Overexpression of the WAPO-A1 gene increases the number of spikelets per spike in bread wheat.

Two homoeologous QTLs for number of spikelets per spike (SPS) were mapped on chromosomes 7AL and 7BL using two wheat MAGIC populations. Sets of lines contrasting for the QTL on 7AL were developed which allowed for the validation and fine mapping of the 7AL QTL and for the identification of a previously described candidate gene, WHEAT ORTHOLOG OF APO1 (WAPO1). Using transgenic overexpression in both a low and a high SPS line, we provide a functional validation for the role of this gene in determining SPS also in hexaploid wheat. We show that the expression levels of this gene positively correlate with SPS in multiple MAGIC founder lines under field conditions as well as in transgenic lines grown in the greenhouse. This work highlights the potential use of WAPO1 in hexaploid wheat for further yield increases. The impact of WAPO1 and SPS on yield depends on other genetic and environmental factors, hence, will require a finely balanced expression level to avoid the development of detrimental pleiotropic phenotypes.

Wheat yield potential can be maximized by increasing red to far-red light conditions at critical developmental stages.

Sensing of neighbours via the Red to Far-Red light ratio (R:FR) may exert a cap to yield potential in wheat. The effects of an increased R:FR inside the canopy were studied in dense wheat mini canopies grown in controlled environments by lowering FR. To distinguish between effects exerted by light sensing and assimilate supply, the treatments were complemented with elevated CO2 , applied between different developmental timepoints to specifically impact tillering, spike growth, floret fertility and grain filling, in different combinations. The yield response to high R:FR was strongly dependent on the developmental stage in all three cultivars and pivoted between positive if applied after the start of stem elongation, and negative or null if applied before. Yield gains of up to 70% and 120% were observed, respectively, in two cultivars, associated with a higher number of tiller spikes and grains per spike in the main shoot. The response to the combination of high R:FR and elevated CO2 or CO2 alone were cultivar dependent. Taken together, our results suggest that R:FR exerts a significant control on yield potential in wheat and achieving a high R:FR from stem elongation to maturity is a promising lever towards a significant increase in grain yield.

The da1 mutation in wheat increases grain size under ambient and elevated CO2 but not grain yield due to trade-off between grain size and grain number.

Grain size is potentially yield determining in wheat, controlled by the ubiquitin pathway and negatively regulated by ubiquitin receptor DA1. We analyzed whether increased thousand grain weight in wheat da1 mutant is translated into higher grain yield and whether additional carbon provided by elevated (e)CO2 can be better used by the da1, displaying higher grain sink strength and size. Yield-related, biomass, grain quality traits, and grain dimensions were analyzed by two-factorial mixed-model analysis, regarding genotype and eCO2. da1 increased grain size but reduced spikes and grains per plant, grains per spike, and spikelets per spike, independent of eCO2 treatment, leaving total grain yield unchanged. eCO2 increased yield and grain number additively and independently of da1 but did not overcome the trade-off between grain size and number observed for da1. eCO2 but not da1 impaired grain quality, strongly decreasing concentrations of several macroelement and microelement. In conclusion, intrinsic stimulation of grain sink strength and grain size, achieved by da1, is not benefitting total yield unless trade-offs between grain size and numbers can be overcome. The results reveal interactions of yield components in da1-wheat under ambient and eCO2, thereby uncovering limitations enhancing wheat yield potential.

Cytokinin regulates the activity of the inflorescence meristem and components of seed yield in oilseed rape.

The number of flowers and seed-bearing structures formed by the inflorescence meristem and the formation of ovules in the female reproductive part of the flowers are important yield-related traits of crop plants. It has been shown that cytokinin is a pivotal factor regulating these traits. Here, we explore the impact of mutation of CYTOKININ OXIDASE/DEHYDROGENASE (CKX) genes encoding cytokinin-degrading enzymes on these yield-related traits in oilseed rape (Brassica napus L.). We describe the identification of four BnCKX3 and two BnCKX5 genes as regulators of reproductive development in the allotetraploid B. napus. RNA-seq analysis and in situ hybridization showed expression of these genes in reproductive organs. Loss-of-function mutants for each of these CKX gene copies were identified by targeting induced local lesions in genomes (TILLING) and combined by crossing. Sextuple ckx3 ckx5 mutants showed an increased cytokinin concentration and larger and more active inflorescence meristems. They also produced up to 72% more flowers with gynoecia containing 32% more ovules and up to 54% more pods on the main stem. The weight of seeds harvested from the main stem of plants grown in the greenhouse or in the field was increased by 20-32%. Our results show that cytokinin regulates inflorescence meristem and placenta activity in oilseed rape. The work demonstrates the potential to achieve yield enhancement in a dicot crop plant by modulating the cytokinin status through mutagenesis of specific CKX genes.