Mutation of the wheat homeobox gene Grain Number Increase 1 increases grain number and grain yield but decreases grain protein content

Inflorescence structure affects final grain yield (GY) in wheat (Triticum aestivum L.). Recent breeding efforts have focused on improving grain number per spike, which is positively correlated with GY. Grain Number Increase 1 (GNI-A1) encodes a homeodomain leucine zipper class I (HD-Zip I) transcription factor that controls the number of grains per spike and GY. However, how this increase in grain number affects grain quality, especially grain protein content (GPC) in wheat, remains elusive. Here we investigated within-spikelet variation in GPC using GNI-A1 near-isogenic lines. Yield trials in two seasons and at two sites demonstrated that lines harboring a reduced-function allele, GNI-A1 (105Y), consistently showed improved GY due to a 27% increase in grain number per spike, along with a 1.7% reduction in GPC compared with lines containing a functional allele, GNI-A1 (105N). We confirmed the positive correlation between GY and grain number and the negative correlation between GY and GPC, but we observed no correlation between GY and thousand-grain weight. The increased grain number conferred by the 105Y allele was due to better floret fertility around the central part of the spike and whole florets. In-depth phenotypic analysis using dissected grain samples revealed that GPC was nearly uniform among spikelets and florets. These results suggest that in plants carrying a mutation in GNI-A1, the increase in the total number of grains is accompanied by a reduction in GPC.