Molecular, biochemical and physiological responses of wheat (Triticum aestivum) to spot blotch disease and salinity

Abstract

Biotic and abiotic stresses are well known to significantly damage crop production and hence reduce yields. In wheat, which is the second major cereal crop and a major source of dietary carbohydrate and vegetable protein for humans, salt stress is recognised as a significant abiotic factor, while spot blotch disease, caused by the fungus Bipolaris sorokiniana, is an important biotic stressor. In order to protect crops from these threats, it is necessary to have a holistic understanding of the plant response towards these particular stress agents. In this study the responses of wheat to salt stress and spot blotch disease, both individually, and in combination with one another, were investigated. Using morphological, physiological, biochemical, and molecular parameters, the responses of four commercial spring wheat genotypes, two developed by KWS (Alderon, Cochsie), and two Saudi varieties (Najran, Sama) to stress, were investigated. The findings show that Najran, Alderon, and Cochise performed better to salt stress than Sama in terms of growth, proline production, and abundance of transcripts for several WRKY genes, when exposed to 80 and 160 mM NaCl for 21 days. TaWRKY53-a, 37, 3, and 71 were identified as key genes that respond differentially between Najran and Sama genotypes, when subjected to salinity (160 mM NaCl). In addition, the findings demonstrated that Najran, Alderon, and Cochsie exhibited less disease severity than Sama to spot blotch. Moreover, Alderon showed greater tolerance than Sama to the combination of salt and spot blotch disease. Biochemical changes (i.e., MDA, SA, JA content) in Alderon in response to B. sorokiniana infection and also to the dual stress were different to those exhibited by Sama. During fungal infection expression of TaWRKY3 and 37 were upregulated in Alderon, while expression of the other WRKY genes investigated were downregulated. In contrast, expression of most WRKY genes were upregulated in Sama, although TaWRKY19 and 45 were downregulated. The majority of glucan synthase (GLS) genes were shown to be upregulated after 48 hours of infection in Alderon, whereas most of these genes were downregulated in Sama. Alderon and Sama exhibited different expression patterns for TaPAL, TaNPR1, TaAOS and TaLOX2. Under the combined stress (salt and spot blotch disease) most WRKY genes, including TaPAL, TaNPR1, TaAOS and TaLOX2, were downregulated in Alderon, but upregulated in Sama. Furthermore, bio-informatic analysis suggests that W-box elements found in the promoter regions of TaWKY53-a, and 71 genes are regulated by WRKY genes. TaWRKY71 was shown to contain motifs related to light response. In addition, TaPAL was shown to contain the highest frequency of motifs that were related to drought. Moreover, TaNPR1 and TaLOX2 were shown to have a high frequency for cis- elements relating to wounding and biotic stress. Finally, studies showed that Alderon and Sama respond differently to spot botch disease and salinity, either when applied individually, or when combined. Knowledge gained will help inform the development or more resilient wheat varieties with enhanced tolerance to biotic and abiotic stress.