A quantitative histological and ultrastructural analysis of interactions between the flax rust and near-isogenic host lines varying in their degree of incompatibility

Abstract

Histological differences were evident in the leaves of 8 near-isogenic lines of flax [Linum usitatissimum]infected with the rust fungus Melampsora lini. Compared with the compatible L9 and M1 interactions, fungal growth was progressively more restricted in the moderately incompatible K and M4, incomparable M and P, and highly incompatible L and M3 interactions. This restriction took place in advance of appreciable necrosis of host cells in K, M4, M and P. At 72 h the proportion of haustoria-containing cells which were necrotic was only 10-15% in K and M4. In M at 72 h necrosis was 80% or more at infection sites with small colonies but was negligible at sites with large colones. In P, by contrast, a similar proportion of necrosis, 40% at 72 h, was present at all infection sites. However, in L and M3 host necrosis was much more rapid and the fungus was restricted to a few host cells. An early ultrastructural event was the appearance of extensive fibrillar deposits in the initially electron-lucent extrahaustorial matrices of both the incompatible M and P and moderately incompatible M4 and K interactions. A positive reaction with silver proteinate indicated that these matrical deposits contained carbohydrate, possibly a mucopolysaccharide or glycoprotein, but they were not extracted by either cellulose, pectinase or chitinase. The extrahaustorial membrane, surrounding the haustoria in the compatible L9 and M1 interactions and the moderately incompatible K interaction was not stained by the periodic acid phosphotungstic acid-chromic acid (PACP) procedure believed specific for the plasmalemma. In incompatible reactions clusters of electron-dense particles sometimes replaced starch in plastids of infected host cells. This event usually coincided with the appearance of extensive matrical deposits around fungal haustoria. At the same time particles were also found in plastids in uninfected host cells immediately adjacent to infection sites, particularly in the M and P interactions. These particles were extracted from thin sections by using pullulanase followed by .alpha.-amylase, indicating that they consisted of highly branched amylopectin.