The Ha-ROXL gene is required for initiation of axillary and floral meristems in sunflower

Axillary meristems (AMs) contribute to the growth of a plant, determining adult architecture and reproductive success in response to environmental stimuli. The missing flowers (mf) mutant of sunflower (Helianthus annuus) is defective in AM development. mf lacks shoot branching and ray flowers, occasionally producing few disk flowers. Here we demonstrated that a point mutation in the REGULATOR OF AXILLARY MERISTEM FORMATION-LIKE (Ha-ROXL) gene of mf generates a premature stop codon and therefore a nonfunctional bHLH transcription factor, no longer localized in the nucleus, where it should exert its function. Virus-induced gene silencing of Ha-ROXL also causes defects in disk and ray flower development. Ha-ROXL mRNA accumulates at the adaxial boundaries of leaves and AMs. During inflorescence development, Ha-ROXL is expressed in small arcs of cells before a clear separation between abaxial bracts and disk flower primordia. No Ha-ROXL mRNA accumulates in mf inflorescences. Several genes known to play roles in plant architecture, auxin transport, and flower development are differentially expressed in mf and Ha-ROXL-silenced plants. These results highlight the predominant role of Ha-ROXL in regulating AMs in sunflower. In dicot, mf is the first mutant for which the ROXL gene is also required for initiation of flower meristems.