Dissecting the Genetic Basis of Extremely Large Grain Shape in Rice Cultivar ‘JZ1560’

Rice grain shape, grain length (GL), width (GW), thickness (GT) and length-to-width ratio (LWR), are usually controlled by multiple quantitative trait locus (QTL). To elucidate the genetic basis of extremely large grain shape, QTL analysis was performed using an F2 population derived from a cross between a japonica cultivar ‘JZ1560’ (extremely large grain) and a contrasting indica cultivar ‘FAZ1’ (small grain). A total number of 24 QTLs were detected on seven different chromosomes. QTLs for GL, GW, GT and LWR explained 11.6%, 95.62%, 91.5% and 89.9% of total phenotypic variation, respectively. Many QTLs pleiotropically controlled different grain traits, contributing complex traits correlation. GW2 and qSW5/GW5, which have been cloned previously to control GW, showed similar chromosomal locations with qGW2-1/qGT2-1/qLWR2-2 and qGW5-2/qLWR5-1 and should be the right candidate genes. Plants pyramiding GW2 and qSW5/GW5 showed a significant increase in GW compared with those carrying one of the two major QTLs. Furthermore, no significant QTL interaction was observed between GW2 and qSW5/GW5. These results suggested that GW2 and qSW5/GW5 might work in independent pathways to regulate grain traits. ‘JZ1560’ alleles underlying all QTLs contributed an increase in GW and GT and the accumulation of additive effects generates the extremely large grain shape in ‘JZ1560’.