Induction and identification of a small-granule, high-amylose mutant in cassava (Manihot esculenta Crantz)

Only two mutations have been described in the literature, so far, regarding starch and root quality traits in cassava. This article reports on an induced mutation in this crop, first identified in 2006. Botanical seed from five different cassava families were irradiated with gamma rays. Seed was germinated, transplanted to the field (M-1 plants), and self-pollinated to produce the M-2 generation. Abnormal types regarding starch granule morphology were identified during the single plant evaluation of M2 genotypes. To confirm these characteristics, selected genotypes were cloned and a second evaluation, based on cloned plants obtained from vegetative multiplication, was completed in September 2007. Two M2 genotypes presented small starch granules, but only one could be fully characterized, presenting a granule size of 5.80 +/- 0.33 mu m compared with three commercial clones with granule sizes ranging from 13.97 +/- 0.12 to 18.73 +/- 0.10 mu m and higher-than-normal amylose content (up to 30.1% in cloned plants harvested in 2007, as compared with the typical values for "normal" cassava starch of around 19.8%). The gels produced by the starch of these plants did not show any viscosity when analyzed with the rapid viscoanalyzers (5% suspension), and the gels had low clarity. Low viscosity could be observed at higher concentrations (8 or 10% suspensions). Preliminary results suggest that the mutation may be due to a lesion in a gene encoding one of the isoforms of isoamylase (probably isa1 or isa2).