A molecular analysis of flowering in Metrosideros : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Biology at Massey University, Palmerston North, New Zealand
A calendar of floral and vegetative developmental events has been previously determined for the woody perennial Metrosideros excelsa. M. excelsa flowers just once a year in summer and bud development has been related to seasonality and gene expression. M. collina cv. Tahiti is closely related to M. excelsa but appeared to flower sporadically through out the year. Using histology, the bud development of M. collina has been analysed and a developmental sequence determined. Periodicity of bud development has also been observed by obtaining bud measurements. While it is difficult to relate bud size to developmental phase or determine whether the bud is vegetative or floral, it appears that M. collina goes through two periods of bud development in one year and that flowering may be related to warm temperature. The genetic interactions between pathways controlling floral meristem development have been analysed in the herbaceous annual model plant Arabidopsis thaliana. In Arabidopsis, expression of the floral meristem identity gene, LEAFY (LFY), is regulated directly by the florally promotive gibberellin and photoperiodic pathways. LFY expression in Arabidopsis was upregulated in response to gibberellin application, which hastened the transition from vegetative to floral meristem. Another floral meristem identity gene, APETALA1 (AP1) is upregulated by LFY to promote formation of the floral meristem. However, exogenous application of gibberellic acid (GA3
) to woody perennial trees, including M. collina, inhibits flowering. Due to the differences between Arabidopsis and woody perennial species, it is important to determine if the Arabidopsis model is a useful tool to analyse flowering in Metrosideros species. LFY and AP1 homologues were isolated previously from M. excelsa, and from M. collina in this project. The responsiveness of MEL (M. excelsa LFY-like) and MESAPI (M. excelsa AP1-like) and MCL (M. collina LFY-like) and MTAP1 (M. collina AP1-like) to GA3
, and other growth regulators and hormones was examined in juvenile and adult Metrosideros. MEL or MCL expression was not detected in juvenile or adult GA3
-treated Metrosideros, which supports GA3
inhibition of flowering in perennials. It appears that the gibberellin component of the Arabidopsis model does not represent appropriately, the response of perennial woody species to gibberellins in terms of flowering.