Genetics and development of pseudanthia in the genus Helianthus: A review

This review summarizes the development of inflorescences in the genus Helianthus, which includes ∼50 species of the subtribe Helianthinae in the Asteraceae family. Inflorescence architecture depends on the activity of a plethora of genes, which regulate the timing of floral meristem formation, their number, and their arrangement within the inflorescence. The coordinated expression of genes for organ identity establishes the differentiation of each floral organ type, and genes responsible for positional identity specify patterns of symmetry. The most iconic inflorescence type in the genus Helianthus is the heterogamous radiate pseudanthium, characterized by actinomorphic hermaphroditic disk florets surrounded by conspicuous zygomorphic sterile marginal ray florets. The homogamous discoid inflorescence, consisting only of disk florets, is exclusive to H. radula. The liguliflorous-like head bearing only ray florets is typical of some interspecific hybrids of Helianthus × multiflorus originating from unreduced eggs of H. decapetalus fertilized by reduced male gametes of H. annuus. Some spontaneous sunflower mutants generate liguliflorous-like head, which nevertheless maintain a morphological distinction between marginal true ray florets and internal ray-like florets with partially functional reproductive organs. Genetic analysis of mutants with altered initiation and development of floral meristems and floral organs suggests a close homology between disk and ray florets. A sunflower mutant defective in floret primordia initiation indicates an intriguing parallelism between axillary and floral meristems. The different inflorescence patterns found in species and mutants within Helianthus genus will be useful in analysing how the genes involved may control fundamental aspects of pseudanthium development and overall plant architecture.