QUANTITATIVE STUDIES WITH PLANT MEMBRANES. I. AUXIN-MEMBRANE INTERACTIONS: CALCIUM RELEASE. II. ENDOPLASMIC RETICULUM BIOGENESIS IN SEED GERMINATION: AN ELECTRON MICROSCOPY STUDY
Abstract
(I) Auxin-Membrane Interactions: Calcium Release. Previous work showed an antagonism between calcium and auxin. This study was conducted to investigate the auxin-calcium antagonism in isolated plant membranes. Membranes from etiolated hypocotyls of soybean (Glycine max (L.) Merr.) were loaded with 45-calcium in the presence of the ionophore A23187. Upon subsequent treatment with 2,4-dichlorophenoxyacetic acid or indole-3-acetic acid, calcium was released. A weak or inactive auxin analogue, 2,3-dichlorophenoxyacetic acid, did not promote release from the hypocotyl membrances, nor did red blood cells treated with auxin release calcium. The concentration dependency of auxin-promoted calcium release paralleled that of auxin-induced growth and was biphasic with an optimum at 1 (mu)M. Membrane vesicles from mungbean hypocotyls and pea epicotyls also released calcium in response to auxin. Calcium release was hormone specific since release occurred with auxin but not a weak or inactive auxin, and the release was tissue specific since release occurred with auxin responsive tissues but not with red cells, an auxin unresponsive tissue. The auxin promoted calcium release was not, however, ion specific. Hypocotyl membranes loaded with 54-manganese released manganese in an analogous manner to the released calcium, and excess manganese or magnesium inhibited the auxin-promoted calcium release. Heavy membranes were somewhat more responsive in the auxin-promoted calcium release. Calcium binding to low speed differential membrane pellets occurred at two sites. Auxin treated membranes had a 40 percent decreased number of high affinity calcium binding sites and a 20 percent decreased number of low affinity binding sites. Thus, the auxin-promoted decrease in membrane-associated calcium was a result of a decreased number of specific calcium binding sites on the membrane. (II) Endoplasmic Reticulum Biogenesis in Germinating Seeds: An Electron Microscopy Study. In most cell types, the investigation of de novo endoplasmic was hampered by the presence of existing endoplasmic reticulum. Aleurone cells in dry seeds, however, contained little endoplasmic reticulum making the investigation of the de novo synthesis of this organelle possible. Aleurone cells from wheat seeds (Triticum aestivum L.) were imbided in water and incubated with 1 (mu)M gibberellic acid. Imbided cells contained endoplasmic reticulum in vesicles or short segments. Cells incubated for 10 h developed extensive regions of stacked endoplasmic reticulum. By quantitative morphometry, the endoplasmic reticulum in the aleurone cells increased nearly 3-fold when comparing imbibed aleurone cells with the 10 h incubated cells. The cellular development was described by two periods. During the early period of incubation from 0 to 4 h, aleurone cells changed little morphologically. Following 4 h of incubation, a second period of development occurred. The aleurone cells underwent changes where lamellar bodies and globoids appeared and disappeared and endoplasmic reticulum increased. The first period was interpreted as a hydration and biosynthetic period for precursor molecules, while the second period was one of membrane synthesis. Aleurone cells also contained conspicuous lamellar bodies and globoids. They were found free in the cytoplasm or asociated with endoplasmic reticulum or protein bodies. Based on morphometric analysis, a precursor-product relationship existed between protein body-associated lamellar bodies and endoplasmic reticulum-associated lamellar bodies. No similar relationship could be proposed for globoids. Lamellar bodies were suggested to function in lipid transport and metabolism in an analogous manner to the function proposed for lamellar bodies in adipocytes.
Degree
Ph.D.
Subject Area
Botany
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