The chromosome regions for increasing early growth in rice: role of sucrose biosynthesis and NH 4 + uptake

To elucidate the mechanism of plant growth we investigated chromosome region affecting traits (CRATs) using the chromosome segment substitution lines derived from a cross between japonica ‘Koshihikari’ and indica ‘Kasalath’ using rice ( Oryza sativa L.). Four CRATs associated with increased plant growth rate were identified which were derived from ‘Kasalath’ alleles on chromosomes 1, 6, 8 and 12. This was achieved by evaluating the net dry weight gain from seed germination to 30 day-old seedlings. Physiological analysis of substitution lines containing a ‘Kasalath’ allele chromosome segment in the ‘Koshihikari’ background revealed that the CRAT on chromosome 1 ( EGR1 ) increased the relative growth rate by increasing the net assimilation rate (NAR), and increased expression of the OsSPS1 gene that encodes the rate-limiting enzyme in sucrose synthesis. In contrast, CRATs on chromosomes 6, 8 and 12 ( EGR6 , EGR8 and EGR12 ) were associated with an increased uptake of NH 4 + , the major nitrogen source for rice. These results suggest that early growth of rice is controlled by multiple traits and sucrose biosynthesis and NH 4 + uptake play key roles.