A NOVEL CALLOSE SYNTHASE FROM POLLEN TUBES OF NICOTIANA

Pollen-tube cell walls are unusual in that they are composed almost en tirely of callose, a (1,3)-beta-linked glucan with a few 6-linked bran ches. Regulation of callose synthesis in pollen tubes is under develop mental control, and this contrasts with the deposition of callose in t he walls of somatic plant cells which generally occurs only in respons e to wounding or stress. The callose synthase (uridine-diphosphate glu cose: 1,3-beta-D-glucan 3-beta-D-glucosyl transferase, EC 2.4.1.34) ac tivities of membrane preparations from cultured pollen tubes and suspe nsion-cultured cells of Nicotiana alata Link et Otto (ornamental tobac co) exhibited different kinetic and regulatory properties. Callose syn thesis by membrane preparations from pollen tubes was not stimulated b y Ca2+ or other divalent cations, and exhibited Michaelis-Menten kinet ics only between 0.25 mM and 6 mM uridine-diphosphate glucose (K(m) 1. 5-2.5 mM); it was activated by beta-glucosides and compatible detergen ts. In contrast, callose synthesis by membrane preparations from suspe nsion-cultured cells was dependent on Ca2+, and in the presence of 2 m M Ca2+ exhibited Michaelis-Menten kinetics above 0.1 mM uridine-diphos phate glucose (K(m) 0.45 mM); it also required a beta-glucoside and lo w levels. of compatible detergent for full activity, but. was rapidly inactivated at higher levels of detergent. Callose synthase activity i n pollen-tube membranes increased ten fold after treatment of the memb ranes with trypsin in the presence of detergent, with no changes in co factor requirements. No increase in callose synthase activity, however , was observed when membranes from suspension-cultured cells were trea ted with trypsin. The insoluble polymeric product of the pollen-tube e nzyme was characterised as a linear (1,3)-beta-D-glucan with no 6-link ed glucosyl branches, and the same product was synthesised irrespectiv e of the assay conditions employed.