Increased pericarp cell length underlies a major quantitative trait locus for grain weight in hexaploid wheat

Abstract

Crop yields must increase to address food insecurity. Grain weight, determined by grain length and width, is an important yield component, but our understanding of the underlying genes and mechanisms is limited. We used genetic mapping and near isogenic lines (NILs) to identify, validate and fine-map a major quantitative trait locus (QTL) on wheat chromosome 5A associated with grain weight. Detailed phenotypic characterisation of developing and mature grains from the NILs was performed. We identified a stable and robust QTL associated with a 6.9% increase in grain weight. The positive interval leads to 4.0% longer grains, with differences first visible 12 d after fertilization. This grain length effect was fine-mapped to a 4.3 cM interval. The locus also has a pleiotropic effect on grain width (1.5%) during late grain development that determines the relative magnitude of the grain weight increase. Positive NILs have increased maternal pericarp cell length, an effect which is independent of absolute grain length. These results provide direct genetic evidence that pericarp cell length affects final grain size and weight in polyploid wheat. We propose that combining genes that control distinct biological mechanisms, such as cell expansion and proliferation, will enhance crop yields.

A reductionist approach to dissecting grain weight and yield in wheat

“…Studies in wheat showing a correlation between carpel size and dry matter accumulation support this hypothesis, suggesting that increasing the size of the maternal tissues allows an enhanced capacity for grain filling i.e. increased source strength (Calderini and Reynolds ; Xie et al ; Brinton et al ). The maternal parent will also contribute to final seed size through other mechanisms including responses to the environment during grain development and the imprinting of genes after fertilization, both of which have been shown to influence final grain size (discussed in Zhang et al ).…”

“…Understanding the precise mechanism by which the grain weight is being controlled can help to make these experiments possible. In the case of the 5A grain weight QTL, we showed that the effect on pericarp cell size was independent of individual grain size, suggesting that this more robust phenotype could be used to assess the F 1 grains (Brinton et al ).…”

“…Quantitative trait loci (QTL) for grain weight have been identified on almost every wheat chromosome (Huang et al ; Breseghello and Sorrells ; Gegas et al ; Kumar et al ). However, only a subset of these QTLs have been validated and none have yet been cloned (Röder et al ; Simmonds et al ; Huang et al ; Farré et al ; Brinton et al ). One of the major challenges to cloning grain size QTL in wheat is the subtle nature of the phenotypic effects, whereby single loci often increase average grain weight by <10%, and to an even lesser degree for the individual sub‐components (Figure A).…”

“…We recently showed that a grain weight QTL, on wheat chromosome 5A, is associated with increased grain length due to increased cell size in the pericarp. Whether this is a direct or indirect effect remains to be determined (Brinton et al ).…”

“…For example, the grain weight QTL on wheat chromosome 5A increases grain length due to longer cells in the pericarp tissue. However, the QTL does not influence carpel size and the first differences in grain size are only observed several days post‐fertilization (Brinton et al ). In these cases, assigning the effect as maternal, or zygotic, requires experiments involving reciprocal crosses and phenotypic assessment in F 1 hybrid seed.…”