SummaryA rice nuclear gene, Rf-1, restores the pollen fertility disturbed by the BT-type male sterile cytoplasm, and is widely used for commercial seed production of japonica hybrid varieties. Genomic fragments carrying Rf-1 were identi®ed by conducting chromosome walking and a series of complementation tests. Isolation and analysis of cDNA clones corresponding to the fragments demonstrated that Rf-1 encodes a mitochondrially targeted protein containing 16 repeats of the 35-aa pentatricopeptide repeat (PPR) motif. Sequence analysis revealed that the recessive allele, rf-1, lacks one nucleotide in the putative coding region, presumably resulting in encoding a truncated protein because of a frame shift. Rice Rf-1 is the ®rst restorer gene isolated from cereal crops that has the property of reducing the expression of the cytoplasmic male sterility (CMS)-associated mitochondrial gene like many other restorer genes. The present ®ndings may facilitate not only elucidating the mechanisms of male sterility by the BT cytoplasm and its restoration by Rf-1 but also isolating other restorer genes from cereal crops, especially rice.
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.
After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.Ethylene regulates a variety of physiological and biochemical processes in plants, such as fruit ripening, senescence, abscission, sex determination, root initiation, and cell elongation (Abeles et al., 1992). Ethylene also regulates the size and stature of plants in response to various environmental cues (Abeles, 1973;Kieber, 1997). Its effects on dark-grown seedling development, described initially as the triple response, include increased radial growth of the root and stem, reduced root and stem elongation, and abnormal horizontal stem growth (Neljubow, 1901;Knight et al., 1910). The term triple response, as it is now commonly used, includes exaggeration in the curvature of the apical hook instead of abnormal horizontal growth (Bleecker et al., 1988;Guzman and Ecker, 1990 and references therein). The triple response of Arabidopsis has been exploited for characterizing the ethylene signal transduction system by identifying numerous mutants that either phenocopy the triple response in the absence of exogenous ethylene or fail to respond properly to ethylene (Guzman and Ecker, 1990).The dramatic stimulation of radial swelling of stems and roots by ethylene has led to physiological studies and cellular analyses of this phenomenon (Burg and Burg, 1966;Lang et al., 1982;Bleecker et al., 1988;Abeles et al., 1992;Baskin and Williamson, 1992;Kieber, 1997). Depending on the timing of ethylene's application, the physiological condition, seedling age, and the plant species concerned, either promotive (Ku et al., 1970;Lehman et al., 1996;Smalle et al., 1997) or inhibitory (Kieber et al., 1993) effects on organ elongation have been reported. For young developing dicot seedlings, ethylene has ...
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