Scanning Electron Microscopy of Virulent and Avirulent Strains ofXanthomonas campestrispv.oryzaeon Rice Leaves

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Abstract
Multiplication of strains of X. campestris pv. oryzae on rice (O. sativa) cultivars and penetration of the bacteria through water pores of leaf blades were investigated with scanning electron microscopy. No marked difference in numbers of cells was noted on the leaf surface 1 h after spray-inoculation on all cultivar-strain combinations. Twenty-four hours after inoculation, bacteria of strain PX061, which is virulent to cultivars TN1 and CAS 209, multiplied immediately outside the water pores and some bacteria had gained entrance through these pores. Cells of PX0101, a strain that has lost its virulence, did not multiply significantly on the leaf surface and apparently were embedded in a thin layer of exudate secreted by the water pores. The exudate eventually sealed the opening of these pores. Bacterial cells of PX086, which is virulent to TN1 but avirulent to CAS 209, multiplied on the water pores of TN1 but were trapped in the exudate of CAS 209 forty-eight hours after inoculation. Bacterial cells of all 3 strains were not observed to multiply on stomata of either cultivar. Bacteria are probably immobilized and inhibited from diving by excretions from water pores in incompatible host-bacteria combinations. Site specificity of bacterial multiplication on leaf surface was also observed.

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