CYTOCHEMISTRY OF DEFENSE RESPONSES IN CASSAVA INFECTED BY XANTHOMONAS-CAMPESTRIS PV MANIHOTIS

Stems of susceptible and resistant cassava plants have been cytologica lly investigated for their defense reactions to an aggressive strain o f Xanthomonas campestris pv. manihotis. Histochemistry, in conjunction with gold cytochemistry, revealed that in susceptible and resistant p lants, phloem and xylem parenchyma cells displayed a wide range of res ponses that limited the bacterial growth within the infected plants. L ignification and suberization associated with callose deposition were effective mechanisms that reinforced host barriers in the phloem. In t he infected xylem, vessels were plugged by a material of pectic and (o r) lignin-like origin. Flavonoids have been seen to be incorporated in secondary cell wall coatings. These reactions occurred at a higher in tensity in the resistant plants. The number of phoem and xylem cells p roducing autofluorescent compounds was higher in infected resistant pl ants than in susceptible plants. Reactions have been observed in the r esistant variety only, such as secretion of phenol-like molecules by t yloses and hyperplasic activity of phloem cells that compartmentalized bacterial lysis pockets, which are potent secondary inoculum sources.