Morphogenetic mechanism regulating phyllochron in rice

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Morphogenetic processes in sequentially growing leaves and tiller buds are highly regular in grasses, resulting in the ordered development of the shoot system in relation to the appearance of successive leaves. Thus, the interval between appearance of successive leaves, or phyllochron, has been used extensively as a time unit to quantify the development and growth of grasses. Genetic variation in phyllochron has long been recognized among grass cultivars. However, specific genetic factors influencing the phyllochron have not been identified. The objective of this study was to identify quantitative trait loci (QTLs) controlling phyllochron in rice. We used F_2 population from a cross between two elite cultivars, Lemont (japonica) and IR36 (indica). Phyllochrons for seedlings of Lemont and IR36 averaged 4.0 and 3.1 days per leaf, respectively. The range of the F_2 phenotype was within that of the parental values. An interval mapping detected five QTLs located on chromosomes 1, 2, 9, and 11. Individually, these QTLs accounted for 8 to 16% of the phenotypic variation. At these QTLs, the presence of a Lemont allele increased phyllochron, except for the QTL on chromosome 9 where the Lemont allele decreased phyllochron. These results demonstrate that phyllochron is controlled by multiple QTLs rather than by a major gene system. Based on the synteny between rice and maize genomes, the ortholog of the maize terminal ear 1 gene was considered to be a candidate gene for the QTL on chromosomes 1.