Abstract
Sterols are integral components of membrane lipid bilayers in eukaryotic organisms and serve as precursors to steroid hormones in vertebrates and brassinosteroids (BR) in plants. In vertebrates, cholesterol is the terminal sterol serving both indirect and direct roles in cell signaling. Plants synthesize a mixture of sterols including cholesterol, sitosterol, campesterol, and stigmasterol but the signaling role for the free forms of individual plant sterols is unclear. Since stigmasterol is the terminal sterol in the sitosterol branch and produced from a single enzymatic step, modifying stigmasterol concentration may shed light on its role in plant metabolism. Although Arabidopsis has been the model of choice to study sterol function, the functional redundancy of AtCYP710A genes and the presence of brassicasterol may hinder our ability to test the biological function of stigmasterol. We report here the identification and characterization of ZmCYP710A8, the sole maize C-22 sterol desaturase involved in stigmasterol biosynthesis and the identification of a stigmasterol-free Zmcyp710a8 mutant. ZmCYP710A8 mRNA expression pattern correlated with transcripts for several sterol biosynthesis genes and loss of stigmasterol impacted sterol composition. Exogenous stigmasterol also had a stimulatory effect on mRNA for ZmHMGR and ZmSMT2. This demonstrates the potential of Zmcyp710a8 in understanding the role of stigmasterol in modulating sterol biosynthesis and global cellular metabolism. Several amino acids accumulate in the Zmcyp710a8 mutant, offering opportunity for genetic enhancement of nutritional quality of maize. Other cellular metabolites in roots and shoots of maize and Arabidopsis were also impacted by genetic modification of stigmasterol content. Yet lack of obvious developmental defects in Zmcyp710a8 suggest that stigmasterol might not be essential for plant growth under normal conditions. Nonetheless, the Zmcyp710a8 mutant reported here is of great utility to advance our understanding of the additional roles of stigmasterol in plant metabolism. A number of biological and agronomic questions can be interrogated using this tool such as gene expression studies, spatio-temporal localization of sterols, cellular metabolism, pathway regulation, physiological studies, and crop improvement.
Highlights
Sterols are important components of plasma membranes in eukaryotic organisms
Identification of a Maize Sterol C-22 Desaturase Involved in Stigmasterol Biosynthesis
These findings suggested that ZmCYP710A8 may encode the maize sterol C-22 desaturase
Summary
Sterols are important components of plasma membranes in eukaryotic organisms They are precursors for synthesis of steroid hormones such as testosterone, estrogen, glucocorticoids and mineral corticoids in mammals, ecdysteroids in insects and crustaceans, antheridiol and oogoniol (mating hormones of fungi), and BR in plants (Fujioka et al, 1997; Noguchi et al, 1999; Nomura et al, 1999; Friedrichsen and Chory, 2001; Bishop and Koncz, 2002; Clouse, 2002). Plants produce a mixture of sterols including cholesterol, sitosterol, stigmasterol, and campesterol, with sitosterol being the most abundant (Hartmann, 1998, 2004; Chappell, 2002; Clouse, 2002; Schrick et al, 2002; Lindsey et al, 2003; Schaller, 2003). Sitosterol can be transformed to stigmasterol via a reaction catalyzed by cytochrome P450 CYP710A sterol C-22 desaturase (Benveniste, 2002; Morikawa et al, 2006; Figure 1)
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