MISSING FLOWERS gene controls axillary meristems initiation in sunflower.

The initiation and growth of axillary meristems are fundamental components of plant architecture. Here, we describe the mutant missing flowers (mf) of Helianthus annuus characterized by the lack of axillary shoots. Decapitation experiments and histological analysis indicate that this phenotype is the result of a defect in axillary meristem initiation. In addition to shoot branching, mutation affects floral differentiation. The indeterminate inflorescence of sunflower (capitulum) is formed of a large flat meristem which produces floret primordia in multiple spirals. In wildtype plants a bisecting crease divides each primordium in two distinct bumps that adopt different fate. The peripheral (abaxial) part of the primordium becomes a small leaf-like bract and the adaxial part becomes a flower. In the mf mutant, the formation of flowers at the axil of bracts is precluded. Histological analyses show that in floret primordia of the mutant a clear subdivision in dyads is not established. The primordia progressively bend inside and only large involucral floral bracts are developed. The results suggest that the MISSING FLOWERS gene is essential to provide or perceive an appropriate signal to the initiation of axillary meristems during both vegetative and reproductive phases.