Cell Wall Alterations in the Arabidopsis emb30 Mutant

Abstract

The Arabidopsis EMB30 gene is essential for controlling the polarity of cell growth and for normal cell adhesion during seedling development. In this article, we show that emb30 mutations also affect the growth of undifferentiated plant cells and adult tissues. EMB30 possesses a Sec7 domain and, based on similarities to other proteins, presumably functions in the secretory pathway. The plant cell wall depends on the secretory pathway to deliver its complex polysaccharides. We show that emb30 mutants have a cell wall defect that sometimes allows material to be deposited into the interstitial space between cells instead of being restricted to cell corners. In addition, pectin, a complex polysaccharide important for cell adhesion, appears to be abnormally localized in emb30 plants. In contrast, localization of epitopes associated with xyloglucan or arabinogalactan was similar in wild-type and emb30 tissues, and the localization of a marker molecule to vacuoles appeared normal. Therefore, emb30 mutations do not cause a general defect in the secretory pathway. Together, these results suggest that emb30 mutations result in an abnormal cell wall, which in turn may account for the defects in cell adhesion and polar cell growth control observed in the mutants. INTRODUCTIONIn Arabidopsis, the normal body pattern of a seedling is determined during embryogenesis. Mature embryos contain a shoot apical meristem, two cotyledons, a hypocotyl, and a root apical meristem. Several genes affecting this pattern formation have been cloned. One of these genes, EMB30 , is required for the formation of roots and inflorescence stems and encodes a protein similar to the yeast Sec7 protein (Sec7p), which functions in the secretory pathway (Shevell et al., 1994). knolle mutants have an abnormal epidermal cell layer and are defective in cytokinesis. The KNOLLE protein sequence shares similarity with syntaxins, which are also involved in the secretory pathway (Lukowitz et al., 1996). The SHOOT MERI-STEMLESS gene is required for the formation of a shoot apical meristem and encodes a homeodomain protein (Long et al., 1996). MONOPTEROS affects formation of the vascular system and the body pattern; it encodes a putative transcription factor (Hardtke and Berleth, 1998).The fate of a cell in plants is largely defined by its position. Except during pollen tube formation, cell migration usually does not occur during plant development. Therefore, the position of a cell is determined by controlling patterns of cell division and cell expansion. Cell morphogenesis and differentiation, in turn, are affected by the interactions of a plant cell with its neighbors (Lyndon, 1990). Plant cells communicate with each other using cytoplasmic connections called plasmodesmata. In addition, plant cells adhere to each other via the middle lamella, a layer of the cell wall. The complex polysaccharide pectin is a major component of the middle lamella and is thought to be important in cell adhesion. Pectin is also found in other parts of the cell wall and can exist in ...

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