Browsing by Author "Zhen, Zeng"

Rhizobia are agriculturally important bacteria that possess the ability to fix nitrogen for their host legumes, an attribute ascribed to the presence of symbiosis-related genes usually clustered on plasmids called symbiotic plasmids (pSyms). Many pSyms have been proven self-transmissible, capable of transferring themselves to other bacteria through conjugation, thereby propagating their symbiotic features. Rhizobium galegae symbiovar (sv.) officinalis has a pSym, on which typical conjugation genes have been revealed. A Type IV secretion system (T4SS) functioning as a conjugation system has also been computationally predicted on a chromid, another replicon in R. galegae sv. officinalis. In addition, the transfer of the pSym of R. galegae sv. officinalis to a non-nodulating mutant strain of R. galegae sv. orientalis has been previously observed under laboratory conditions. Therefore, this thesis was aimed at investigating the self-transmissibility of the pSym of R. galegae sv. officinalis and the necessity of the T4SS’ presence for the pSym transfer. Two derivatives of the R. galegae sv. officinalis were generated with one strain cured of its pSym by using Tn5-mob-sacB transposon and the other strain excised the T4SS from the chromid by Cre-lox site specific recombination system. Conjugation were then performed between these two derivatives as well as between the wild-type strain and the plasmid-cured derivative, followed by the host plant nodulation tests. The tests showed no formation of a single nodule in either pair, which was unexpectedly inconsistent with the previous experimental observation. No solid explanations could be proposed at this stage. It might be due to the low transfer frequency resulted from complex associations with subtle environmental signal molecules or recipient cell recognition that presumably disabled the transmissibility of the pSym.