The effect of friabilin on wheat grain hardness: a review

A wheat marketing system established the primary classification of hexaploid wheat based on the endosperm texture, i.e. hardness or softness of the grain. Hardness affects a range of characters including the milling (tempering, milling yield, flour particle size, shape and density of flour particles), baking and end-use properties. Wheat grain hardness is largely controlled by genetic factors but it can also be affected by the environmental and other factors. The endosperm texture is primarily associated with the Hardness (Ha) locus on the short arm of chromosome 5D. It is regulated by friabilin. This 15 kDa starch surface protein complex is present in larger amounts in soft wheats compared to hard ones and consists of three major polypeptides: puroindoline a (Pina), puroindoline b (Pinb) and grain softness protein 1 (Gsp-1). The soft grain texture in wheat is a result of both puroindoline genes being in the wild type active form and bound to starch. When one of the puroindolines is either absent or altered by mutation, then the result is a hard texture. Gene sequence variation and mutation of both puroindoline genes account for the majority of variation in the wheat grain texture. The latter may serve as the potential for improvement of milling and baking wheat quality. However, many wheat varieties have the intermediately (mixed) hard endosperm and there is a wide variation between soft and hard grain texture. Grain hardness is affected by a number of factors beyond genetics including N management, tillage system, pest infestations, environment (location of growth, temperature and rainfall during the growing season) and their interactions, and factors such as moisture, gliadin composition, and content of lipids, starch and pentosans.