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Abscisic acid (ABA) is an important plant hormone that modulates seed germination and plant growth and stress responses, but its signaling remains poorly understood. We investigated the role of ROP10, a member of the Arabidopsis Rop subfamily of Rho GTPases, in ABA signaling. A null rop10 mutant exhibits enhanced responses to ABA in seed germination, root elongation, and stomatal closure assays and in the induction of expression of the transcription factor MYB2, but it shows wild-type levels of ABA and normal responses to other hormones. Consistently, transgenic expression of a constitutively active form of ROP10 reduces ABA inhibition of seed germination, whereas dominant-negative mutants of ROP10 enhance ABA response and partially suppress abi2. Furthermore, ABA specifically downregulates ROP10 transcription in root tips. ROP10 is localized to the plasma membrane (PM), and PM localization is crucial for its function. These results suggest that ROP10 is a PM-localized signaling molecule that is involved specifically in the negative regulation of ABA signaling.

We describe a family of stress-induced, developmentally regulated soybean genes for which cDNAs have been obtained from two different cultivars (Glycine max cv. Mandarin and Glycine max cv. Williams). The mRNAs corresponding to these cDNAs, called SAM22 and H4, respectively, accumulate predominantly in the roots of soybean seedlings but are present at high levels in the roots and leaves of mature plants. SAM22 accumulation is especially dramatic in senescent leaves. In addition, SAM22 accumulation can be induced on young leaves by wounding or by transpiration-mediated uptake of salicylic acid, methyl viologen, fungal elicitor, hydrogen peroxide or sodium phosphate (pH 6.9). Taken together, these data indicate that the genes corresponding to SAM22 and H4 are induced by various stresses and developmental cues. Southern blot analysis indicates that multiple copies of sequences related to SAM22 exist in the soybean genome. We also show that the nucleotide sequences of the cDNAs corresponding to SAM22 and H4 are 86% identical at the nucleotide level to each other and 70% identical at the amino acid level to the 'disease resistance response proteins' of Pisum sativum.

(Q.Z., M.P.R.)Arabidopsis (Arabidopsis thaliana) mutants lacking a functional ERA1 gene, which encodes the b-subunit of protein farnesyltransferase (PFT), exhibit pleiotropic effects that establish roles for protein prenylation in abscisic acid (ABA) signaling and meristem development. Here, we report the effects of T-DNA insertion mutations in the Arabidopsis GGB gene, which encodes the b-subunit of protein geranylgeranyltransferase type I (PGGT I). Stomatal apertures of ggb plants were smaller than those of wild-type plants at all concentrations of ABA tested, suggesting that PGGT I negatively regulates ABA signaling in guard cells. However, germination of ggb seeds in response to ABA was similar to the wild type. Lateral root formation in response to exogenous auxin was increased in ggb seedlings compared to the wild type, but no change in auxin inhibition of primary root growth was observed, suggesting that PGGT I is specifically involved in negative regulation of auxin-induced lateral root initiation. Unlike era1 mutants, ggb mutants exhibited no obvious developmental phenotypes. However, era1 ggb double mutants exhibited more severe developmental phenotypes than era1 mutants and were indistinguishable from plp mutants lacking the shared a-subunit of PFT and PGGT I. Furthermore, overexpression of GGB in transgenic era1 plants partially suppressed the era1 phenotype, suggesting that the relatively weak phenotype of era1 plants is due to partial redundancy between PFT and PGGT I. These results are discussed in the context of Arabidopsis proteins that are putative substrates of PGGT I.Protein prenylation involves the formation of a thioether bond between a farnesyl (C15) or geranylgeranyl (C20) group and a C-terminal Cys residue (Clarke, 1992;Zhang and Casey, 1996). This posttranslational modification promotes protein-membrane and, in many cases, protein-protein interactions. Three protein prenyltransferases are known to catalyze protein prenylation in plants and other eukaryotes: (1) a heterodimeric protein farnesyltransferase (PFT) that consists of a-and b-subunits and transfers a farnesyl group from farnesyl pyrophosphate (FPP) to proteins bearing a C-terminal CaaX motif, where ''C'' is Cys, ''a'' is often an aliphatic amino acid, and ''X'' is generally Met, Ala, Gln, Ser, or Cys; (2) a heterodimeric type I protein geranylgeranyltransferase (PGGT I) that shares a common a-subunit with PFT but possesses a distinct b-subunit and transfers a geranylgeranyl group from geranylgeranyl pyrophosphate (GGPP) to proteins bearing a C-terminal CaaX motif, where ''X'' is Leu; and (3) a heterotrimeric type II protein geranylgeranyltransferase (PGGT II or Rab PGGT) that consists of distinct a-and b-subunits, as well as a third subunit called the Rab escort protein, and transfers a geranylgeranyl group from GGPP to Rab protein substrates (Clarke, 1992;Zhang and Casey, 1996;Crowell, 2000). PFT subunits have been identified and characterized from pea (Pisum sativum; Yang et al., 1993;Qian et al., 1996), tomato (Lycopersicon ...