The members of the Toc159 family of GTPases act as the primary receptors for the import of nucleus-encoded preproteins into plastids. Toc159, the most abundant member of this family in chloroplasts, is required for chloroplast biogenesis (Bauer, J., K. Chen, A. Hiltbunner, E. Wehrli, M. Eugster, D. Schnell, and F. Kessler. 2000. Nature. 403:203–207) and has been shown to covalently cross-link to bound preproteins at the chloroplast surface (Ma, Y., A. Kouranov, S. LaSala, and D.J. Schnell. 1996. J. Cell Biol. 134:1–13; Perry, S.E., and K. Keegstra. 1994. Plant Cell. 6:93–105). These reports led to the hypothesis that Toc159 functions as a selective import receptor for preproteins that are required for chloroplast development. In this report, we provide evidence that Toc159 is required for the import of several highly expressed photosynthetic preproteins in vivo. Furthermore, we demonstrate that the cytoplasmic and recombinant forms of soluble Toc159 bind directly and selectively to the transit peptides of these representative photosynthetic preproteins, but not representative constitutively expressed plastid preproteins. These data support the function of Toc159 as a selective import receptor for the targeting of a set of preproteins required for chloroplast biogenesis.

Multiple forms of phospholipase D (PLD) were activated in response to wounding, and the expressions of PLD ␣ , PLD ␤ , and PLD ␥ differed in wounded Arabidopsis leaves. Antisense abrogation of the common plant PLD , PLD ␣ , decreased the wound induction of phosphatidic acid, jasmonic acid (JA), and a JA-regulated gene for vegetative storage protein.Examination of the genes involved in the initial steps of oxylipin synthesis revealed that abrogation of the PLD ␣ attenuated the wound-induced expression of lipoxygenase 2 (LOX2) but had no effect on allene oxide synthase (AOS) or hydroperoxide lyase in wounded leaves. The systemic induction of LOX2, AOS, and vegetative storage protein was lower in the PLD ␣ -suppressed plants than in wild-type plants, with AOS exhibiting a distinct pattern. These results indicate that activation of PLD mediates wound induction of JA and that LOX2 is probably a downstream target through which PLD promotes the production of JA. INTRODUCTIONJasmonic acid (JA) and related compounds are a new class of plant hormones that play an important role in regulating many cellular processes, such as wound and defense responses (Farmer and Ryan, 1992;Bell et al., 1995;Creelman and Mullet, 1997;McConn et al., 1997). The production of JA is a tightly regulated process, and the concentrations of JA in unperturbed plant tissues are often very low. However, JA accumulates in wounded plants or in plants and cultured cells treated with pathogen elicitors; it acts as a signal activating the expression of various genes, such as proteinase inhibitors, thionin, and enzymes in phytoalexin metabolism . The pathway for de novo JA biosynthesis, beginning with free ␣ -linolenic acid, has been well elucidated (Vick, 1993;Creelman and Mullet, 1997; also see Figure 1). But when and how linolenic acid is made available for JA synthesis is not well understood. Linolenic acid, the most abundant fatty acid in leaves, is mostly present in esterified glycerolipid form (Browse and Somerville, 1991). Free fatty acids are not generally found in large amounts in healthy, intact plant cells. The release of linolenic acid from membranes has been thought to be an important step in controlling JA synthesis. An increase in free linolenic acid was observed in cultured cells of several plant species after treatment with fungal wall elicitors (Gundlach et al., 1992) and in wounded plants (Conconi et al., 1996;Ryu and Wang, 1998). A phospholipase A (PLA)-like activity has been proposed to mediate the release of linolenic acid from membranes (Farmer and Ryan, 1992), and the presence of such a wound-inducible PLA activity has been noted in tomato and other plant species (Lee et al., 1997;Narváez-Vásquez et al., 1999).Recent studies have suggested that activation of phospholipase D (PLD) also may play an important role in mediating wound-induced lipid hydrolysis Wang, 1996, 1998;Lee et al., 1997). PLD hydrolyzes phospholipids at the terminal phosphoesteric bond, generating phosphatidic acid (PA) and free head groups, such as choline . T...

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Multiple forms of phospholipase D (PLD) were activated in response to wounding, and the expressions of PLD ␣ , PLD ␤ , and PLD ␥ differed in wounded Arabidopsis leaves. Antisense abrogation of the common plant PLD , PLD ␣ , decreased the wound induction of phosphatidic acid, jasmonic acid (JA), and a JA-regulated gene for vegetative storage protein.Examination of the genes involved in the initial steps of oxylipin synthesis revealed that abrogation of the PLD ␣ attenuated the wound-induced expression of lipoxygenase 2 (LOX2) but had no effect on allene oxide synthase (AOS) or hydroperoxide lyase in wounded leaves. The systemic induction of LOX2, AOS, and vegetative storage protein was lower in the PLD ␣ -suppressed plants than in wild-type plants, with AOS exhibiting a distinct pattern. These results indicate that activation of PLD mediates wound induction of JA and that LOX2 is probably a downstream target through which PLD promotes the production of JA. INTRODUCTIONJasmonic acid (JA) and related compounds are a new class of plant hormones that play an important role in regulating many cellular processes, such as wound and defense responses (Farmer and Ryan, 1992;Bell et al., 1995;Creelman and Mullet, 1997;McConn et al., 1997). The production of JA is a tightly regulated process, and the concentrations of JA in unperturbed plant tissues are often very low. However, JA accumulates in wounded plants or in plants and cultured cells treated with pathogen elicitors; it acts as a signal activating the expression of various genes, such as proteinase inhibitors, thionin, and enzymes in phytoalexin metabolism . The pathway for de novo JA biosynthesis, beginning with free ␣ -linolenic acid, has been well elucidated (Vick, 1993;Creelman and Mullet, 1997; also see Figure 1). But when and how linolenic acid is made available for JA synthesis is not well understood. Linolenic acid, the most abundant fatty acid in leaves, is mostly present in esterified glycerolipid form (Browse and Somerville, 1991). Free fatty acids are not generally found in large amounts in healthy, intact plant cells. The release of linolenic acid from membranes has been thought to be an important step in controlling JA synthesis. An increase in free linolenic acid was observed in cultured cells of several plant species after treatment with fungal wall elicitors (Gundlach et al., 1992) and in wounded plants (Conconi et al., 1996;Ryu and Wang, 1998). A phospholipase A (PLA)-like activity has been proposed to mediate the release of linolenic acid from membranes (Farmer and Ryan, 1992), and the presence of such a wound-inducible PLA activity has been noted in tomato and other plant species (Lee et al., 1997;Narváez-Vásquez et al., 1999).Recent studies have suggested that activation of phospholipase D (PLD) also may play an important role in mediating wound-induced lipid hydrolysis Wang, 1996, 1998;Lee et al., 1997). PLD hydrolyzes phospholipids at the terminal phosphoesteric bond, generating phosphatidic acid (PA) and free head groups, such as choline . T...

No abstract