Wheat puroindolines interact to control grain hardness and effect ruminal digestability of starch in cattle
Endosperm texture directly affects end-use and milling qualities of hexaploid wheat. Genetically, the Hardness (Ha) locus controls the majority of endosperm texture variation and contains the puroindoline a (pina) and b (pinb) genes. Soft kernel texture results from both puroindolines being in the wild-type form, while hard wheats have a mutation in either puroindoline. Molecularly, grain texture is regulated by the starch surface protein complex friabilin, consisting primarily of the two proteins, puroindoline A (PINA) and B (PINB). Recent studies suggest that grain hardness is correlated with the presence of both functional PINA and PINB, not total puroindoline content. In addition, endosperm texture amongst cereal grains seems to play a major role in the speed and location of starch degradation in ruminants. The site of starch digestion along the gastrointestinal tract alters the nature of digestive end products (volatile fatty acids in the rumen and glucose in the small intestine) and the efficiency of metabolic utilization by the ruminant. The objectives here were to investigate which puroindoline limits grain softness in soft wheats, determine the limit of grain softness reduction, and to investigate the effect of varying puroindoline content in wheat on the rate of starch digestion in the rumen of beef cattle. Lines containing only additional pina were on average 7.4 units harder in grain texture and had starch that digested up to 10.6% faster than lines containing only additional pinb. Transgenic lines containing additional pinb showed a 2.6 to 4.8 fold increase in both PINA and PINB as bound friabilin, while lines containing additional pina showed only an increase in PINA. Increased expression of puroindolines was correlated with a decreased rate of ruminal digestion and was independent of particle size. The results demonstrated that PINB limits the binding of PINA to starch and is the limiting factor in the reduction of grain softness in soft wheats, that the limit of grain softness has not been reached, and that puroindolines slow the digestion of wheat starch in the rumen. Overall, puroindolines are important proteins in wheat end-use quality and can be manipulated and studied by use of transformation and over-expression.