Comparison of callose plug structure between dicotyledon and monocotyledon pollen germinated<i>in vitro</i>

Abstract

To verify whether the callose plug formation in dicots differs from that in monocots, we examined the shapes and intervals of the callose plugs formed in pollen from 48 dicot and 27 monocot species germinated in vitro. We also investigated changes in the plug components during callose plug formation in the dicot Camellia japonica L. and the monocot Lilium longiflorum Thunb., using immunoelectron microscopy. The callose plugs in pollen of most dicots walled off the tubes, and in some species the plugs displayed ingrowth along the longitudinal axis of the tube. In contrast, the callose plugs in pollen of most monocots showed incomplete walling off of the tubes, and none displayed ingrowth. In all the monocots, the plugs were formed at irregular intervals, compared with the regular intervals seen in about 40% of dicot species examined. In camellia and lily pollen, the growing plugs contained callose, non‐ or low‐esterified pectins and arabinogalactan protein in the early stages of formation. The changes during plug development, including the increase of callose and the presence of other components to some degree, were similar in both types of pollen. These results suggest that the mode of callose plug construction in dicots is distinct from that in monocots, although the plug components of both pollen types are very similar.