Increased and ectopic expression ofTriticum polonicum VRT-A2underlies elongated glumes and grains in hexaploid wheat in a dosage-dependent manner

Abstract

Flower development is a major determinant of yield in crops. In wheat, natural variation for the size of spikelet and floral organs is particularly evident in Triticum polonicum, a tetraploid subspecies of wheat with long glumes, lemmas, and grains. Using map-based cloning, we identified VRT2, a MADS-box transcription factor belonging to the SVP family, as the gene underlying the P1 locus. The causal P1 mutation is a sequence substitution in intron-1 that results in both increased and ectopic expression of the T. polonicum VRT-A2 allele. Based on allelic variation studies, we propose that the intron-1 mutation in VRT-A2 is the unique T. polonicum species defining polymorphism, which was later introduced into hexaploid wheat via natural hybridizations. Near-isogenic lines differing for the T. polonicum long-glume (P1) locus revealed a gradient effect of P1 across florets. Transgenic lines of hexaploid wheat carrying the T. polonicum VRT-A2 allele show that expression levels of VRT-A2 are highly correlated with spike, glume, grain, and floral organ length. These results highlight how changes in expression profiles, through variation in cis-regulation, can impact on agronomic traits in a dosage-dependent manner in polyploid crops.

High expression of VRT2 increases the number of rudimentary basal spikelets in wheat

“…We performed qRT-PCR on a cv Paragon NIL carrying the VRT-A2b allele and compared VRT-A2 expression against the Paragon wildtype NIL described above (Figure 4). Consistent with the results of Adamski et al (2021), we detected significantly higher expression of VRT-A2b compared to the wildtype allele across most of the tissue sections (see Supplemental Table S6 for individual comparisons), and a progressive decrease in VRT-A2b expression over time (P = 0.031). In contrast to the wildtype NILs, ANOVA did not identify a significant interaction effect between spatial section and Waddington stage in VRT-A2b NILs (P = 0.18).…”

“…Overall, expression decreased significantly from DR to GP/TS Waddington stages in the apical (P = 0.00015), basal (P = 0.0074), and peduncle (P = 0.012) sections consistent with the previously reported strong downregulation of VRT-A2 in the early wheat spike development (Li et al, 2021;Adamski et al, 2021;Liu et al, 2021). This is also consistent with the observed downregulation of VRT-A2 orthologs upon floral transition in barley (Trevaskis et al, 2007) and rice (Harrop et al, 2018).…”

“…We focused the expression analysis on the A-genome homoeolog, VRT-A2, as its role in spike, glume and grain development of wheat was recently characterised (Adamski et al, 2021;Liu et al, 2021). S5 for expression data and Supplemental Table S6 for statistical analysis of gene expression differences).…”

“…polonicum VRT-A2b allele on the expression gradient of VRT-A2 and spike morphology. Adamski et al (2021) showed that VRT-A2 in T.…”

“…To validate the phenotypic effect of VRT-A2, we analysed transgenic wheat lines transformed with the complete genomic T. polonicum VRT-A2b sequence (including the native promoter and the intron 1 re-arrangement). Transgenic T1 lines were classified based on the transgene copy number which was previously shown by Adamski et al (2021) to be highly correlated with VRT-A2 expression levels in multiple tissues. We phenotyped lines with zero (n = 2 independent events; 5 plants each), low (1-5 transgene copies; n = 4 independent events; 5 plants each) and high (9-35 transgene copies, n = 2 independent events; 5 plants each) transgene copy number.…”