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.

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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]