Interactions between sterol biosynthesis genes in embryonic development of Arabidopsis

Abstract

SummaryThe sterol biosynthesis pathway of Arabidopsis produces a large set of structurally related phytosterols including sitosterol and campesterol, the latter being the precursor of the brassinosteroids (BRs). While BRs are implicated as phytohormones in post-embryonic growth, the functions of other types of steroid molecules are not clear. Characterization of the fackel (fk) mutants provided the ®rst hint that sterols play a role in plant embryogenesis. FK encodes a sterol C-14 reductase that acts upstream of all known enzymatic steps corresponding to BR biosynthesis mutants. Here we report that genetic screens for fklike seedling and embryonic phenotypes have identi®ed two additional genes coding for sterol biosynthesis enzymes: CEPHALOPOD (CPH), a C-24 sterol methyl transferase, and HYDRA1 (HYD1), a sterol C-8,7 isomerase. We describe genetic interactions between cph, hyd1 and fk, and studies with 15-azasterol, an inhibitor of sterol C-14 reductase. Our experiments reveal that FK and HYD1 act sequentially, whereas CPH acts independently of these genes to produce essential sterols. Similar experiments indicate that the BR biosynthesis gene DWF1 acts independently of FK, whereas BR receptor gene BRI1 acts downstream of FK to promote post-embryonic growth. We found embryonic patterning defects in cph mutants and describe a GC±MS analysis of cph tissues which suggests that steroid molecules in addition to BRs play critical roles during plant embryogenesis. Taken together, our results imply that the sterol biosynthesis pathway is not a simple linear pathway but a complex network of enzymes that produce essential steroid molecules for plant growth and development.

Sterol Methyl Oxidases Affect Embryo Development via Auxin-Associated Mechanisms

“…4; Supplemental Figs. S7 and S8) were very 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). Second, a previous study showed that loss of function of both Arabidopsis CSDs, which are directly downstream of SMO2 (Supplemental Fig.…”

“…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).…”

“…It is also possible that the embryogenesis defects of these upstream sterol mutants are due to a basic requirement for wild-type membrane sterol composition for proper PIN auxin efflux facilitator positioning and auxin-dependent gene expression. Several reports have emphasized the importance of correct membrane sterol composition for PIN polar localization and auxin signaling (Jang et al, 2000;Schrick et al, 2000Schrick et al, , 2002Souter et al, 2002;Willemsen et al, 2003;Men et al, 2008). However, the results from the cpi1-1 mutant do not support this view.…”

“…Downstream mutants, such as dwf7/ste1, dwf5, and dwf1/dim, displayed phenotypes similar to BR-deficient mutants and could be rescued by BR application (Klahre et al, 1998;Choe et al, 1999aChoe et al, , 1999bChoe et al, , 2000. By contrast, upstream mutants, such as smt1/cph, fk, and hyd1, could not be rescued by either endpathway regular sterols or BR (Diener et al, 2000;Jang et al, 2000;Schrick et al, 2000Schrick et al, , 2002Souter et al, 2002). These data suggest that a steroid molecule, rather than BR, is required for embryogenesis, and this yetunidentified sterol could be an SBI derived from one of the biosynthetic steps between HYDRA1 and DWF7.…”

“…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.…”