stem fasciated, a recessive mutation in sunflower (Helianthus annuus L.), 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. We characterised in sunflower (Helianthus annuus L.), a mutation named stem fasciated (stf) that affects various aspects of plant development including SAM shape and auxin level. Methods F1, F2 and F3 generations were obtained through reciprocal crosses between stf and normal plants. For the genetic analysis, a c2 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 (RAM) was also conducted. To evaluate the level of endogenous indole-3-acetic acid (IAA), a capillary gas-chromatography-mass spectrometry-selected ion monitoring (GC-MS-SIM) analysis was performed. Key Results stf is controlled by a single nuclear recessive gene. stf plants are characterised by a dramatic increased number of leaves and vascular bundles in the stem, as well as by a shorten plastochron and an altered phyllotaxis pattern. By histological analysis, we demonstrate that the stf phenotype is related to an enlarged vegetative SAM. Microscopy analysis of the mutant’s apex has also revealed an abnormal enlargement of nuclei in both central (CZ) and peripheral zone (PZ) and a disorganized distribution of cells in the L2 layer of the CZ. The stf mutant showed a high endogenous level of free IAA, whereas auxin perception appears normal.