No abstract

The syntaxin family of integral membrane proteins are thought to function as receptors for transport vesicles, with different isoforms of this family localized to various membranes throughout the cell. The yeast Pepl2

Soluble proteins are transported to the plant vacuole through the secretoty pathway via membrane-bound vesicles.Targeting of vesicles to appropriate organelles requires severa1 membrane-bound and soluble factors that have been characterized in yeast and mammalian systems. For example, the yeast PEPl2 protein is a syntaxin homolog that is involved in protein transport to the yeast vacuole. Previously, we isolated an Arabidopsis thaliana homolog of PEPl2 by functional complementation of the yeast pepl2 mutant. Antibodies raised against the cytoplasmic portion of AtPEP12 have been prepared and used for intracellular localization of this protein. Biochemical analysis indicates that AtPEP12 does not localize to the endoplasmic reticulum, Golgi apparatus, plasma membrane, or tonoplast in Arabidopsis plants; furthermore, based on biochemical and electron microscopy immunogold labeling analyses, AtPEP12 is likely to be localized to a post-Golgi compartment in the vacuolar pathway.

Plants exude a variety of substances through their roots, germinating seeds and aerial parts. Some of these released compounds seem to have an inhibitory effect against pathogens. The aim of this work was to investigate and identify antifungal proteins present in exudates from imbibed cowpea seeds (Vigna unguiculata (L.) Walp). The obtained exudation was analyzed in regard to specific protein activities by enzymatic or immunological assays for plant defense proteins, from 4 h to 48 h of seed imbibition. Our results show that cowpea seeds exudates present several defense related proteins characterized as b-1,3-glucanases, cystatins, vicilins and lipid transfer proteins (LTPs), as well as a storage vacuole membrane a-TIP protein, since the very first hours of imbibition. These exudates also have an ''in vitro'' inhibitory effect on the growth of the fungus Fusarium oxysporum f. sp. phaseoli. Our results suggest that seed exudates should promote seed protection from soil pathogens.

aERD2 and aSAR1 of Arabidopsis are functional homologs of yeast genes encoding proteins essential for endoplasmic reticulum (ER)-to-Golgi transport. The regulation of these secretory pathway genes in yeast, mammals, and plants is not known. High levels of expression of aERD2 and aSAR1 were observed in roots, flowers, and inflorescence stems, with the highest levels being detected in roots. The aSAR1 transcript levels were highest in young leaves and declined during leaf maturation. Low levels of aERD2 were detected in both young and fully mature leaves when compared with roots. In situ hybridization showed that trichomes accumulate more aERD2 transcript as the leaf expands, whereas aSAR1 is expressed equally in all leaf cell types. Treating plants with tunicamycin, a drug that blocks N-glycosylation in the ER, or with cold shock, known to block secretory protein transport, led to a marked accumulation of aERD2 and aSAR1 transcripts. The Arabidopsis ARF gene, which encodes a GTPase probably involved in Golgi vesicle traffic, was not affected by these treatments. This study is an essential first step toward understanding the regulation of genes that encode proteins involved in vesicular trafficking.

SummaryThe 2S albumin genes of Arabidopsis thallana are a model system to study gene expression during late embryogeneeis. The at2Sl gene has previously been shown to be expressed essentially in the embryo axis, unlike at2S2, which is expressed throughout the embryo. Hybrid promoter constructs between at2Sl and at2S2 were Introduced Into Arabidopsla end used to identify a cotyledon regulatory region necessary for 2S albumin expression in palisade paranchyma and specific epidermal cells. Other promoter sequences flanking this tisaue-speclflc promoter element were shown to control mRNA exprosalon levels independently of the mRNA distribution throughout the embryos. Certain hybrid promoters resulted in the alteration of the time course of expression in cotyledons. Differential expression of 2S albumin genes is dlscusaed in terms of layered cellular organization and mitotic activity throughout the embryo.

aERD2 and aSAR1 of Arabidopsis are functional homologs of yeast genes encoding proteins essential for endoplasmic reticulum (ER)-to-Golgi transport. The regulation of these secretory pathway genes in yeast, mammals, and plants is not known. High levels of expression of aERD2 and aSAR1 were observed in roots, flowers, and inflorescence stems, with the highest levels being detected in roots. The aSAR1 transcript levels were highest in young leaves and declined during leaf maturation. Low levels of aERD2 were detected in both young and fully mature leaves when compared with roots. In situ hybridization showed that trichomes accumulate more aERD2 transcript as the leaf expands, whereas aSAR1 is expressed equally in all leaf cell types. Treating plants with tunicamycin, a drug that blocks N-glycosylation in the ER, or with cold shock, known to block secretory protein transport, led to a marked accumulation of aERD2 and aSAR1 transcripts. The Arabidopsis ARF gene, which encodes a GTPase probably involved in Golgi vesicle traffic, was not affected by these treatments. This study is an essential first step toward understanding the regulation of genes that encode proteins involved in vesicular trafficking.

In wild-type Arabidopsis seeds the 2S albumin seed protein gene family members are differentially expressed. In this work it is shown that as predicted by the wild type situation, the at2S2 promoter is much more effective than that of the at2S1 gene in the expression of a transgene. However, unexpectedly high expression levels were obtained using a construct in which the transgene was present as a tandem duplication in the T-DNA. Neither in this case nor in homozygous plants with either construct was epigenetic silencing observed. While transgene mRNA levels were of the same order of magnitude as the endogenous at2S2 gene, protein levels were much lower.