Puroindoline b limits binding of Puroindoline a to starch and grain softness

Endosperm texture affects end-use and milling qualities of wheat (Triticum aestivum L.). The Hardness (Ha) locus controls the majority of grain hardness variation and contains the Puroindoline a (Pina) and b (Pinb) genes. Soft kernel texture results when both Pina and Pinb are present in wild-type form, whereas hard-textured wheats contain a mutation in either Pin. Past studies suggested that grain hardness is correlated with soft type PINA and PINB, not total puroindoline. The primary objective of this study was to determine which PIN limits grain softness in soft wheats. To accomplish this, six transgenic Pin lines created in the hard wheat cultivar Hi-Line were crossed with the soft wheat cultivar Heron. Resulting F-2:3 progeny homozygous for the Pin transgene(s) and the soft-type Ha locus were evaluated in two environments through 2 yr by measuring grain hardness and puroindoline levels. Genotypes containing added PINB were 7.4 units softer in grain texture, as measured with the Single Kernel Characterization System, than genotypes with added PINA (P < 0.001). In genotypes with added PINB, a 2.6- to 4.8-fold increase was detected in both PINA and PINB bound to starch, whereas only an increase in PINA was detected in added PINA genotypes. Results indicate that increased PINB increases total PIN starch levels and decreases grain hardness more than increased PINA. This demonstrates that PINB limits the binding of PINA to starch, and grain softness in soft wheats.