is a conserved primary regulator of flowering.

INTRODUCTION

A fundamental developmental switch for plants is transition from vegetative to floral growth, which integrates external and internal signals. INDETERMINATE1 (Id1) family proteins are zinc finger transcription factors that activate flowering in grasses regardless of photoperiod. Mutations in maize and rice () cause very late flowering. RID1 promotes expression of the flowering activator genes () and (), a rice homolog of CONSTANS (CO).

METHODS AND RESULTS

Mapping of two recessive late flowering mutants from a pedigreed sorghum EMS mutant library identified two distinct mutations in the () homolog, mutant alleles named and . The weaker allele caused a 35 day delay in reaching boot stage in the field, but its effect was limited to 6 days under greenhouse conditions. The strong allele delayed boot stage by more than 60 days in the field and under greenhouse conditions. When and were combined, the delayed flowering phenotype remained and resembled that of , confirming late flowering was due to loss of function. Evaluation of major flowering time regulatory gene expression in showed that is needed for expression of floral activators, like and , and repressors, like and .

DISCUSSION

These results demonstrate a conserved role for in promotion of flowering in sorghum, where it appears to be critical to allow expression of most major flowering regulatory genes.