Heredity and gene fine mapping of a small spikelet and compact glume 1 (sscg1) mutant in wheat

Spike development in wheat (Triticum aestivum L.) is crucial in determining spike type, which affects agronomic traits, such as spikelet number, grain shape, kilo-grain-weight, and ultimately affects wheat yield. Therefore, it is valuable for wheat genetic studies and breeding to mine the genes involved in spike development. A small spikelet and compact glume mutant 1 (sscg1) was obtained from wheat line Shengnong 1 treated with ethyl methane sulfonate (EMS). The typical characteristics of the mutant sscg1 were its smaller spikelets and compacted glumes. In this study, the agronomy traits, karyotype, heredity, and mutated gene mapping of sscg1 were systemically studied. The mutant traits were controlled by a recessive gene named as sscg1, which was fine mapped in a 1.33 Mb region covering 147.28-148.61 Mb on chromosome 3AS in CS1.0. Seven putative candidate genes were identified in this region. Because no spike developmental related genes have been reported in this region, sscg1 was a novel spike development regulation gene. Considered the annotated functions and expression levels of the candidate genes, IAA3-3A encoded an auxin-responsive IAA3-like protein in CS2.1, and almost didn't express in mutant sscg1 during spike development, it most likely was the mutated gene. Transcriptome sequencing, qRT-PCR and determination of the endogenous hormone IAA contents demonstrated that the expressions of most TaARF genes were significantly increased, and the IAA contents were decreased during spike development in sscg1. The result suggested that the IAA signal pathway was a key factor controlling wheat spike development. Using elite alleles of IAA3-3A and regulating IAA signal pathway can be an approach for wheat yield breeding.