Plant hormones associated with increasing grain number and yield potential in wheat (Triticum aestivum L.) and their genetic regulation

Plant hormones are organic substances that influence specific physiological processes, such as floret fertility, and move throughout the plant. Previous studies suggest that genetic variation in grain number in cereals is associated with hormones such as cytokinin, which is crucial in controlling cell division and lateral meristem activity. This has been demonstrated in cereals when QTLs linked with reduced expression of cytokinin oxidase/dehydrogenase (OsCKX2) increased grain number in rice and wheat. However, how hormones regulate grain number traits such as fruiting efficiency (FE, ratio of grain number to spike dry weight at anthesis) and grain dry matter partitioning in wheat is not fully understood. The objectives of this study were, using a high biomass spring wheat panel, to identify novel grain number and partitioning traits for advancing harvest index and grain yield, and to determine how they are influenced by spike hormones. Finally, to understand the genetic regulation of these traits, using a bespoke target sequence capture strategy, to generate SNPs and establish molecular markers for the hormonal traits. A high biomass association panel (HiBAP II) of 150 CIMMYT spring wheat genotypes was phenotyped for grain number and partitioning traits in the field under irrigated conditions and spike hormone levels were sampled at anthesis in two seasons in NW Mexico. A subset of 10 genotypes representative of field variation for FE was grown in the glasshouse under well-watered conditions at the University of Nottingham, UK in three years for detailed hormonal analysis at GS49 (late booting) and 65 (anthesis). To test if certain plant components and stem internodes were competing with the spike at GS49 and GS65, destructive samples were taken where the plants were separated into their constitutive parts. Results showed genetic variation in grain yield correlated with grain number in both the glasshouse and field experiments, and grain yield correlated with harvest index in the field experiments. The increase in grain number amongst genotypes was associated with an increase in FE. The stem internode traits which were competing for most with spike growth was stem internode 2 leaf sheath, suggesting reducing internode 2 leaf sheath length could increase allocation of assimilates to the spike at anthesis. The results also identified novel genetic variation in spike hormones and found associations between higher levels of cytokinins and increases in grain number and yield at both GS49 and GS65 in the glasshouse and field experiments. The genetic variation in spike cytokinins also correlated across the glasshouse and field experiments. Twenty-six traits were analysed in a genome-wide association study and 213 putative marker-trait associations were identified, while 53 candidate genes were suggested including one candidate gene on chromosome 1B for spike cytokinins zeatin riboside and isopentenyladenosine at anthesis, which, in Arabidopsis, codes for an auxin efflux carrier protein PIN3 that is regulated by cytokinins. The next steps include validating the markers through developing KASP markers to be deployed in wider germplasm and using the genetic information from this study to improve traits in plant breeding programmes