Genetic modification of spikelet arrangement in wheat increases grain number without significantly affecting grain weight

Abstract

Crop yield is determined by the acquisition and allocation of photoassimilates in sink organs. Therefore, genetic modification of sink size is essential for understanding the complex signaling network regulating sink strength and source activities. Sink size in wheat depends on the number of spikelets per spike, floret/grain number per spikelet as well as the grain weight or dry matter accumulation. Hence, increasing spikelet number and improving sink size are targets for wheat breeding. The main objective of the present work was to genetically modify the wheat spike architecture, i.e., the sink size by introgressing the 'Miracle wheat' or the bh t-A1 allele into an elite durum wheat cv. Floradur. After four generations of backcrossing to the recurrent parent, Floradur (FL), we have successfully developed Near Isogenic Lines (NILs) with a modified spikelet arrangement thereby increasing spikelet and grain number per spike. Genotyping of bh t-A1 NILs using the Genotyping-By-Sequencing approach revealed that the size of the introgressed donor segments carrying bh t-A1 ranged from 2.3 to 38 cM. The size of the shortest donor segment introgressed into bh t-A1 NILs was estimated to be 9.8 mega base pairs (Mbp). Phenotypic analysis showed that FL-bh t-A1-NILs (BC3F2 and BC3F3) carry up to seven additional spikelets per spike, leading to up to 29% increase in spike dry weight at harvest (SDW h). The increased SDW h was accompanied by up to 23% more grains per spike. More interestingly, thousand kernel weight (TKW) did not show significant differences between FL-bh t-A1-NILs and Floradur, suggesting that besides increasing spikelet number, bh t-A1 could also be targeted for increasing grain yield in wheat. Our study suggests that the genetic modification of spikelet number in wheat can be an entry point for improving grain yield, most interestingly and also unexpectedly without the trade-off effects on TKW. Hence, FL-bh t-A1-NILs are not only essential for increasing grain number, but also for understanding the molecular and genetic mechanism of the source-sink interaction for a clearer picture of the complex signaling network regulating sink strength and source activities.

Inferring vascular architecture of the wheat spikelet based on resource allocation in the branched headt (bht-A1) near isogenic lines

“…We used cv. Floradur Near Isogeneic Lines (FL-bh t -A1-NILs) carrying alleles of the branched head t (bh t -A1) locus derived from tetraploid wheat mutant accession, TRI 19165 or 'Miracle Wheat' (Poursarebani et al 2015;Wolde et al 2019a). Floradur is a German commercial elite spring durum wheat (Triticum durum L.) variety with a standard spike.…”

“…Five different plant families of FL-bh t -A1-NILs (at BC2F3, BC3F2 and BC3F3 generations) were used in this study. Generally, besides carrying the primary standard spikelets (PSt), FL-bh t -A1-NILs are characterised by carrying secondary spikelets (SSt) sharing the same rachis node with that of the PSt (Wolde et al 2019a). First, the five homozygous BC3F2 family plants (n = 25) were evaluated for spike morphology under the field conditions at the Leibniz Institute of Plant Genetics and Crop Plant Research, Gatersleben, Germany (51 49 0 23 00 N, 11 17 0 13 00 E, altitude 112 m above sea level) during the 2015 growing season.…”

“…Here, it is important to note that spike-branching in 'miracle wheat' or TRI 19165, the donor of the bh t -A1 allele on chromosome 2A, is a true type or genuine spike-branching accession, whereupon 'mini spike-like branches' appear from the distichously arranged rachis nodes by replacing the spikelets (Poursarebani et al 2015;Wolde et al 2019b). However, FL-bh-A1 t -NILs predominantly show supernumerary spikelets without genuine spike-branching (Wolde et al 2019a). This suggests the presence of other spike-branching modifiers suppressing genuine spike-branching in the recurrent parent, Floradur, background.…”

“…The source of the caloriesthe grainis produced on a sessile specialised branch, called the spikelet, which contains a collection of grain-producing florets that are arranged on the main axis of the spikelet called the rachilla. Grain number in wheat is therefore strongly influenced by the number and arrangement of spikelets (Boden et al 2015;Dobrovolskaya et al 2015;Poursarebani et al 2015;Dixon et al 2018;Wolde et al 2019a), as well as the fertility of the spikelet (Guo et al 2017(Guo et al , 2018Prieto et al 2018;Sakuma et al 2019). Hence, the simultaneous increase in spikelet number and spikelet fertility is important for increasing grain yield in wheat.…”

“…Therefore, developing the appropriate genetic material and the mechanistic working model are important to further study the phloem anatomy and structure in wheat spikelets. Previously, we developed FL-bh t -A1-NILs, which are typically characterised by the formation of supernumerary secondary spikelets (SSt) sharing the same rachis node with the primary spikelets (PSt) (Wolde et al 2019a). The unique spikelet arrangement in FL-bh t -A1-NILs, i.e.…”