The wheat transcription factor bZIPC1 interacts with FT2 and affects spikelet and grain number per spike. We identified a natural allele with positive effects on these two economically important traits. Loss-of-function mutations and natural variation in the gene FLOWERING LOCUS T2 (FT2) in wheat have previously been shown to affect spikelet number per spike (SNS). However, while other FT-like wheat proteins interact with bZIP-containing transcription factors from the A-group, FT2 does not interact with any of them. In this study, we used a yeast-two-hybrid screen with FT2 as bait and identified a grass-specific bZIP-containing transcription factor from the C-group, designated here as bZIPC1. Within the C-group, we identified four clades including wheat proteins that show Y2H interactions with different sets of FT-like and CEN-like encoded proteins. bZIPC1 and FT2 expression partially overlap in the developing spike, including the inflorescence meristem. Combined loss-of-function mutations in bZIPC-A1 and bZIPC-B1 (bzipc1) in tetraploid wheat resulted in a drastic reduction in SNS with a limited effect on heading date. Analysis of natural variation in the bZIPC-B1 (TraesCS5B02G444100) region revealed three major haplotypes (H1-H3), with the H1 haplotype showing significantly higher SNS, grain number per spike and grain weight per spike than both the H2 and H3 haplotypes. The favorable effect of the H1 haplotype was also supported by its increased frequency from the ancestral cultivated tetraploids to the modern tetraploid and hexaploid wheat varieties. We developed markers for the two non-synonymous SNPs that differentiate the bZIPC-B1b allele in the H1 haplotype from the ancestral bZIPC-B1a allele present in all other haplotypes. These diagnostic markers are useful tools to accelerate the deployment of the favorable bZIPC-B1b allele in pasta and bread wheat breeding programs.

Key message

This study identified Rht25, a new plant height locus on wheat chromosome arm 6AS, and characterized its pleiotropic effects on important agronomic traits. Understanding genes regulating wheat plant height is important to optimize harvest index and maximize grain yield. In modern wheat varieties grown under high-input conditions, the gibberellin-insensitive semi-dwarfing alleles Rht-B1b and Rht-D1b have been used extensively to confer lodging tolerance and improve harvest index. However, negative pleiotropic effects of these alleles (e.g., poor seedling emergence and reduced biomass) can cause yield losses in hot and dry environments. As part of current efforts to diversify the dwarfing alleles used in wheat breeding, we identified a quantitative trait locus (QHt.ucw-6AS) affecting plant height in the proximal region of chromosome arm 6AS (< 0.4 cM from the centromere). Using a large segregating population (~ 2800 gametes) and extensive progeny tests (70-93 plants per recombinant family), we mapped QHt.ucw-6AS as a Mendelian locus to a 0.2 cM interval (144.0-148.3 Mb, IWGSC Ref Seq v1.0) and show that it is different from Rht18. QHt.ucw-6AS is officially designated as Rht25, with Rht25a representing the height-increasing allele and Rht25b the dwarfing allele. The average dwarfing effect of Rht25b was found to be approximately half of the effect observed for Rht-B1b and Rht-D1b, and the effect is greater in the presence of the height-increasing Rht-B1a and Rht-D1a alleles than in the presence of the dwarfing alleles. Rht25b is gibberellin-sensitive and shows significant pleiotropic effects on coleoptile length, heading date, spike length, spikelet number, spikelet density, and grain weight. Rht25 represents a new alternative dwarfing locus that should be evaluated for its potential to improve wheat yield in different environments.