INTERACTIONS BETWEEN BACTERIAL DIAZOTROPHS AND NON-LEGUME DICOTS - ARABIDOPSIS-THALIANA AS A MODEL-PLANT

When interactions between diazotrophic bacteria and non-legume plants are studied within the context of trying to extend biological nitrogen fixation to non-legume crops, an important first step is to establish reproducible internal colonization at high frequency of these plants. Using Azorhizobium caulinodans ORS571 (which induces stem and root no dules on the tropical legume Sesbania rostrata), tagged with a constit utively expressed lacZ reporter gene, we have studied the possibilitie s of internal colonization of the root system of the model dicot Arabi dopsis thaliana. ORS571 was found to be able to enter A. thaliana root s after first colonizing lateral root cracks (LRCs), at the points of emergence of lateral roots. Cytological studies showed that after LRC colonization, bacteria moved into the intercellular space between the cortical and endodermal cell layers of roots. In our experimental cond itions, this LRC and intercellular colonization are reproducible and o ccur at high frequency, although the level of colonization at each sit e is low. The flavonoids naringenin and daidzein, at low concentration s, were found to significantly stimulate (at the p=0.01 level) the fre quency of LRC and intercellular colonization of A. thaliana roots by A . caulinodans. The role in colonization of the structural nodABC genes , as well as the regulatory gene nodD, was studied and it was found th at both colonization and flavonoid stimulation of colonization are nod gene-independent. These systems should now enable the various genetic and physiological factors which are limiting both for rhizobial colon ization and for endophytic nitrogen fixation in non-legumes, to be inv estigated. In particular, the use of A. thaliana, which has many advan tages over other plants for molecular genetic studies, to study intera ctions between diazotrophic bacteria and non-legume dicots, should pro vide the means of identifying and understanding the mechanisms by whic h plant genes are involved in these interactions.