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Budding gene-ius
The variety of life forms amongst herbaceous angiosperms is well illustrated in the paper by Alvarez et al. (Palmerston North, New Zealand, pp. 953–963) dealing with branching patterns in Lotus japonicus. This species often has a ‘straggly’ appearance, at least partly due to its unusual branching pattern. It is well known that lateral branches arise in the axils of leaves. This may include seed leaves – cotyledons – although in many species the meristems in cotyledonary axils are inactive or rudimentary. However, in L. japonicus it is the axillary meristems of cotyledons that are major determinants of the branching pattern. Not only are these meristems active, but they also give rise to additional (accessory) meristems that are themselves active. Extensive lateral shoot formation thus originates from the cotyledonary axils. The authors have studied several aspects of the formation and activity of these meristems; we concentrate here on genetic control. The origin of axillary meristems is still uncertain: do they arise de novo in the axils, or are they derived originally from the shoot apical meristem (SAM) (the ‘detached meristem’ hypothesis)? Study by in situ hybridization of the expression of the L. japonicus version of the gene SHOOT-MERISTEMLESS (STM) suggests very strongly that during embryogenesis axillary meristems are indeed derived from the SAM and continue to express STM during the life of the plant. The accessory meristems then arise between the axillary bud and the cotyledon in part of the zone in which STM is expressed. However, the reason that these particular axillary meristems continue to throw off further meristems is not clear, although, as the authors state, the Lateral Suppressor (LS) gene may well be involved. Finally, the discovery of a mutant, super-accessory-branches, sac (which exhibits increased lateral branch formation from the axils of all leaves, not just cotyledons) provides a further tool for investigating this fascinating phenomenon.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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