Loss of function of <i>SSIIIa</i> and <i>SSIIIb</i> coordinately confers high RS content in cooked rice

Abstract

The sedentary lifestyle and refined food consumption significantly lead to obesity, type 2 diabetes, and related complications, which have become one of the major threats to global health. This incidence could be potentially reduced by daily foods rich in resistant starch (RS). However, it remains a challenge to breed high-RS rice varieties. Here, we reported a high-RS mutant <i>rs4</i> with an RS content of ~10.8% in cooked rice. The genetic study revealed that the loss-of-function <i>SSIIIb</i> and <i>SSIIIa</i> together with a strong <i>Wx</i> allele in the background collaboratively contributed to the high-RS phenotype of the <i>rs4</i> mutant. The increased RS contents in <i>ssIIIa</i> and <i>ssIIIa ssIIIb</i> mutants were associated with the increased amylose and lipid contents. SSIIIb and SSIIIa proteins were functionally redundant, whereas <i>SSIIIb</i> mainly functioned in leaves and <i>SSIIIa</i> largely in endosperm owing to their divergent tissue-specific expression patterns. Furthermore, we found that <i>SSIII</i> experienced duplication in different cereals, of which one <i>SSIII</i> paralog was mainly expressed in leaves and another in the endosperm. <i>SSII</i> but not <i>SSIV</i> showed a similar evolutionary pattern to <i>SSIII</i>. The copies of endosperm-expressed <i>SSIII</i> and <i>SSII</i> were associated with high total starch contents and low RS levels in the seeds of tested cereals, compared with low starch contents and high RS levels in tested dicots. These results provided critical genetic resources for breeding high-RS rice cultivars, and the evolutionary features of these genes may facilitate to generate high-RS varieties in different cereals.