Polyamines mediate the effect of post-anthesis soil drying on starch granule size distribution in wheat kernels

Polyamines (PAs) are important endogenous plant growth regulators responding to environmental stress and mediating many physiological processes including grain filling in cereals. This study investigated whether PAs mediate the effect of post-anthesis soil drying on starch granule size distribution, starch content, and weight of superior and inferior kernels of wheat (Triticum aestivum L.). Two wheat cultivars were grown in pots. Three treatments, well-watered (WW), moderate soil drying (MD) and severe soil drying (SD), were imposed from 9 days post-anthesis until maturity. PA levels in kernels and small, medium and large granules were measured. The results showed that superior kernels had much higher free spermidine (Spd) and free spermine (Spm) concentrations, larger volumes of medium starch granules, and smaller-sized large granules than did inferior kernels under all the treatments. Compared to WW, MD significantly increased the concentrations of free Spd and free Spm, activities of soluble starch synthase and granule-bound starch synthase, volume of medium granules, and starch content and kernel weight of inferior kernels, and decreased the size of large granules. SD produced the opposite effect. Application of Spd or Spm to spikes produced effects similar to those of MD, and application of an inhibitor of Spd and Spm synthesis produced effects similar to those of SD. These results suggest that PAs mediate the effect of post-anthesis soil drying on starch biosynthesis in wheat kernels by regulating key enzymes in starch synthesis and that elevated PA levels under MD increase the volume of medium granules and kernel weight of inferior kernels.