Developmental expression of the cucumber <Emphasis Type="Italic">Cs-XTH1</Emphasis> and <Emphasis Type="Italic">Cs-XTH3</Emphasis> genes,encoding xyloglucan endotransglucosylase/hydrolases,can be influenced by mechanical stimuli

The expression of two genes encoding xyloglucan endotransglucosylase/hydrolases (XTHs), Cs-XTH1 and Cs-XTH3, was upregulated during the onset of cucumber somatic embryogenesis. As a means of characterising the developmental regulation of these genes, the activity of the respective upstream regulatory regions was investigated in seedlings and somatic embryos of Arabidopsis thaliana and Cucumis sativus. GUS assays revealed that both genes are under developmental control. In addition, elevated promoter activity was found in the tension-bearing regions of the plant and in response to touch and wounding, which is consistent with the existence of numerous stress-related cis elements in the 5′-regulatory regions. In vivo xyloglucan endotransglucosylase (XET) action assays were performed to gain an overview on the role of XTHs during somatic embryogenesis. The highest XET action was observed in the external cell layers of somatic embryos in the cotyledonary region and in the presumptive region of peg formation. Based on the results, we propose a dual mechanism (one developmental and the second adaptive) for the regulation of Cs-XTH1 and Cs-XTH3 activity wherein the developmental pattern can be modified by mechanical stimuli.     

Embryogenesis in flowering plants: Recent approaches and prospects

The overall architectural pattern of the mature plant is established during embryogenesis. Very little is known about the molecular processes that underlie embryo morphogenesis. Last decade has, nevertheless, seen a burst of information on the subject. The synchronous somatic embryogenesis system of carrot is largely being used as the experimental system. Information on the molecular regulation of embryogenesis obtained with carrot somatic embryos as well as observations on sandalwood embryogenic system developed in our laboratory are summarized in this review. The basic experimental strategy of molecular analysis mostly relied on a comparison between genes and proteins being expressed in embryogenic and non-embryogenic cells as well as in the different stages of embryogenesis. Events such as expression of totipotency of cells and establishment of polarity which are so critical for embryo development have been characterized using the strategy. Several genes have been identified and cloned from the carrot system. These include sequences that encode certain extracellular proteins (EPs) that influence cell proliferation and embryogenesis in specific ways and sequences of the abscisic acid (ABA) inducible late embryogenesis abundant (LEA) proteins which are most abundant and differentially expressed mRNAs in somatic embryos. That LEAs are expressed in the somatic embryos of a tree flora also is evidenced from studies on sandalwood. Several undescribed or novel sequences that are enhanced in embryos were identified. A sequence of this nature exists in sandalwood embryos was demonstrated using aCuscuta haustorial (organ-specific) cDNA probe. Somatic embryogenesis systems have been used to assess the expression of genes isolated from non-embryogenic tissues. Particular attention has been focused on both cell cycle and histone genes     

Two azuki bean XTH genes,VaXTH1 and VaXTH2, with similar tissue-specific expression profiles,are differently regulated by auxin

To gain insight into the functional diversity of the XTH (xyloglucan endotransglucosylase/hydrolase) gene family, we analyzed the expression profiles of two azuki bean genes, VaXTH1 and VaXTH2, which share a striking resemblance in their amino acid sequences. The two XTH genes exhibit essentially similar tissue-specific expression profiles, in that both mRNAs are found predominantly in the phloem fibers of growing internodes. However, their expression profiles are not identical. Whereas VaXTH1 is expressed in xylem cells in the basal part of the internode, little or no expression of VaXTH2 is found in the xylem. Furthermore, they exhibit spatially divergent RNA distribution profiles along the internode, VaXTH1 being expressed nearer to the top of the internode than VaXTH2. This indicates their temporally divergent expression profiles during development of the phloem fiber. Indole-3-acetic acid (IAA) up-regulates both of the mRNA levels. However, this effect of IAA on the VaXTH1 gene is nullified in 0.25 M mannitol, which prevents cell expansion without affecting auxin action per se. In contrast, the IAA-induced up-regulation of the VaXTH2 gene is not affected by mannitol. Furthermore, fusicoccin, which promotes acidification and growth, up-regulates VaXTH1 expression, but not VaXTH2 expression. Thus, the two XTH genes are committed to different steps of the cell wall dynamics in the same cell type at different stages of phloem fiber development, and are regulated by IAA in different ways.