Novel Extracellular Chitinases Rapidly and Specifically Induced by General Bacterial Elicitors and Suppressed by Virulent Bacteria as a Marker of Early Basal Resistance in Tobacco

Early basal resistance (EBR, formerly known as early induced resistance) is triggered by general bacterial elicitors. EBR has been suggested to inhibit or retard expression of the type III secretion system of pathogenic bacteria and may also prevent nonpathogenic bacteria from colonizing the plant tissue. The quickness of EBR here plays a crucial role, compensating for a low bactericidal efficacy. This inhibitory activity should take place in the cell wall, as bacteria do not enter living plant cells. We found several soluble proteins in the intercellular fluid of tobacco leaf parenchyma that coincided with EBR under different environmental (light and temperature) conditions known to affect EBR. The two most prominent proteins proved to be novel chitinases (EC 3.2.1.14) that were transcriptionally induced before and during EBR development. Their expression in the apoplast was fast and not stress-regulated as opposed to many pathogenesis-related proteins. Nonpathogenic, saprophytic, and avirulent bacteria all induced EBR and the chitinases. Studies using these chitinases as EBR markers revealed that the virulent Pseudomonas syringae pv. tabaci, being sensitive to EBR, must suppress it while suppressing the chitinases. EBR, the chitinases, as well as their suppression are quantitatively related, implying a delicate balance determining the outcome of an infection.