Cytochemistry of the wall of infected cells in Casuarina actinorhizae

Abstract

Development of the wall of infected cells in Casuarina actinorhizae differs from that of many actinorhizae. After the endophyte penetrates the wall of a cortical cell, that (primary) cell wall becomes lignified, based on histochemical (autofluorescence, phloroglucinol staining) and cytochemical (permanganate staining, enzyme etching) evidence. Subsequently, the remaining walls of the infected cell become lignified. Adjacent noninfected cells somehow are stimulated to deposit a lignified secondary wall only on those walls bordering the infected cell. This remarkable participation of all adjacent noninfected cells in the development of a given infected cell results in an increased thickness and strength of the wall material surrounding infected cells. When they mature, there is a further modification of some of the wall layers surrounding infected cells, manifested in a relative impermeability to en bloc staining with permanganate. Unlike lignified walls, the permanganate-impermeable region is selectively stained by osmium or ferricyanide-reduced osmium and is relatively resistant to concentrated chromic acid digestion. A region that remains permeable to (and stained by) permanganate (part of the secondary wall of bordering noninfected cells) may be developmentally related to phi thickenings. An earlier contention that the permanganate-impermeable region contains suberin is unconfirmed. This region is most likely an unusual lignin modification or results from unidentified material impregnated in its ligninlike matrix.