The Identification of CVP1 Reveals a Role for Sterols in Vascular Patterning

Abstract

Vascular cell axialization refers to the uniform alignment of vascular strands. In the Arabidopsis cotyledon vascular pattern1 ( cvp1 ) mutant, vascular cells are not arranged in parallel files and are misshapen, suggesting that CVP1 has a role in promoting vascular cell polarity and alignment. Characterization of an allelic series of cvp1 mutations revealed additional functions of CVP1 in organ expansion and elongation. We identified CVP1 and found that it encodes STEROL METHYLTRANSFERASE2 (SMT2), an enzyme in the sterol biosynthetic pathway. SMT2 and the functionally redundant SMT3 act at a branch point in the pathway that mediates sterol and brassinosteroid levels. The SMT2 gene is expressed in a number of developing organs and is regulated by various hormones. As predicted from SMT2 enzymatic activity, the precursors to brassinosteroid are increased at the expense of sterols in cvp1 mutants, identifying a role for sterols in vascular cell polarization and axialization. INTRODUCTIONThe plant vascular system is formed progressively during embryogenesis and postembryonic development. In organ primordia, provascular cells appear among ground cells in hierarchical patterns that give rise to the patterns of veins (Nelson and Dengler, 1997). Provascular cells proliferate and elongate in axialized files that appear to depend on the intercellular polar transport of auxin through the developing tissue. Screens for pattern-defective mutants have identified several genes that regulate or influence this process. The monopteros mutation causes a discontinuous venation pattern associated with improper vascular differentiation (Przemeck et al., 1996) and corresponds to an auxin response transcription factor (Hardtke and Berleth, 1998).In gnom/emb30 mutants, which correspond to a brefeldin A-sensitive ADP-ribosylation factor guanine-nucleotide exchange factor (ARF-GEF), veins are unaxialized and patterned irregularly (Mayer et al., 1993;Shevell et al., 1994;Steinmann et al., 1999;Grebe et al., 2000;Koizumi et al., 2000). Additional venation pattern mutants have been described, such as the van mutants, lop1 / tornado1 , and scarface , but their gene products have not been identified (Carland and McHale, 1996;Cnops et al., 2000;Deyholos et al., 2000;Koizumi et al., 2000).We recently described a number of additional mutants with various effects on cotyledon vascular pattern (Carland et al., 1999). One of these, cotyledon vascular pattern1 ( cvp1 ), resembles the monopteros phenotype in that a discontinuous, poorly axialized venation pattern is formed. Here, we show that cvp1 corresponds to a defect in sterol biosynthesis.Derivatives of the sterol pathway in plants and animals include both membrane sterols and signaling steroids. Recently, the membrane sterol cholesterol was shown to influence signaling and vectorial processes through its key role in the organization of lipid rafts (Simons and Ikonen, 1997). Lipid rafts appear to provide scaffolding for protein localization and distribution, membrane trafficking, and membrane si...

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“…This alteration of sterol profile is consistent with the loss or reduction of SMT2 function and is similar to the situation described for co-suppressed 35S::SMT2 Arabidopsis plants (Schaeffer et al 2001), and cvp1 (Carland et al 2002) and frl1 (Hase et al 2005) mutants that have a mutation in SMT2. Similar alterations were observed in SMT3-RNAi lines (#1, #2, and #3), with increased levels of campesterol (1.8-fold), campestanol (1.9-fold), and brassicasterol (2.0-fold).…”

“…The Arabidopsis genome contains three distinct genes encoding sterol-C24-methyltransferases (SMTs) involved in sterol biosynthesis (Figure 4). SMT2 and the functionally redundant SMT3 mainly catalyze a second methyl transfer, as a branch point distinguishing the sterols from brassinosteroid precursors (Bouvier-Navé et al 1997;Carland et al 2002).…”

Arabidopsis ribosomal proteins control vacuole trafficking and developmental programs through the regulation of lipid metabolism

“…However, lipid biosynthesis pathways have been elucidated in the regulation of endomembrane trafficking of PIN proteins. In Arabidopsis, sphingolipids and sterols have been shown to be involved in the regulation of PINs and AUX1 to maintain their polar subcellular localization (24,28,38,39). Sterols have been implicated in trafficking of PIN2 in its membrane domain.…”

“…In this mutant, PIN2 endocytosis and root gravitropism was compromised as the result of altered sterol composition (27). Another mutant, cotyledon vascular pattern 1(cvp1), which is a knock-out mutant of STEROL METHYLTRANSFERASE2 (SMT2), a key enzyme in the sterol biosynthetic pathway, displayed abnormal vascular cell polarity and defectiveness in cell elongation (39,40). The results from ceramide mutants indicate that sphingolipids with very long acyl chains define a trafficking pathway with specific endomembrane compartments and polar auxin transport proteins (25).…”

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“…However, a recent study has shown that polarized localization of the AtPIN1 and AtPIN3 proteins requires STEROL METHYLTRANSFERASE1 (SMT1) function, possibly providing clues as to the origins of some of the developmental defects, especially regarding vascular development, which are observed in the smt1 (orc) mutant (Willemsen et al 2003). Interestingly the STEROL METHYLTRANSFERASE2 (SMT2/CVP1) gene has also been implicated in vascular patterning, suggesting that it may be involved in similar processes (Carland et al 2002).…”