Deduced amino acid sequences encoded by the cDNAs related to the MIP gene family from Nicotiana excelsior were characterized. Phylogenetic characterization of the products of corresponding genes named NeMip1, NeMip2, and NeMip3 strongly suggested that they are water channel proteins localized in the plasma membrane. Organ specificity of the gene expression was examined in leaves, roots, and reproductive organs. NeMip1 was expressed in roots and reproductive organs; however, it was hardly detectable in leaves. Two other genes, NeMip2 and NeMip3, were expressed in all of organs examined. mRNA accumulation from the genes was investigated in leaves under salt- and drought-stresses. The results demonstrated that mRNA accumulation from all three genes increased under salt- and drought-stresses within one day. However, they showed different accumulation patterns. In addition to their up-regulation under salt- and drought-stresses, daily changes in NeMip2 and NeMip3 mRNA accumulation was observed under unstressed conditions in leaves.
The response of developing maize (Zea mays L.) endosperm to elevated levels of abscisic acid (ABA) was investigated. Maize kernels and subtending cob sections were excised at 5 days after pollination (DAP) and placed in culture with or without 90 micromolar (±)-ABA in the medium. A decreased number of cells per endosperm was observed at 10 DAP (and later sampling times) in kernels cultured in medium containing ABA from 5 DAP, and in kernels transferred at 8 DAP to medium containing ABA, but not in kernels transferred at 11 DAP to medium containing ABA. The number of starch granules per endosperm was decreased in some treatments, but the reduction, when apparent, was comparable to the decreased number of endosperm cells. The effect on endosperm fresh weight was slight, transient, and appeared to be secondary to the effect on cell number. Mature endosperm dry weight was reduced when kemels were cultured continuously in medium containing ABA. Endosperm (+)-ABA content of kernels cultured in 0, 3, 10, 30, 100, or 300 micromolar (±)-ABA was measured at 10 DAP by indirect ELISA using a monoclonal antibody. Content of (+)-ABA in endosperms correlated negatively (R = -0.92) with endosperm cell number. On the basis of these studies we propose that during early kernel development, elevated levels of ABA decrease the rate of cell division in maize endosperm which, in turn, could limit the storage capacity of the kernel.
The influence of osmoticum treatments on early kernel development of maize (Zea mays L.) was studied using an in vitro culture method. Kernels with subtending cob sections were placed in culture at 5 days after pollination. Sucrose (0.29, 0.44, or 0.58 molar) and sorbitol (0, 0.15, or 0.29 molar) were used to obtain six media with water potentials of -1.1, -1.6, or -2.0 megapascals. Kernel water potential declined in correspondence with the water potential of the medium; however, fresh weight growth was not significantly inhibited from 5 to 12 days after pollination. In stress treatments with media water potentials of -1.6 or -2.0 megapascals, endosperm tissue accumulated water and solutes from 10 and 12 days after pollination at a rate similar to or greater than that of the control (-1.1 megapascals). In contrast, endosperm cell division was inhibited in all treatments relative to control. At 10 days after pollination, endosperm sucrose concentration was greater in two of the -2.0 megapascal treatments with 0.44 or 0.58 molar media sucrose compared to control kernels cultured in 0.29 molar sucrose at -1.1 megapascals. Significant increases in abscisic acid content per gram of fresh weight were detected in two -2.0 megapascal treatments (0.29 molar sucrose plus 0.29 molar sorbitol and 0.58 molar sucrose) at 10 days after pollination. We conclude that in cultured maize kernels, endosperm cell division was more responsive than fresh weight accumulation to low water potential treatments. Data were consistent with mechanisms involving abscisic acid or lowered tissue water potential, or an interaction of the two factors.also be considered (reviewed by Kermode [11]). Perhaps the effects of ABA observed on endosperm cell division could be duplicated or attenuated by altered solute concentration. In support of this possibility, studies have shown that the effects of ABA on embryo development can be mimicked to some extent by increased solute concentration (1,4,12). However, in some systems, such responses to solute concentration require a concomitant rise in endogenous ABA (21,25). Thus, to evaluate the effects of water deficit on endosperm development, both tissue water status (0', r, Op) and ABA levels need to be considered.In maize kemel culture, the level of carbohydrates available to the developing kemel is controlled by medium composition. In this study, we altered the i, of the medium by adding increments of sucrose and/or sorbitol. Sorbitol was used to impose the /w treatments in an attempt to separate the effects of increased carbohydrate nutrition from the effects of decreased 0,. Individual kemels with a small piece of subtending cob tissue were placed on the media and cultured for 3 to 7 d. This method excludes most matemal tissue, thereby decreasing import of ABA from leaves and roots. The goals of the present studies were to assess the extent to which endosperm cell division and FW accumulation respond to elevated osmoticum concentration and to evaluate the potential role of changes in tissue wat...
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