| ||||||||||||||||||||||
Fat SAM tests positive for hormone overdose
The changes in morphogenesis required to generate a strongly fasciated stem are very dramatic and it is of great interest that they may be caused by mutation of a single gene. Such is the case with sunflower, Helianthus annuus, where one such gene is known as stem fasciated (stf), as described by Fambrini et al. (Pisa and Bologna, Italy, pp. 715–730). Mutations are recessive, implying that they cause loss of the function(s) controlled by the wild-type gene. The authors have set out to find, by comparing mutant with wild type, what this gene does, using morphological, histological and biochemical analyses. The phenotype of the mutant raises expectations that the shoot apical meristem (SAM) is likely to be distorted, leading to stem thickening and probably also to disturbance of the normal pattern of primordia formation. In the authors’ very extensive study these expectations were indeed met. The vegetative SAM of the mutant was larger than that of the wild type; there was some distortion of one of the cell layers in the central zone, and the nuclei of both the peripheral and central zones were enlarged. Normal phyllotaxis was lost (perhaps because of the shape of the SAM) and, more surprisingly, the plastochron was shortened so that leaves were produced more frequently. Very interestingly, stf plants were significant over-producers of auxin, even allowing for the greater size of the apex. Further, decapitation of stf plants did not abolish apical dominance. The implication is that over-production of auxin in the apex led to higher concentrations further down the stem. However, the data did not bear this out: there are clearly other factors at work here. stf is thus a very interesting gene with effects on, amongst other things, the shape and size of the SAM, on tissue patterning and organogenesis in the SAM, and on the hormonal physiology of the apex.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
| ||||||||||||||||||||||