Overexpression of an Arabidopsis cDNA encoding a sterol-C24(1)-methyltransferase in tobacco modifies the ratio of 24-methyl cholesterol to sitosterol and is associated with growth reduction.
Higher plants synthesize 24-methyl sterols and 24-ethyl sterols in defined proportions. As a first step in investigating the physiological function of this balance, an Arabidopsis cDNA encoding an S-adenosyl-L-methionine 24-methylene lophenol-C24(1)-methyltransferase, the typical plant enzyme responsible for the production of 24-ethyl sterols, was expressed in tobacco (Nicotiana tabacum L.) under the control of a constitutive promoter. Transgenic plants displayed a novel 24-alkyl-Delta5-sterol profile: the ratio of 24-methyl cholesterol to sitosterol, which is close to 1 in the wild type, decreased dramatically to values ranging from 0.01 to 0.31. In succeeding generations of transgenic tobacco, a high S-adenosyl-L-methionine 24-methylene lophenol-C24(1)-methyltransferase enzyme activity and, consequently, a low ratio of 24-methyl cholesterol to sitosterol, was associated with reduced growth compared with the wild type. However, this new morphological phenotype appeared only below the threshold ratio of 24-methyl cholesterol to sitosterol of approximately 0.1. Because the size of cells was unchanged in small, transgenic plants, we hypothesize that a radical decrease of 24-methyl cholesterol and/or a concomitant increase of sitosterol would be responsible for a change in cell division through as-yet unknown mechanisms.
Institut de Biologie Moléculaire des Plantes, Département d'Enzymologie Cellulaire et Moléculaire, Institut de Botanique, 28 rue Goethe, 67083 Strasbourg, France. schaller@medoc.u-strasb.fr
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D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics*; D001483:Base Sequence; D002784:Cholesterol / Q000031:analogs & derivatives* / Q000378:metabolism; D003001:Cloning, Molecular; D017931:DNA Primers; D018076:DNA, Complementary; D008780:Methyltransferases / Q000235:genetics*; D010840:Phytosterols; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D012855:Sitosterols / Q000378:metabolism*; D014026:Nicotiana / Q000235:genetics* / Q000254:growth & development
D017931:DNA Primers; D018076:DNA, Complementary; D010840:Phytosterols; D012855:Sitosterols; C021273:campesterol; C025473:gamma-sitosterol; D002784:Cholesterol; D008780:Methyltransferases; C022670:delta 24-sterol methyltransferase