Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2001
Publication Date: N/A
Citation: N/A Interpretive Summary: Soybean root-rot is caused by a harmful soil fungus, Phytophtohora sojae. A beneficial bacterium, Bradyrhizobium japonicum, also "infects" soybean roots, but it helps the host plant by providing nitrogen. Thus, an analysis of how plants respond differently to harmful vs. beneficial microorganisms is critical to our understanding of agricultural microbiology. We examined the role of cellular calcium levels (in soybean) to understand how plants fight-back invading microbes. Root-rot causing fungus (P. sojae) as well as beneficial, nitrogen-providing bacterium (B. japonicum) synthesize complex sugar compounds (glucan, a specific glucose polymer having hormone-like properties). We have discovered that soybean plants respond differently to glucans from beneficial vs. harmful bacteria. Correlating the structure of various glucans and understanding their function would help us to design better beneficial bacteria as well as to develop strategies in controlling harmful microorganism.
Technical Abstract: Soybean cell suspension cultures have been used to analyze the role of cellular Ca (+2) influx and the increase of cytosolic Ca (+2) concentration in defence responses induced by b-glucan elicitors from the phytopathogenic oomycete Phytophthora sojae. Ca (+2) chelators and the Ca (+2) channel inhibitor La (+3) inhibited (i) b-glucan-elicited increase of intracellular Ca (+2) concentration, monitored in transgenic cells expressing the Ca (+2) sensing aequorin, (ii) b-glucan-elicited production of hydrogen peroxide, and (iii) b-glucan-induced phytoalexin synthesis. The elicitor-induced increase of cytosolic Ca (+2) levels and phytoalexin synthesis were similarly inhibited upon treatment with the anion channel inhibitors anthracene-9-carboxylate (A9C) and 5-nitro- 2-(3-phenylpropylamino)-benzoate (NPPB). However, at the same concentrations, the anion channel inhibitors had no effect on the b- glucan-induced hydrogen peroxide production. Cyclic b-glucans from Bradyrhizobium japonicum, known to suppress the elicitor-induced increase of the cytosolic Ca (+2) concentration as well as phytoalexin synthesis in soybean, did not inhibit elicitor-induced accumulation of hydrogen peroxide in these cells but acted as elicitors of the oxidative burst themselves. These results indicate that the signalling pathways leading to (i) rapid defence reactions, namely the production of reactive oxygen species, and (ii) the longer lasting phytoalexin response diverge at an early stage of elicitor-recognition and signal transduction.