BRACHYCYTES IN FUNARIA PROTONEMATE - INDUCTION BY ABSCISIC-ACID AND FINE-STRUCTURE

Brachycytes are thick-walled, drought-tolerant brood cells or parts of brood bodies that develop in protonemata of Funaria (and other mosses ). Their formation can be induced by ABA in the chloronema of normally growing protonema, either after spore germination, or after protoplas t regeneration or in the auxin-deficient mutant 87.25, The expression of brachycyte characteristics and the number of brachycytes depends on ABA concentration and the length of the treatment. ABA induces in the mutant also the differentiation of unema cells in an all-or-nothing r eaction. Brachycytes germinate when ABA is bring removed, Brachycytes are surrounded by a thick, massive wall, store lipids instead of starc h and have only tiny vacuoles, In mature brachycytes the chloroplasts are transformed into cup-shaped organelles with unstacked, long thylak oids in the ''cup'' parts and small grana in the ''brim''. Each rediff erentiation, namely the formation of brachycytes In the chloronema, th e outgrowth of a regenerated, wall surrounded former protoplast, and t he germination of a brachycyte, is connected with the deposition of a new wall layer. When maturating brachycytes become globular, the wall between adjacent cells splits peripherally. The ''stumps'' of the disr upted plasmodesmata remain in the new ''outer wall'' for some days.