LOCATION OF CELLULOSE AND CALLOSE IN POLLEN TUBES AND GRAINS OF NICOTIANA-TABACUM

The distribution of cellulose and callose in the walls of pollen tubes and grains of Nicotiana tabacum L. was examined by electron microscop y using gold-labelled cellobiohydrolase for cellulose and a (1,3)-beta -D-glucan-specific monoclonal antibody for callose. These probes provi ded the first direct evidence that cellulose co-locates with callose i n the inner, electron-lucent layer of the pollen-tube wall, while both polymers are absent from the outer, fibrillar layer. Neither cellulos e nor callose are present in the wall at the pollen-tube tip or in cyt oplasmic vesicles. Cellulose is first detected approximately 5-15 mu m behind the growing tube tip, just before a visible inner wall layer c ommences, whereas callose is first observed in the inner wall layer ap proximately 30 mu m behind the tip. Callose was present throughout tra nsverse plugs, whereas cellulose was most abundant towards the outer r egions of these plugs. This same distribution of cellulose and callose was also observed in pollen-tube walls of N. alata Link et Otto, Bras sica campestris L. and Lilium longiflorum um Thunb. In pollen grains o f N. tabacum, cellulose is present in the intine layer of the wall thr oughout germination, but no callose is present. Callose appears in gra ins by 4 h after germination, increasing in amount over at least the f irst 18 h, and is located at the interface between the intine and the plasma membrane. This differential distribution of cellulose and callo se in both pollen tubes and grains has implications for the nature of the beta-glucan biosynthetic machinery.