HRP MUTANT OF PSEUDOMONAS-SYRINGAE PV PHASEOLICOLA INDUCES CELL-WALL ALTERATIONS BUT NOT MEMBRANE DAMAGE LEADING TO THE HYPERSENSITIVE REACTION IN LETTUCE

Both wild-type (S21-WT) and hrpD(-) (S21-533) strains of Pseudomonas s yringae pv phaseolicola induced the formation of large paramural papil lae in lettuce (Lactuca sativa) mesophyll cells adjacent to bacterial colonies. Localized alterations to the plant cell wall included deposi tion of hydroxyproline-rich glycoproteins, phenolics, and callose, and were associated with proliferation of the endoplasmic reticulum and m ultivesicular bodies. Tissue collapse during the hypersensitive reacti on caused by S21-WT was associated with electrolyte leakage and rapid accumulation of the phytoalexin lettucenin A, both of which followed m embrane damage indicated by the failure of mesophyll cells to plasmoly ze. A few cells lost the ability to plasmolyze after inoculation with S21-533, and low levels of lettucenin A were recorded, but neither lea kage of electrolytes nor tissue collapse were detected. Dysfunction of the plasma membrane in cells adjacent to colonies of S21-WT led to ex tensive vacuolation of the cytoplasm, organelle disruption, and cytopl asmic collapse-changes unlike those occurring in cells undergoing apop tosis. Strain S21-533 remained viable within symptomless tissue, where as cells of S21-WT were killed as a consequence of the hypersensitive reaction. Our observations emphasize the subtle coordination of the pl ant's response occurring at the subcellular level.