Wheat NILs contrasting in grain size show different expansin expression, carbohydrate and nitrogen metabolism that are correlated with grain yield

Abstract

Two wheat near-isogenic lines (NILs; -GS (small grain size) and + GS (large grain size)) were obtained by backcrossing. We found that the grain volume and weight of + GS were significantly higher than those of -GS. There was a corresponding increase in grain yield associated with the + GS line. For analysis of causes, we investigated and compared the anatomic and physiological characteristics of two NILs during the grain filling stage. The bigger grain size in + GS likely resulted from the longer endosperm cells, which is related to the higher IAA content and expansin activity accompanied by the up-regulation of some expansin-related genes. For the higher grain weight of + GS, starch accumulation was higher in + GS than in -GS, which was consistent with relatively higher glucose (Glu), fructose (Fru), and sucrose (Suc) contents. Elevated sugar level may be correlated with sucrose synthase (SuSy) activity and up-regulation of GBSS I, SSS III, and SBE I. The amount of soluble protein in + GS grains also exceeded that in -GS grains, possibly because of increased NR, NiR, and GS activities and up-regulation of nitrogen assimilation and nitrate transporter genes in + GS grains. Furthermore, the photosynthetic rate was higher in + GS and -GS at the early filling stage before 21 DAA. The same trends were observed in sugar content (Suc, Fru, and Glu) and SPS activity. However, the increase in the rate of grain length and width in + GS plants at the late filling stage was significantly higher than that at the early filling periods. These suggest that high accumulation of dry matter (such as starch and protein) at early filling stage was the important factor in achieving the final increased size of wheat grains, besides cell expansion.