Chitin/ lipo-chitin recognition by plants
Agonist recognition by a receptor protein leading to the activation of a signal transduction pathway is a key event in many important cellular processes (e.g., organogenesis). The molecular mechanisms of sensory perception in plants are largely unknown. We are studying the nitrogen-fixing symbiosis between the bacterium, Bradyrhizobium japonicum, and soybean. B. japonicum infects soybean roots and induces the formation of a nodule, a new organ, in which the bacteria reside. Nodule organogenesis is induced by substituted lipo-chitin molecules synthesized by the products of the bacterial nodulation genes. Past work from our laboratory has identified the lipo-chitin molecules produced by B. japonicum and explored their biosynthesis. Structure/ function studies suggest that the lipo-chitin Nod signals are recognized by two receptors. One receptor shows limited specificity, since unsubstituted, chitin oligomers are active. The second receptor requires a specific Nod signal.
The lipo-chitin Nod signals resemble simple chitin oligomers, which are known to induce plant defense responses. Indeed, it is our hypothesis that the Nod signal recognition pathway likely evolved from a more ancient chitin sensing system found in all plants. This has led us to examine the chitin response in plants. Using DNA microarray analysis, we have identified a number of genes in the model plants Arabidopsis thaliana and Medicago truncatula that respond to chitin treatment (Ramonell et al., 2002; Zhang et al., 2002). Our data suggest that Arabidopsis possesses two chitin-recognition pathways; one, responds to higher molecular weight chitin oligomers resulting in induction of pathogen defense pathways and two, a pathway that responds to lower molecular weight oligomers. Zhang et al. (2002) demonstrated that the first pathway is unique and unrelated to the well characterized pathways involving salicylic acid, ethylene or jasmonic acid. Most recently, Wan et al. (2004) demonstrated that this pathway activates a MAP kinase cascade resulting in the activation of WRKY family transcriptional regulators.
Hierarchical cluster of 166 Arabidopsis ESTs with
significant changes (>2.5-fold) in transcription level in response to chitin
treatment. Each gene is represented by a single row of colored boxes, and
each time point is represented by a single column. Induction (or repression)
ranges from pale to saturated red (or green). The data represent an average
of two independent experiments.
©Copyright, 2002.
