TaKMT-7A Gene Positively Regulates Spike Number in Wheat

Wheat (Triticum aestivum L.) is a crucial global food crop that plays a central role in agricultural production and food security. The spike number per unit area (SN) is one of the three component factors of grain yield. In this study, we combined the UG-Map with 27 environments of a recombinant inbred line (RIL) population, and mapped a quantitative trait locus (QTL) for SN, QSn-7A-9048, in which the meta-QTL interval contains only one candidate gene, TraesCS7A02G-364700 (TaKMT-7A). Using the CRISPR/Cas9 system, we generated two homozygous mutant lines, aa-1 and aa-2 of TaKMT-7A, which resulted in frameshift mutations, leading to the premature termination of the translation process. The SN values for the wild type (WT), aa-1, and aa-2 were 4.48, 3.43, and 3.48, respectively. Compared with the WT, the SN of the two mutant lines significantly decreased, and no significant differences for grain number per spike (GNS) and thousand-grain weight (TGW) were detected. We also obtained two overexpression (OE) lines of TaKMT-7A, OE-1 and OE-2. The SN values for the negative control (NC), OE-1, and OE-2 were 2.31, 3.33, and 3.00, respectively. Compared with NC, the SN values in the OE lines significantly increased. The phenotypes of the knockout (KO) lines and OE lines demonstrate that TaKMT-7A acts as a positive regulator of SN in wheat. We performed RNA-Seq analysis using young tiller buds from the WT and aa-1 mutant lines at the tillering stage, and a total of 2315 differentially expressed genes (DEGs) were identified. We screened 22 wheat genes, of which 18 orthologous genes have previously been cloned and are associated with branching in rice and Arabidopsis. These genes included nitrogen transporter, amino metabolism, auxin transporter, auxin homeostasis, auxin response, auxin biosynthesis, strigolactone biosynthesis, and repress gibberellin responses. These genes may represent potential downstream targets of TaKMT-7A.