Calcium changes and the response to methyl jasmonate in rice lodicules during anthesis

Potassium pyroantimonate precipitation was used to locate loosely bound calcium in rice (Oryza sativa L.) lodicules before and after anthesis, and flowering of panicles was accelerated by treatment with methyl jasmonate. From 1 day to 4 h before anthesis, the number of calcium precipitates in the cell walls and vacuole membranes decreased gradually, whereas they increased remarkably in the cytoplasm and nucleolus. At the beginning of anthesis, the number of calcium granules in lodicules reduced sharply, but there was a large accumulation of flocculent precipitates in the vacuoles. After anthesis, the flocculent precipitates decreased in number until they disappeared, whereas the granular precipitates started to accumulate once again. The rice florets treated with 2 mM methyl jasmonate were induced to open within 10-30 min and they then closed 0.5-1 h later. The nucleolus, cytoplasm, and vacuole membrane of the lodicule cells contained many calcium granules during flowering, although the cell walls lacked calcium. At 1 h after treatment, the number of calcium granules had decreased, while flocculent precipitates were regularly observed in the nondegenerated cells. At 6 h after treatment, calcium grains started to reappear in the cell walls. These changes in calcium precipitates before and after anthesis indicate that the opening and closing of florets correlates with the calcium level in lodicule cells. In addition, excised panicles, with florets judged to be nearing anthesis, were soaked in 2-200 mM EGTA solution for 2 min after treatment with 2 mM methyl jasmonate. The results indicate that EGTA had an antagonistic effect on the methyl jasmonate-induced floret opening in rice.