stem fasciated, a Recessive Mutation in Sunflower (Helianthus annuus), Alters Plant Morphology and Auxin Level

� Background and Aims Plant lateral organs such as leaves arise from a group of initial cells within the flanks of the shoot apical meristem (SAM). Alterations in the initiation of lateral organs are often associated with changes in the dimension and arrangement of the SAM as well as with abnormal hormonal homeostasis. A mutation named stem fasciated (stf) that affects various aspects of plant development, including SAM shape and auxin level, was characterized in sunflower (Helianthus annuus). � Methods F1, F2 and F3 generations were obtained through reciprocal crosses between stf and normal plants. For the genetic analysis, a x 2 test was used. Phenotypic observations were made in field-grown and potted plants. A histological analysis of SAM, hypocotyl, epicotyl, stem and root apical meristem was also conducted. To evaluate the level of endogenous indole-3-acetic acid (IAA), a capillary gas chromatography‐mass spectrometry‐selected ion monitoring analysis was performed. � Key Results stf is controlled by a single nuclear recessive gene. stf plants are characterized by a dramatically increased number of leaves and vascular bundles in the stem, as well as by a shortened plastochron and an altered phyllotaxis pattern. By histological analysis, it was demonstrated that the stf phenotype is related to an enlarged vegetative SAM. Microscopy analysis of the mutant’s apex also revealed an abnormal enlargement of nuclei in both central and peripheral zones and a disorganized distribution of cells inthe L2 layer of the central zone. The stfmutant showed a high endogenous free IAA level, whereas auxin perception appeared normal. � Conclusions The observed phenotype and the high level of auxin detected in stf plants suggest that the STF gene is necessary for the proper initiation of primordia and for the establishment of a phyllotactic pattern through control of both SAM arrangement and hormonal homeostasis.