The chromosome regions for increasing early growth in rice : role of sucrose biosynthesis and NH4 (+) 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 NH₄⁺, the major nitrogen source for rice. These results suggest that early growth of rice is controlled by multiple traits and sucrose biosynthesis and NH₄⁺ uptake play key roles