Histological Studies of Callus Induction in Rice Seed

Abstract

The experiments were performed to clarify the tissues from which callus is induced in rice seed. Dehusked seeds (var. Aichiasahi) were inoculated aseptically on Maeda's medium containing 3% sugar, 0.3% casein hydrolysate and 10-5 M 2, 4-dichlorophenoxy acetic acid (2, 4-D), and cultured for 30 days at 30°C under light condition. Morphological and histological changes of the tissues during culture period were as follows. The growths of the plumula and radicle were suppressed, although the scutellum and mesocotyl tissues proliferated abnormally. Tissue enlargement occurred in the scutellum, in which the parenchyma cells expanded and the epithelium cells divided 3 or 4 times for the first 7 days. After 20 days of culture epithelium bigan rapid cell proliferation to produce callus tissue. The other hand, in the epithelium on the medium without 2, 4-D a cell division occurred once. In the mesocotyl, cell divisions bigan at the outermost layer of the central cylinder leading to that of the radicle for 2 days of culture and the cell divisions expanded to the radicle and the vascular tissue of the scutellum. After that they proliferated consistently during culture period and produced callus tissue. The fresh weight of an embryo became 225 fold at the end of culture. On the contrary, a vascular system fully developed in the mesocotyl of the control seedling lacking 2, 4-D which grew normally. The endosperm digestion of callus forming seeds delayed considerably since the some dehydrogenase activities in thc epithelium cells would fail. The morphological changes accompanying with callus formation occurred even when the seeds removed from callus inducing medium containing 2, 4-D to the medium lacking 2, 4-D after 2 or 3 days of culture, suggesting that irreversible change of the tissues had already occurred for this period. Mesocotyl, scutellum and radicle dissected from embryos of dry seeds were cultured under above condition, but they did not produce callus tissue. Dissected plumula and the tissue having both plumula and mesocotyl produced callus tissue. This suggests that plumula would play physiologicaly important roles and that callus formation in mesocotyl and scutellum tissues in a seed would occure under the physiological interactions between plumula and these tissues. Furthermore, when 2, 4-D was decreased to 3×10-5 M the outermost layer of the central cylinder in mesocotyl formed stunted primordium of lateral roots, showing that callus tissue of mesocotyl is formed by abnormal proliferation of the cells which should form lateral roots.