Molecular and anatomical studies were conducted on Metrosideros excelsa to determine if the current genetic models for flowering with regard to inflorescence and floral meristem identity genes in annual plants applied for a woody perennial. Microscopy studies revealed that floral initiation as cymule primordia began in May. Cymules began to develop by August and by October all the floral organs were fully differentiated. MEL, MESAP1 and METFL1, the partial orthologues of LEAFY, APETALA1 and TERMINAL FLOWER1 respectively, were then isolated from M. excelsa buds through RT-PCR. RT-PCR analysis and expression Southerns showed that MEL and MESAP1 were present at low levels as early as March, and that they were both upregulated at the time cymule primordia were initiated and again during floral organogenesis. As AP1 is considered an indicator of floral determination, the expression of both MEL and MESAP1 as early as March indicated that floral commitment had occurred by then. The results from microscopy studies supported this conclusion. Studies on juvenile M. excelsa plants revealed that GA3 application caused upregulation of MEL but not MESAP1 indicating that meristem competence was also probably required to promote flowering in M. excelsa as has been suggested for Arabidopsis (Weigel and Nilsson, 1995). In situ hybridisation studies revealed that MEL expression shifted from the apex of the distal axillary bud in May to cymule primordia in early June and subsequently to the sepals, petals, anthers and the gynoecium and ovules in the later stages of floral development. MESAP1 expression was seen in young floral meristems, but during the later stages official development it was confined to the sepals, petals and the perianth region, which is typical of a Class A gene. METFL1 was expressed throughout the period of inflorescence development. It was expressed in the inflorescence meristem and not in the floral meristems, as is the case with TFL1 in Arabidopsis. Thus the key floral and inflorescence meristem identity genes in the woody perennial M. excelsa showed similar spatial expression patterns as their equivalents in herbaceous plants. However, there were differences in temporal expression patterns such as the bimodal pattern of expression seen with MEL and MESAP1.
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Sreekantan, L., McKenzie, M. J., Jameson, P. E., & Clemens, J. (2001). Cycles of floral and vegetative development in metrosideros excelsa (myrtaceae). International Journal of Plant Sciences, 162(4), 719-727.