Metabolomic profiling of SQ-1-induced changes in starch metabolism in sterile anthers of wheat

The metabolome and starch contents are closely related with the normal pollen development in plants. Thus, in this study, metabolome profiling, including principal component analysis, hierarchical cluster analysis, and metabolite–metabolite correlations were performed, and the changes induced by SQ-1 in the expression of starch synthetase genes in a male sterility line (PHYMS-1376) and male fertility line (MF-1376) were analysed to elucidate the relationship between the metabolite contents (metabolomes and starches) and pollen abortion and, consequently, explore the mechanisms underlying male-sterility induced by SQ-1. Results from this study showed that PHYMS-1376 displayed a high male-sterility rate (up to 99.07%), accompanied by low starch content in pollen. Metabolomic profiling revealed 93 metabolites present at significantly different levels using two-tailed unpaired Student's t-test (p-value ≤ 0.05) between the anthers of PHYMS-1376 and MF-1376, which were classified into five clusters. These 93 differential metabolites were analysed using principal component analysis and partial least squares-discriminant analysis found that all 60 samples were separated into four classes based on their developmental stage: (1) tetrad, (2) early uninucleate, (3) late uninucleate, and (4) binucleate and trinucleate stages in MF-1376 and PHYMS-1376 anthers. A total of 4278 correlations were identified among these 93 differential metabolites; thus, 107 significantly correlated pairs were found. The pathway analysis of the 93 differential metabolites showed that 67 metabolites play roles in the aminoacyl-transfer ribonucleic acid biosynthesis, tricarboxylic acid cycle, glycolysis, starch and sucrose metabolism, and other metabolic pathways. Meanwhile, an integrated metabolic map revealed relationships in terms of metabolic pathways among 84 metabolites from the 93 differential metabolites. Furthermore, the expression of key starch synthetase genes was dysregulated during the anther development from the tetrad to trinucleate stage in PHYMS-1376. In conclusion, we hypothesised that the expression patterns of most analysed metabolites (e.g. amino acids, fatty acids, and sugars) affected the numerous metabolic pathways, thereby probably harbouring insufficient nutrients for the abnormal regulation of starch synthetase genes for pollen development, leading to an abnormal or lack of starch formation and ultimately resulting in pollen abortion in the male sterility line induced by SQ-1.