Increased puroindoline levels slow ruminal digestion of wheat (Triticum aestivum L.) starch by cattle

Starch is the primary nutrient in ruminant diets used to promote high levels of performance. The site of starch digestion alters the nature of digestive end products (VFA in the rumen vs. glucose in the small intestine) and the efficiency of use. Cereal grain endosperm texture plays a major role in the rate and extent of starch degradation in ruminants. Wheat grain texture is regulated by the starch surface protein complex friabilin that consists primarily of puroindoline (PIN) A and B. Soft kernel texture in wheat is a result of both PIN genes being in the wild type active form and bound to starch. The objective of this study was to investigate the effect of varying PIN content in wheat on the rate of starch digestion in the rumen of beef cattle. In Exp. 1, 6 transgenic soft pin a/b isolines created in a hard wheat background, and 2 hard wheat controls were milled to yield a wide range of mean particle sizes across all lines. Milled samples were incubated in situ for 3 h. Increased expression of both PINA and PINB decreased DM digestibility (DMD) by 29.2% (P < 0.05) and decreased starch digestibility by 30.8% (P < 0.05). Experiment 2 separated the effects of particle size and total PIN content on digestion by milling the hardest and softest lines such that the mean particle size was nearly identical. Increased PIN decreased DMD by 21.7% (P < 0.05) and starch digestibility by 19.9% (P < 0.05) across particle sizes smaller than whole kernel. Experiment 3 addressed the time course of PIN effects in the rumen by observing ground samples of the hardest and softest lines over a 12-h in situ period. Increased PIN decreased DMD by 10.4% (P < 0.05) and starch digestibility by 11.0% (P < 0.05) across all time points. Dry matter and starch digestibility results demonstrated that increased expression of PIN was associated with a decreased rate of ruminal digestion independent of particle size. Puroindolines seem to aid in the protection of starch molecules from microbial digestion in the rumen, potentially increasing the amount of starch entering the small intestine.