We studied the expression of the four genes encoding 2S albumin seed storage proteins (at2S1 to at2S4) in Arabidopsis thaliana. All four genes followed similar temporal profiles throughout development, but at2S2 and at2S3 were expressed at significantly higher levels than at2S1 or at2S4. In situ hybridization showed that at2S2 to at2S4 mRNAs were present throughout the embryo, whereas at2S1 was expressed at levels similar to at2S2 and at2S3 in the embryo axis but at only insignificant levels in the cotyledons. The different members of the gene family are, thus, likely to be regulated by different combinations of cis-acting elements, but it cannot be ruled out that post-transcriptional factors play a role. We studied the effect of enlarging the gene family by introducing an extra, nearly identical gene driven by the promoter of at2S1. The data were consistent with a model in which the expression of at2S2 to at2S4 is not affected by that of at2S1, and in which, at least at low copy numbers of the introduced gene, there is no limit on the overall amount of RNA that the at2S gene family can produce.
The rhal gene from Arabidopsis encodes a small GTP binding protein belonging to the YpVRab family. Transgenic Arabidop-sis plants containing the promoter region of the rhal gene fused to the P-glucuronidase (gus) reporter gene revealed gus expression limited mainly to the guard cells of stomata, the stipules, and the mot tip of young plants. In flowering plants, expression was found predominantly in the receptacle and in guard cells of the different flower organs. High GUS activity could also be seen in callus tissue and developing seeds. No detectable activity was present in other plant tissues ; activity could not be induced by various treatments. GUS activity was visualized histochemically using both 5-bromo-4-chloro-3-indolyl P-D-glucuronide and a newly developed GUS substrate: Sudan Il-8-glucuronide. The latter precipitates as red crystals at the site of GUS activity. Results obtained by the gus analysis were confirmed by whole-mount mRNA in situ hybridization. A hypothesis for the function of the Rhal protein is discussed.
When sedentary endoparasitic nematodes infect plants, they induce complex feeding sites within the root tissues of their host. To characterize cell wall changes induced within these structures at a molecular level, we studied the expression of an extensin gene (coding for a major structural cell wall protein) in nematode-infected tobacco roots. Extensin gene expression was observed to be induced very early upon infection. This induction was weak, transient, and probably due to wounding during penetration and migration of the tobacco cyst nematode Globodera tabacum ssp solanacea-rum. In contrast, high extensin gene expression was observed during the whole second larval stage (an ~2-week-long phase of establishment of the feeding site) of the root knot nematode Meloidogyne javanica. During later stages of this interaction, expression gradually decreased. Extensin gene expression was found in at least three different tissues of the gall. We propose that distinct mechanisms lead to induced expression in these different cell types. The significance of these results for the understanding of plant-nematode interactions as well as the function of structural cell wall proteins, such as extensin, is discussed.
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