Photo: Courtesy of M. Matsuoka, Reprinted with permission from Nature ©2001
During the summer of 1997, two research groups succeeded in bringing closure to a classic tale in genetics. After 131 years, they identified one of Mendel's pea genes at the molecular level. The gene, called Le, controls stem length--plants with defective copies are shorter than normal. Characterized in both pea and Arabidopsis, the gene encodes an enzyme called gibberellin 3b-hydroxylase, which converts two precursors of the growth regulator gibberellin into active forms, including gibberellin 1.1,2 Le was the second of Mendel's genes to be identified. Re-searchers pulled out the first--for a starch-branching enzyme that in defective form causes the wrinkled seed trait--12 years ago.3
Now a gene called GA20ox-2 encoding another enzyme in the gibberellin biosynthetic pathway has been conquered, this time in rice.4 The enzyme is GA20 oxidase, which catalyzes three steps in the pathway leading to production of gibberellin 20, a substrate for the b- hydroxylase. Four alleles of a semi-dwarf rice mutant called sd1 encode defective enzyme, thereby depleting stems of precursor. No precursor means no active hormone. Part of the gene, 383 base-pairs, is missing in one of the alleles, while three others encode oxidases with amino acid substitutions that impede activity. By inserting the wild type gene into mutant plants, the researchers restored normal growth.
Gibberellin also controls flower development, so the fact that sd1 rice plants reproduce normally seems odd. However, the rice genome contains a second oxidase gene called GA20ox-1 that is active in flowers. The two genes produce enzymes almost 48% identical at the amino acid level.
With recently completed rice genome drafts in hand,5,6 scientists can now mine the sequence for other interesting genes important in plant growth regulation and development.
Barry A. Palevitz (email@example.com) is a contributing editor.
1. D.R. Lester et al., "Mendel's stem length gene (Le) encodes a gibberellin 3b-hydroxylase," Cell, 9:1435-43, 1997.
2. D.N. Martin et al., "Mendel's dwarfing gene: cDNAs from the Le alleles and function of the expressed proteins," Proceedings of the National Academy of Sciences, 94:8907-11, 1997.
3. M.K. Bhattacharyya et al., "The wrinkled-seed character of pea described by Mendel is caused by a transp0son-like insertion in a gene encoding starch-branching enzyme," Cell, 60:115-22, 1990.
4. A. Sasaki et al., "A mutant gibberellin-synthesis gene in rice," Nature, 416:701-702, April 18, 2002.
5. J. Yu et al., "A draft sequence of the rice genome (Oryza sativa L. ssp. indica)," Science, 296:79-92, April 5, 2002.
6. S.A. Goff et al., "A draft sequence of the rice genome (Oryza sativa L. ssp. japonica)," Science, 296:92-100, April 5, 2002.