Hrp Mutant of Pseudomonas syringae pv phaseolicola Induces Cell Wall Alterations but Not Membrane Damage Leading to the Hypersensitive Reaction in Lettuce

Abstract

Both wild-type (521 -WT) and hrpD-621-533) strains of Pseudomonas syringae pv phaseolicola induced the formation of large paramural papillae in lettuce (Lacfuca sativa) mesophyll cells adjacent to bacterial colonies. Localized alterations to the plant cell wall included deposition of hydroxyproline-rich glycoproteins, phenolics, and callose, and were associated with proliferation of the endoplasmic reticulum and multivesicular bodies. Tissue collapse during the hypersensitive reaction caused by S21 -WT was associated with electrolyte leakage and rapid accumulation of the phytoalexin lettucenin A, both of which followed membrane damage indicated by the failure of mesophyll cells to plasmolyze. A few cells lost the ability to plasmolyze after inoculation with S21-533, and low levels of lettucenin A were recorded, but neither leakage of electrolytes nor tissue collapse were detected. Dysfunction of the plasma membrane in cells adjacent to colonies of S21-WT led to extensive vacuolation of the cytoplasm, organelle disruption, and cytoplasmic collapse-changes unlike those occurring in cells undergoing apoptosis. Strain S21-533 remained viable within symptomless tissue, whereas cells of S21-WT were killed as a consequence of the hypersensitive reaction. Our observations emphasize the subtle coordination of the plant's response occurring at the subcellular Ievel.

The hypersensitive response and the induction of cell death in plants

“…There are only a few examples correlating disease symptoms with cytologically defined necrosis (as defined in animal systems) and resistance with apoptosis-like cell death (Levine et al, 1996). In other cases, resistance is associated with cytological changes reminiscent of animal necrosis (Bestwick et al, 1995). Although cell death in plants could functionally play the same role as in animals, it may be that the mechanisms underlying this process evolved differently .…”

“…Alternatively, cell death and the associated cellular decompartmentalization could provoke the release of toxic compounds (phytoalexins) accumulated in the vacuoles and precede the arrest of the pathogen. Finally, the desiccation process accompanying the HR may generate an anti-microbial environment (Bestwick et al, 1995). Because cell death per se may be insufficient to Figure 4 Suppression and negation of the plant defenses.…”

Infection of mottled stripe disease‐susceptible and resistant sugar cane varieties by the endophytic diazotroph Herbaspirilium

“…B-4362, the bacteria appeared to be et al (1989) and Boher et al (1995). VanderMolen emerging from the stomata and colonizing the leaf et al (1977) proposed that the 'gels' and 'gums', surface; hence the stomatal colonization observed in which probably contain lectins, glycoproteins, phen-the less diseased parts of leaves at 20 d could have olics and callose, amongst other components (Ouchi, been the result of a secondary infection from the 1983; Bestwick, Bennett & Mansfield, 1995), orig-surfaces of adjacent heavily diseased tissues. Howinate from the expansion of primary walls and ever, a number of cv.…”

Ensnaring microbes: the components of plant disease resistance

“…Parallels between this plant cell death phenotype and the programmed, apoptotic, cell death observed in other eukaryotes (Martin, 1993;Collins & Rivas, 1993), have been suggested (Lamb, 1994), However, the first detailed histological study undertaken specifically to address this equation using a non-host bacterially induced HR on lettuce to Pseudomonas syringae pv, phaseolicola, found no evidence to indicate that the plant cells underwent apoptosis (Bestwick, Bennett & Mansfield, 1995), But as this was non-host mediated HR it could be distinct from race-specific R-Avrmediated HR, The HR might only be a consequence of other rapidly induced defence responses, for example the oxidative burst or elevated lipoxygenase activity which eventually overcome the cellular protection mechanisms of cells within a certain distance from the original challenge site. Indeed, in several plant-pathogen interactions, if the environmental conditions are modified, for example by increasing humidity levels, microbial arrest can occur in the absence of HR (Klement, 1982;Hammond-Kosack & Jones, 1995).…”