Variation in Faba Bean Amylose Content

Abstract

In peas, a wide range of amylose contents has been avail-able for a very long time. One of Mendel’s original genes, forwrinkled seeds, was shown to encode a lesion in the starchbranching enzyme I [1]. Wrinkled pea starch contains 60–90% amylose whereas smooth pea starch has 30–45%[2–5]. Much more recently, a waxy (0% amylose) genotypewas developed by mutation breeding [6].In faba beans (Vicia faba L.), however, no mutations inthe starch synthetic enzymes, leading to either a wrinkled ora waxy phenotype, have been reported. This may be becauseof the outbreeding nature of the faba bean, which worksagainst the fixation of homozygous recessive genes. In addi-tion, the generally lower economic profile of faba beans incomparison to peas may have meant that less effort has beenexpended on searching for this variation. Quantitative varia-tion in amylose content has been estimated at 20–22% intwo cultivars [7] through 33% in one cultivar [4] to 38–39%in three cultivars [8]. An understanding of Vavilov’s 1920law of homologous series [9], along with the principles be-hind genomics, suggests that we should be able to isolateboth wrinkled and waxy mutants in faba beans.Altered starch composition would be useful in faba beans,which are often higher yielding than peas, particularly onclay-rich soils. Firstly, a waxy genotype would be desirablefor feeding monogastric animals, if it behaved similarly towaxy maize which has higher feeding values than the nor-mal-starch equivalent [10]. Secondly, high amylose starch isadvantageous for application in plastics as well as other uses,so in combination with protein extraction for processing,would increase the value of the crop. Thirdly, if the otherproperties of the high amylose genotype were equivalent tothose of the wrinkled pea, fresh eating quality would begreatly increased due to the higher free sugar content.