ULTRASTRUCTURAL AND CYTOCHEMICAL ASPECTS OF THE RESPONSE OF EGGPLANT PARENCHYMA CELLS IN DIRECT-CONTACT WITH VERTICILLIUM-INFECTED XYLEM VESSELS

Changes that occurred in paratracheal parenchyma cells immediately adjacent to infected xylem vessels in eggplant stem tissues inoculated with Verticillium albo-atrum were investigated ultrastructurally and cytochemically. The response of these cells to vessel colonization was characterized by a rapid sealing-off of attempted sites of fungal penetration and a marked cytoplasmic reorganization. Accumulation of electron-opaque, globular structures in paramural spaces and formation of intracytoplasmic vesicles filled with osmiophilic deposits were among the most typical features of reaction. Large amounts of callose (beta-1-3-glucan) were detected in the electron-opaque globules by means of gold labelling with a purified beta-1,3-glucanase from tobacco. These callose-rich structures were found to be deposited over an amorphous matrix which was almost free of cellulose and pectin, lour contained substantial amounts of glucosides and galactose residues. The occurrence of galactose in the matrix is discussed in relation to the possible accumulation of hydroxyproline-rich glycoproteins. Lipids and phospholipids were detected in the intracytoplasmic vesicles. Considering the known lipophilic nature of phytoalexins, the positive reaction observed upon treatment with the gold-complexed lipase suggested the presence of newly-synthesized secondary products. Support for this speculation came from the labelling pattern obtained with a laccase-gold complex specific for phenolic compounds. Finally, chitinase and beta-1,3-glucanase were localized in vacuoles where they probably accumulated as a non-specific response to infection. Our results bring additional evidence that paratracheal parenchyma cells in direct contact with invaded vessels are signalled to mobilize a number of defence strategies among which formation of physical barriers at specific sites is one of the earliest and key events involved in protection against invasion of the xylem parenchyma cells.