Unleashing floret fertility in wheat through the mutation of a homeobox gene

Abstract

Floret fertility is a key determinant of the number of grains per inflorescence in cereals. During the evolution of wheat (<i>Triticum</i> sp.), floret fertility has increased, such that current bread wheat (<i>Triticum aestivum</i>) cultivars set three to five grains per spikelet. However, little is known regarding the genetic basis of floret fertility. The locus <i>Grain Number Increase 1</i> (<i>GNI1</i>) is shown here to be an important contributor to floret fertility. <i>GNI1</i> evolved in the Triticeae through gene duplication. The gene, which encodes a homeodomain leucine zipper class I (HD-Zip I) transcription factor, was expressed most abundantly in the most apical floret primordia and in parts of the rachilla, suggesting that it acts to inhibit rachilla growth and development. The level of <i>GNI1</i> expression has decreased over the course of wheat evolution under domestication, leading to the production of spikes bearing more fertile florets and setting more grains per spikelet. Genetic analysis has revealed that the reduced-function allele <i>GNI-A1</i> contributes to the increased number of fertile florets per spikelet. The RNAi-based knockdown of <i>GNI1</i> led to an increase in the number of both fertile florets and grains in hexaploid wheat. Mutants carrying an impaired <i>GNI-A1</i> allele out-yielded WT allele carriers under field conditions. The data show that gene duplication generated evolutionary novelty affecting floret fertility while mutations favoring increased grain production have been under selection during wheat evolution under domestication.