Photos from the 2003 East Tennessee Collegiate Division Meeting of the Tennessee Academy of Science held at Pellissippi State Technical Community College
4/11/2003
Crystal McAlvin, University of Tennessee-Knoxville
First Place Outstanding Graduate Student Presentation
ABSTRACT
Effects Of Salicylic Acid On Root Growth And Nodulation In The Model Legume, Lotus japonicus
The infection of legume roots by rhizobia occurs without the apparent induction of a plant defense response. Salicylic acid (SA) is well established as a signal molecule involved in plant defense. Previous research showed that addition of exogenous SA to plant roots could inhibit nodulation, suggesting a possible role of this signal in nodulation. We initially attempted to address this hypothesis by adding exogenous SA to roots of the model legume, Lotus japonicus (Handberg and Stougaard, 1992; Jiang and Gresshoff, 1997). However, we found that the levels of SA, which affected nodulation, also strongly inhibited growth of the symbiont, Mesorhizobium loti. Since endogenous SA is likely the mediator of any physiologically relevant effect on nodulation, we sought to modulate these levels by expression of salicylate hydroxylase, encoded by the bacterial nahG gene, in transgenic Lotus japonicus plants. Several independent transgenic lines, expressing NahG, were analyzed for their nodulation and root growth phenotype after inoculation with Mesorhizobium loti. Expression of the transgene correlated with a significant increase in mean nodule number per plant when compared to wild-type controls. Examination of infection thread formation indicated that the nahG plants did not have enhanced infection, but rather an increase in the length of the root infection zone. Interestingly, the nahG plants also showed a significant increase in root growth that correlated to the nodulation phenotype. Analysis of plant extracts indicated that SA levels in the nahG transgenic plants were significantly reduced, compared to wild-type controls. These data suggest that SA may play a role in suppression of nodulation. However, interpretation of these results is complicated by the concomitant effects on root growth. The nahG transgenic plants should be a valuable resource for further investigating the mechanism of SA action in legumes.
