Mutations of corolla symmetry affect carpel and stamen development in Helianthus annuus

The inflorescence of sunflower (Helianthus annuus L.) is heterogamous with zygomorphic ray flowers located in the outermost whorl of the head and actinomorphic disk flowers arrayed in arcs radiating from the center of the head. Two mutants with altered corolla symmetry have been described. The Chrysanthemoides (Chry) mutant is characterized by a shift from the polysymmetric corolla of disk flowers into a monosymmetric ray-like corolla. The tubular ray flower (turf) mutant is characterized by a shift from the zygomorphic corolla of ray flowers into a nearly actinomorphic tubular-like corolla. We performed a genetic analysis of turf, showing that a single nuclear recessive gene controls the trait. Furthermore, we characterized in detail the morphological floral features of Chry and turf, demonstrating that both mutations also affect the development of stamens and carpels. Most disk flowers found in the peripheral whorls of Chry heads showed drastic reduction in stamen length, as well as absence of ovules, and developed an unbranched style. By contrast, tubular-like ray flowers of turf achieved the ability to differentiate both fertile stamens and ovules. Homeotic transformations were also identified in the tubular-like ray flowers of turf, affecting both filaments and anthers that displayed petaloid-like traits. Our results point to a primary role for TURF and CHRY in the programming of the corolla symmetry and suggest a key interaction of both genes with floral organ identity genes.Key words: Helianthus annuus, floral symmetry, floral mutants, actinomorphic flowers, zygomorphic flowers.