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Tuesday, January 06, 2009

Mendel's Stem Length Gene (Le)

The seven traits that Gregor Mendel worked with were: seed shape (R/r), cotyledon color (I/i), seed and flower color (A/a), pod shape (V/v), pod color (Gp/gp), flower position (Fa/fa), and stem length (Le/le). The last trait is also known as Tall (T) and short (t).

The gene responsible for cotyledon color has been identified. It encodes an enzyme that degrades chlorophyll [Identity of the Product of Mendel's Green Cotyledon Gene (Update)]. The gene giving rise to the wrinkled phenotype (r) encodes the starch branching enzyme [Biochemist Gregor Mendel Studied Starch Synthesis].

The gene responsible for stem length has also been identified and cloned (Lester et al., 1997). It encodes an enzyme called 3β-hydroxylase. This enzyme is responsible for one of the last steps in the synthesis of the gibberallin GA1. See Monday's Molecule #102 for the structure of a similar gibberellin.

Gibberellins are plant growth hormones and GA1 is required to stimulate the grown of the stem in pea plants. The wild-type enzyme synthesizes GA1 from its substrate GA20. The pea gene (Le) is related to a similar gene in other flowering plants. Defects in those genes produce dwarf plants.

Lester et al. (1997) cloned the pea gene and identified a restriction length polymorphism that was associated with a mutant version of the gene (le). In the mutant, a single alanine residue was replaced by a threonine residue and this resulted in a 3β-hydoxylase activity that was 20-fold less than the wild type level. The reduced amount of gibberellin GA1 could account for the smaller plants.

The restriction length polymorphism was used as a genetic marker in crosses between Le/Le plants and le/le plants. It segregated with the le genotype as expected. This experiment establishes that the gene for 3β-hydoxylase is the Le gene that Mendel studied and the phenotype is due to differing levels of the plant hormone gibberellin GA1.


Lester, D.R., Ross, J.J., Davies, P.J., and Reid, J. (1997) Mendel’s Stem Length Gene (Le) Encodes a Gibberellin 3β-Hydroxylase. The Plant Cell 9:1435-1443. [PDF]

2 comments :

Christine said...

Thanks, this was very helpful!

Kantharaj said...

you have to explain the biochemistr or mol biol of the gene expressions-it is 2012 not 1856