Down-regulation of RFL, the FLO/LFY homolog of rice, accompanied with panicle branch initiation

Abstract

FLORICAULA (FLO) of Antirrhinum and LEAFY (FLY) of Arabidopsis regulate the formation of floral meristems. To examine whether same mechanisms control floral development in distantly related species such as grasses, we isolated RFL, FLO-LFY homolog of rice, and examined its expression and function. Northern analysis showed that RFL is expressed predominantly in very young panicle but not in mature florets, mature leaves, or roots. In situ hybridization revealed that RFL RNA was expressed in epidermal cells in young leaves at vegetative growth stage. After the transition to reproductive stage, RFL RNA was detected in all layers of very young panicle including the apical meristem, but absent in the incipient primary branches. As development of branches proceeds, RFL RNA accumulation localized in the developing branches except for the apical meristems of the branches and secondary branch primordia. Expression pattern of RFL raised a possibility that, unlike FLO and LFY, RFL might be involved in panicle branching. Transgenic Arabidopsis plants constitutively expressing RFL from the cauliflower mosaic virus 35S promoter were produced to test whether 35S-RFL would cause similar phenotype as observed in 35S-LFY plants. In 35S-RFL plants, transformation of inflorescence meristem to floral meristem was rarely observed. Instead, development of cotyledons, rosette leaves, petals, and stamens was severely affected, demonstrating that RFL function is distinct from that of LFY. Our results suggest that mechanisms controlling floral development in rice might be diverged from that of Arabidopsis and Antirrhinum.

Figure 1

Comparison of amino acid sequences encoded by RFL, FLO, and LFY. Conserved amino acids are shown by asterisks.

Figure 2

Distribution of RFL RNA in rice plants. (A) Northern blot analysis. MF, mature florets; YP, young panicles; LS, leaf sheaths; LB, leaf blades; R, roots. (B–G) RFL expression analyzed by in situ hybridization. (B) A vegetative shoot apex. (C and D) A young panicle at primary branch primordia differentiation stage. (E) A young panicle at secondary branch primordia differentiation stage. (F) A developing panicle. Four primary branches at various developmental stages in a panicle are shown. The oldest primary branch (arrow) is in the floret differentiation stage. (G) Developing florets. All floral organis have developed by this stage. pb, primary branch; sb, secondary branch; pa, panicle apex; f, floret. (Bar = 50 μm.)

Figure 3

Transgenic Arabidopsis plants transformed with 35S-RFL. (A) A 35S-RFL plant with an abnormal terminal flower. (B–D) Flowers with short petals and short stamens in 35S-RFL plants (B and D) and wild-type flowers (C and E). A sepal and a petal were removed from a flower in D and E. (F–H) Seedlings of 35S-RFL (F and H) and wild-type (G and I) plants. (J–L) Rosette leaves in 35S-RFL plants (J and K) and a wild-type plant at 19 days (L). (Bar = 5 mm.)