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KMID : 0578320120340010077
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Molecules and Cells
2012 Volume.34 No. 1 p.77 ~ p.84
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Overexpression of 3β-Hydroxysteroid Dehydrogenases/C-4 Decarboxylases Causes Growth Defects Possibly Due to Abnormal Auxin Transport in Arabidopsis
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Kim Bo-Kyung
Kim Gyu-Sik
Shozo Fujioka
Suguru Takatsuto
Choe Sung-Hwa
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Abstract
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Sterols play crucial roles as membrane components and precursors of steroid hormones (e.g., brassinosteroids, BR). Within membranes, sterols regulate membrane per-meability and fluidity by interacting with other lipids and proteins. Sterols are frequently enriched in detergent-insoluble membranes (DIMs), which organize molecules involved in specialized signaling processes, including auxin transporters. To be fully functional, the two methyl groups at the C-4 position of cycloartenol, a precursor of plant sterols, must be removed by bifunctional 3?-hy-droxysteroid dehydrogenases/C-4 decarboxylases (3?HSD/D). To understand the role of 3?HSD/D in Arabidopsis devel-opment, we analyzed the phenotypes of knock-out mu-tants and overexpression lines of two 3?HSD/D genes (At1g47290 and At2g26260). Neither single nor double knock-out mutants displayed a noticeable phenotype; however, overexpression consistently resulted in plants with wrinkled leaves and short inflorescence internodes. Interestingly, the internode growth defects were opportunistic; even within a plant, some stems were more severely affected than others. Endogenous levels of BRs were not altered in the overexpression lines, suggesting that the growth defect is not primarily due to a flaw in BR biosynthesis. To determine if overexpression of the sterol biosynthetic genes affects the functions of membrane-localized auxin transporters, we subjected plants to the auxin efflux carrier inhibitor, 1-N-naphthylphthalamic acid (NPA). Where- as the gravity vectors of wild-type roots became randomly scattered in response to NPA treatment, those of the overexpression lines continued to grow in the direction of gravity. Overexpression of the two Arabidopsis 3?HSD/D genes thus appears to affect auxin transporter activity, possibly by altering sterol composition in the membranes.
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KEYWORD
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3beta-hydroxysteroid dehydrogenase, brassinosteroids, membrane raft, NPA, sterol
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