Evolution of floral meristem identity genes. Analysis of Lolium temulentumgenes related to APETALA1 and LEAFY of Arabidopsis

Flowering (inflorescence formation) of the grass Lolium temulentum is stric tly regulated, occurring rapidly on exposure to a single long day (LD). Dur ing floral induction, L. temulentum differs significantly from dicot specie s such as Arabidopsis in the expression, at the shoot apex, of two APETALA1 (AP1)-like genes, LtMADS1 and LtMADS2, and of L. temulentum LEAFY (LtLFY). As shown by in situ hybridization, LtMADS1 and LtMADS2 are expressed in th e vegetative shoot apical meristem, but expression increases strongly withi n 30 h of LD floral induction. Later in floral development, LtMADS1 and LtM ADS2 are expressed within spikelet and floret meristems and in the glume an d lemma primordia. It is interesting that LtLFY is detected quite late (abo ut 12 d after LD induction) within the spikelet meristems, glumes, and lemm a primordia. These patterns contrast with Arabidopsis, where LFY and AP1 ar e consecutively activated early during flower formation. LtMADS2, when expr essed in transgenic Arabidopsis plants under the control of the AP1 promote r, could partially complement the organ number defect of the severe ap1-15 mutant allele, confirming a close relationship between LtMADS2 and AP1.