No abstract

Mutations in the Arabidopsis ETHYLENE-INSENSITIVE3 (EIN3) gene severely limit a plant's response to the gaseous hormone ethylene. ein3 mutants show a loss of ethylene-mediated effects including gene expression, the triple response, cell growth inhibition, and accelerated senescence. EIN3 acts downstream of the histidine kinase ethylene receptor, ETR1, and the Raf-like kinase, CTR1. The EIN3 gene encodes a novel nuclear-localized protein that shares sequence similarity, structural features, and genetic function with three EIN3-LIKE (EIL) proteins. In addition to EIN3, EIL1 orEIL2 were able to complement ein3, suggesting their participation in the ethylene signaling pathway. Overexpression of EIN3 or EIL1 in wild-type or ethylene-insensitive2 plants conferred constitutive ethylene phenotypes, indicating their sufficiency for activation of the pathway in the absence of ethylene.

The response of Arabidopsis thaliana etiolated seedlings to the plant hormone ethylene is a conspicuous phenotype known as the triple response. We have identified genes that are required for ethylene perception and responses by isolating mutants that fail to display a triple response in the presence of exogenous ethylene. Five new complementation groups have been identified. Four of these loci, designated ein4, ein5, ein6 and ein7, are insensitive to ethylene. The fifth complementation group, eir1, is defined by a novel class of mutants that have agravitropic and ethylene-insensitive roots. Double-mutant phenotypes have allowed the positioning of these loci in a genetic pathway for ethylene signal transduction. The ethylene-response pathway is defined by the following loci: ETR1, EIN4, CTR1, EIN2, EIN3, EIN5, EIN6, EIN7, EIR1, AUX1 and HLS1. ctr1-1 is epistatic to etr1-3 and ein4, indicating that CTR1 acts after both ETR1 and EIN4 in the ethylene-response pathway. Mutations at the EIN2, EIN3, EIN5, EIN6 and EIN7 loci are all epistatic to the ctr1 seedling phenotype. The EIR1 and AUX1 loci define a root-specific ethylene response that does not require EIN3 or EIN5 gene activity. HLS1 appears to be required for differential cell growth in the apical hook. The EIR1, AUX1 and HLS1 genes may function in the interactions between ethylene and other plant hormones that occur late in the signaling pathway of this simple gas.

Four rice (Oryza sativa) genomic actin genes have been characterized. The rice actin genes show a conservation of intron number and position that is characteristic of plant actins. Sequence comparisons revealed that the plant actins generally have a high degree of inter- and intraspecific sequence heterogeneity. However, one rice actin gene has a much higher degree of nucleotide sequence identity to a previously isolated actin sequence from Arabidopsis thaliana than to any other plant actin gene. This leads us to suggest that the two sequences may represent functionally homologous genes which arose from an ancient actin gene lineage that was separated by the divergence of the dicot and monocot plants. Genomic DNA blot analysis showed that the rice actin gene family contains at least eight unique members. RNA hybridization analysis revealed that individual rice actin genes can display different patterns of transcript accumulation. The observed differences in sequence and transcript accumulation patterns suggest that the individual rice actin genes may differ in their transcriptional regulation and/or cellular function.

We have isolated and sequenced a full-length cDNA clone containing information for the rice actin gene RAc1. Transcript terminus mapping and sequence alignment between the RAc1 cDNA clone and a previously isolated RAc1 genomic clone were used to determine the structure of the RAc1 gene. This allowed us to make the first complete structural characterization of a plant actin gene. The analysis revealed the presence of a 5'-noncoding exon, separated by an intron, from the first translated exon of the RAc1 gene. This is one of the few reported cases of a plant gene containing such a 5'-noncoding exon. Sequence comparison between the previously isolated plant actin genes suggests that such an exon may be a common feature of plant actin gene structure. The present study also confirms that the rice actin gene family is composed of at least eight unique members.