The elimination of Starch Synthase IIa (SSIIa) in durum wheat seeds increases amylose
and protein but decreases starch and seed size. To mitigate the negative impact of
increased amylose upon seed size we identified partially functional SSIIa alleles
and combined them with SSIIa null alleles. The first step was to mutagenize durum
with ethyl methanesulfonate (EMS) and select SSIIa-A1 and SSIIa-B1 missense alleles
and combine them by crossing with SSIIa null alleles. Segregating lines possessing
a SSIIa null mutation in one genome and a missense mutation in the other genome were
compared to single SSIIa null mutant sister lines over two field seasons. Lines that
carried one SSIIa null and one missense allele showed elevated amylose compared to
single null sister lines and controls. In most SSIIa haplotypes tested, increased
amylose content was negatively correlated with seed size and starch content. However,
two SSIIa missense alleles were identified that increased amylose to 34 % when combined
with a SSIIa null allele. Thus, moderate amylose increases are possible if a small
amount of SSIIa activity remains in developing wheat seeds without negatively impacting
seed size or starch.
