hydra Mutants of Arabidopsis Are Defective in Sterol Profiles and Auxin and Ethylene Signaling

Abstract

The hydra mutants of Arabidopsis are characterized by a pleiotropic phenotype that shows defective embryonic and seedling cell patterning, morphogenesis, and root growth. We demonstrate that the HYDRA1 gene encodes a ⌬ 8-⌬ 7 sterol isomerase, whereas HYDRA2 encodes a sterol C14 reductase, previously identified as the FACKEL gene product. Seedlings mutant for each gene are similarly defective in the concentrations of the three major Arabidopsis sterols. Promoter::reporter gene analysis showed misexpression of the auxin-regulated DR5 and ACS1 promoters and of the epidermal cell file-specific GL2 promoter in the mutants. The mutants exhibit enhanced responses to auxin. The phenotypes can be rescued partially by inhibition of auxin and ethylene signaling but not by exogenous sterols or brassinosteroids. We propose a model in which correct sterol profiles are required for regulated auxin and ethylene signaling through effects on membrane function. INTRODUCTIONSterols are essential components of fungal, plant, and animal membranes. They regulate fluidity and interact with lipids and proteins within the membrane, and they are the precursors for the brassinosteroid (BR) hormones in plants (Hartmann, 1998). The sterol biosynthetic pathway in plants, therefore, can be viewed as comprising two parts: one branch produces the bulk membrane sterols (the principal sterols in Arabidopsis being stigmasterol, campesterol, and sitosterol), and the second part represents the BR synthesis branch. Sterol biosynthesis has been well characterized in yeast, supported by a powerful system of genetic analysis. In animals, and more recently in plants, sterol biosynthetic enzyme function has been confirmed via the functional complementation of yeast mutants (Gachotte et al., 1996). Functional analysis of sterol function in plants has involved a range of approaches, but recently, genetic studies have provided useful information on the requirement for particular enzymes in sterol and BR biosynthesis and, for BRs, perception and signal transduction (Clouse, 2000;Diener et al., 2000;Schaeffer et al., 2001).In animals, sterols appear to be important to maintain correct cell-signaling activities. For example, drugs such as the ligand SR31747A, which inhibits the activity of the receptor (emopamil binding protein [EBP], which has ⌬ 8-⌬ 7 sterol isomerase activity), cause defects in a diversity of cellular processes, including the inhibition of mammalian lymphocyte proliferation in response to mitogens (Derocq et al., 1995) and the inhibition of graft rejection in mouse via the modulation of gene expression (Carayon et al., 1995), and they may influence lipoprotein functions leading to immunosuppressive effects (Dussossoy et al., 1999). In plants, a lack of detailed pharmacological studies has precluded analogous investigations of the role of sterols in plant cell biology.However, mutational and transgenic studies have given new insight into the roles of sterols in plant development. sterol methyltransferase1 ( smt1 ) mutants accumulate cholesterol...

Sterol Methyl Oxidases Affect Embryo Development via Auxin-Associated Mechanisms

“…4, B-H). These arrested embryos showed patterning defects similar to those of the fk, hyd1, and smt1/cph mutants (Diener et al, 2000;Jang et al, 2000;Schrick et al, 2000Schrick et al, , 2002Souter et al, 2002;Willemsen et al, 2003) and did not develop the characteristic heart shape (Supplemental Fig. S7A).…”

“…Because we were unable to obtain 24-ethyl lophenol, we could not test its effects on Arabidopsis growth. In any case, modification of the sterol composition observed in smo2-1/+ smo2-2 and smo2-1 smo2-2/+ mutants is probably responsible for the observed phenotype, because the proper membrane sterol composition has already been shown to be essential for correct PIN auxin efflux facilitator positioning and auxin-dependent developmental processes (Souter et al, 2002;Willemsen et al, 2003;Men et al, 2008;Pullen et al, 2010). SMO2-1 and SMO2-2 share 91% amino acid sequence identity, and both can rescue the yeast erg25 mutant (Darnet et al, 2001;Darnet and Rahier, 2004), suggesting that they exert similar functions in yeast.…”

“…However, increasing evidence indicates that plant sterols per se can act as signaling molecules. Plant sterol biosynthetic mutants, such as smt1/cph, cpi1-1, cyp51A2, fackel (fk), and hyd1, could not be rescued by BR application (Diener et al, 2000;Jang et al, 2000;Schrick et al, 2000Schrick et al, , 2002Souter et al, 2002;Willemsen et al, 2003;Kim et al, 2005;Men et al, 2008). Sterols were found to have intimate interactions with plant hormones, such as auxin and ethylene.…”

PIN Polar Targeting

“…Depletion of cholesterol, the main animal sterol, decreases the polar delivery of target proteins (Keller and Simons, 1998), while depletion of plant sterols leads to cell polarity defects followed by reduced auxin transport and auxin-related developmental defects. Studies carried out on sterol-deficient orc and cyclopropylsterol isomerase1 (cpi1) mutants have detected defects in the polar localization of PIN and AUX1 proteins, indicating that the polar delivery of cargos in plants also depends on the sterol composition of the PM (Souter et al, 2002;Grebe et al, 2003;Willemsen et al, 2003;Kleine-Vehn et al, 2006). Recent work has highlighted the essential role of sterols in the reiteration of PIN polarity after the division of polarized cells (Men et al, 2008).…”

Murata, N., Yamada, M., Nishida, I., Okuyama, H., Sekiya, J. and Hajime, W. (eds). Advanced research on plant lipids

“…This is important because as more Arabidopsis mutants are characterized, some researchers discover (usually to their surprise) that they have become lipidologists! This can be exempli®ed by the discovery of the key role of sterols in Arabidopsis embryo development, with the characterization of the hydra1 and fackel/hydra2 mutants as defective in D 8 -D 7 sterol isomerase and sterol C14 reductase, respectively (Souter et al, 2002). Other examples are the emerging role of sphingolipids in signalling networks (Coursol et al, 2003), and lipid transfer proteins involved in systemic resistance to pathogen attack (Maldonado et al, 2002).…”