Interactions of AtRGL1, a negative regulator of gibberellic acid signalling : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Biochemistry, Massey University, Palmerston North, New Zealand

Abstract

Arabidopsis thaliana AtRGL1 (repressor of ga1-3 like-1) is a negative regulator of the signal transduction pathway of the plant hormone gibberellin. AtRGL1 belongs to the DELLA subfamily within the GRAS family of plant regulatory proteins. There are four other DELLA proteins, including AtRGA (repressor of ga1-3) and AtRGL2, encoded by the A. thaliana genome. Previous studies provided evidence that the DELLA proteins are nuclear localised and are functionally divided into N- and C- terminal domains. The N-terminal domain perceives the gibberellin signal, while the C-terminal domain functions as a negative regulator of transcription and also as a possible dimerisation domain. Previous studies have also shown that AtRGA, AtRGL1, and AtRGL2 function together in the regulation of the development of the inflorescence and that AtRGL1 is primarily expressed in this tissue. To investigate how DELLA proteins function in gibberellin signalling. I sought plant proteins that interact with AtRGL1. Two proteins. p24 (24 kDa) and p64 (64 kDa), were isolated from wild-type plant nuclear extracts by affinity to the N-tenninal 121 amino acid residues of AtRGL1. The identity of these two proteins remains to be established. To investigate the interactions of the C-terminal domain of AtRGL1 an anti-AtRGL1 polyclonal antiserum was developed for co-immunoprecipitation experiments. However, AtRGL1 was not detectable in plant nuclear extracts from the inflorescence of wild-type plants, precluding this approach. The possibility of DELLA protein dimerisation was also investigated using AtRGA, AtRGL1, and AtRGL2 in yeast 2-hybrid experiments. Yeast 2-hybrid protein interaction results suggest that AtRGA, AtRGL1, and AtRGL2 do not form homo- or hetero-dimers. Complexities encountered with this approach could make these results invalid, so these interactions require further investigation.