledge about the effect of mechanisms of adaptation to stress of wheat plants on nutritional and quality traits of grain and provides a global picture of the effects of the heat stress on metabolies and transcriptome of two different genotypes of durum wheat. Moreover, although the genetic backgrounds of two durum-wheat genotypes were different, it cannot be excluded that some of the differences observed in the responses to heat stress can be due to anthocyanin accumulation, which can be considered a trait of interest for future breeding activities for durum wheat.