Morphological, physiological and molecular studies of Pachycladon exilis (Brassicaceae) : 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
Pachycladon exilis is a highly endangered endemic species of New Zealand. Due to interest in preventing the extinction of this species and because it is related to the model plant, Arabidopsis, floral induction and floral development were investigated. The effects of environmental signals on growth and flowering were investigated by growing plants under factorial combinations of daylength and chilling in controlled environment rooms. The two daylengths were long days (16 h daylength) and short days (8 h daylength), to which plants were exposed following exposure of imbibed seeds for 0, 10, 20 or 30 d at 4°C. The influence of daylength and duration of chilling on the quantitative expression of the key meristem identity genes, LEAFY (LFY) and TERMINAL FLOWER 1 (TFL1), was also studied using real-time reverse transcriptase-PCR. Morphological studies on P. exilis showed that the plants produced rosette leaves alternately on short internodes and cauline leaves separated by longer internodes. Dissection of whole plants showed that the shoot comprised three levels of branching in a hierarchy, with the leaves of one level subtending the shoots of the next level. Further, it was observed that the plants produced adventitious shoots from roots. Daylength and chilling treatments influenced development and floral induction. Although flowering started at the same time in plants exposed to long and short days, plants that were exposed to long days and that had passed through cold treatment as imbibed seeds flowered earlier after producing fewer leaves, and flowered more synchronously and to a higher percentage, than plants maintained under short days following chilling or plants with no cold treatment. At the molecular level, partial sequences of LFY and TFL1 were isolated in P. exilis and the expression levels of these genes were investigated using quantitative real-time PCR. It was observed that the up regulation of PeLFY coincided with the down regulation of PeTFL1 at the time of the transition from vegetative to inflorescence development. As expected, PeTFL1 expression was continued in the vegetative axillary meristems, which suggests that PeTFL1 is responsible for, or at least involved in, perenniality of P. exilis.