Tuesday, November 25, 2008

Identity of the Product of Mendel's Green Cotyledon Gene (Update)

Last year I drew your attention to the identification of one of Gregor Mendel's original seven mutants [Identity of the Product of Mendel's Green Cotyledon Gene]. At the time, I thought that a recent 2007 paper by Sato et al. was the first publication to make this identification. Now I realize that I slighted the original discoverers Armstead et al. (2007) and earlier publications. The Armstead et al. paper is referenced in the later Sato et al. paper but the reference does not give appropriate credit to the work done by Armstead et al. in identifying the Mendelian gene. It was easy to get the impression that the Japanese group was the first to make the definite connection between Mendel's gene and work done in other species. I suspect that this was deliberate.

Here's a revised version of my original posting. Thanks to a reader who alerted me to the proper credit.

Another of Mendel's seven genes has been identified. This one is described in his 1865 paper Experiments in Plant Hybridization [MendelWeb] as character number 2.
2. To the difference in the color of the seed albumen (endosperm). The albumen of the ripe seeds is either pale yellow, bright yellow and orange colored, or it possesses a more or less intense green tint. This difference of color is easily seen in the seeds as their coats are transparent.
Mendel's reference to the color of albumin, or endosperm, is inaccurate. He was actually observing the color of the cotyledons—the "seed leaves" that surround the embryo in the pea seed. These tiny leaves are covered by a seed coat that is partially transparent.

In wild-type peas the seeds turn yellow as they mature (I) but certain mutants exhibit a "stay-green" phenotype where the peas retain their green color (i). The figure shows seeds from a plant with the II genotype (top) and the ii genotype (bottom). The seed coat has been removed from the lower pair of each group of four peas.

A paper published in Science identifies the "stay-green" gene that Mendel worked with (Armstead et al. 2007). This work is based on several decades of study of the "stay-green" phenotype by Howard Thomas and his colleagues at the Institute of Grassland and Environmental Research, Aberystwyth, UK (Armstead et al. 2006; Thomas, 1987; Thomas et al. 1996; Thomas and Stoddart, 1975).

It turns out that the gene, called sgr (stay-green), encodes an enzyme that is localized to chloroplasts and plays a role in the degradation of chlorophyll during senescence and maturation of seeds. When the enzyme is defective chlorophyll isn't broken down and the tissue stays green.

This brings to three the number of Mendel's genes that have a known function. The wrinkled pea phenotype is caused by a defect in the gene for starch branching enzyme (Bhattacharya et al., 1990) [Biochemist Gregor Mendel Studied Starch Synthesis]. The tall/short phenotypes are caused by defects in the gene for gibberellin 3β-hydroxylase (Martin et al., 1997). Gibberellins are plant growth hormones.

[Photo Credit: The photograph of mutant and wild-type pea seeds is taken from Figure 1 of Sato et al. (2007)]

Armstead, I., Donnison, I., Aubry, S., Harper, J., Hörtensteiner, S., James, C., Mani, J., Moffet, M., Ougham, H., Roberts, L., Thomas, A., Weeden, N., Thomas, H., and King, I. (2007) Cross-species identification of Mendel's I locus. Science 315: 73. [DOI: 10.1126/science.1132912]

Armstead, I., Donnison, I., Aubry, S., Harper, J., Hörtensteiner, S., James, C., Mani, J., Moffet, M., Ougham, H., Roberts, L., Thomas, A., Weeden, N., Thomas, H., and King, I. (2006) From crop to model to crop: identifying the genetic basis of the staygreen mutation in the Lolium/Festuca forage and amenity grasses. New Phytologist 172: 592-597.

Bhattacharyya, M. K., Smith, A. M., Ellis, T. H., Hedley, C., and Martin, C. (1990) The wrinkled-seed character of a pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branching enzyme. Cell 60:115-122.

Martin D.N., Proebsting W.M., Hedden P. (1997) Mendel's dwarfing gene: cDNAs from the Le alleles and function of the expressed proteins. Proc. Natl. Acad. Sci. (USA) 94:8907–8911.

Sato Y., Morita R., Nishimura M., Yamaguchi H., and Kusaba M. (2007) Mendel’s green cotyledon gene encodes a positive regulator of the chlorophyll-degrading pathway. Proc. Natl. Acad. Sci. (USA) 104: 14169-14174. [doi: 10.1073/pnas.0705521104].

Thomas, H. (1987) Sid: a Mendelian locus controlling thylakoid membrane disassembly in senescing leaves of Festuca pratensis. Theoretical and Applied Genetics 73: 551 555.

Thomas, H., Schellenberg, M., Vicentini, F., Matile, P. (1996) Gregor Mendel's green and yellow pea seeds. Botanica Acta 109: 3-4.

Thomas, H., and Stoddart, J.L. (1975) Separation of chlorophyll degradation from other senescence processes in leaves of a mutant genotype of meadow fescue (Festuca pratensis). Plant Physiology 56: 438-441.

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