NADPH: protochlorophyllide oxidoreductase (POR) is an essential enzyme that catalyzes the photoreduction of protochlorophyllide to chlorophyllide, which is ultimately converted to chlorophyll in developing leaves. Rice has two POR isoforms, OsPORA and OsPORB. OsPORA is expressed in the dark during early leaf development; OsPORB is expressed throughout leaf development regardless of light conditions. The faded green leaf (fgl) is a loss-of-function osporB mutant that displays necrotic lesions and variegation in the leaves due to destabilized grana thylakoids, and has increased numbers of plastoglobules in the chloroplasts. To investigate whether the function of OsPORA can complement that of OsPORB, we constitutively overexpressed OsPORA in fgl mutant. In the 35S:OsPORA/fgl (termed OPAO) transgenic plants, the necrotic lesions of the mutant disappeared and the levels of photosynthetic pigments and proteins, as well as plastid structure, were recovered in developing leaves under natural long days in the paddy field and under short days in an artificially controlled growth room. Under constant light conditions, however, total chlorophyll and carotenoid levels in the developing leaves of OPAO plants were lower than those of wild type. Moreover, the OPAO plants exhibited mild defects in mature leaves beginning at the early reproductive stage in the paddy field. The physiological function of OsPORB in response to constant light or during reproductive growth cannot be completely replaced by constitutive activity of OsPORA, although the biochemical functions of OsPORA and OsPORB are redundant. Therefore, we suggest that the two OsPORs have differentiated over the course of evolution, playing distinct roles in the adaptation of rice to the environment.

Background: Gibberellic acid (GA; or gibberellin) affects the development of floral organs, especially anthers and pollen, and perturbation of development of male floral organs can cause sterility. Many studies of GA signaling have concentrated on anther development, but the effect of GA on grain production remains to be examined.Results: Using a cross of ‘Milyang23 (M23)’, which has a functional allele ofEarly flowering1(EL1), and ‘H143’, which has a nonfunctionalel1allele, we generated heterogeneous inbred family-near isogenic lines (HNILs) that are homozygous forEL1[HNIL(M23)] orel1[HNIL(H143)]. Here, we found that HNIL(H143) exhibited anther deformities and low pollen viability. The expression ofGAMYB, a major activator of GA signaling, and its downstream genesCYP703A3andKAR, mainly involved in pollen formation, increased abnormally during spikelet development; this activation of GA signaling may cause the sterility. To confirm the negative effect of theel1mutation on spikelet fertility, we examined a line carrying a T-DNA insertionel1mutant [hereafter ZH11(el1)] and its parental cultivar ‘Zhonghua11 (ZH11)’. ZH11(el1) showed nearly identical defects in anther development and pollen viability as HNIL(H143), leading to decreased seed setting rate. However, the elitejaponicacultivar Koshihikari, which has a nonfunctionalel1allele for early flowering in long days, produces fertile spikelets and normal grain yields, like other elitejaponicacultivars. This indicates that as-yet-unknown regulator(s) that can overcome the male sterile phenotype of theel1mutation must have been introduced into Koshihikari.Conclusions: Theel1mutation contributes to early flowering injaponicarice under long days but fails to limit GA signaling, thus negatively affecting spikelet fertility, which results in a loss of grain yield. Thus,EL1is essential for photoperiod sensitivity in flowering as well as spikelet fertility in grain production.