HORMONAL STUDIES OF FASS, AN ARABIDOPSIS MUTANT THAT IS ALTERED IN ORGAN ELONGATION

We have isolated an allele of fass, an Arabidopsis thaliana mutation t hat separates plant development and organ differentiation from plant e longation, and studied its hormonal regulation. Microsurgically isolat ed fass roots elongate 2.5 times as much as the roots on intact mutant plants. Wild-type heart embryos, when cultured with a strong auxin, n aphthaleneacetic acid, phenocopy fass embryos. fass seedlings contain variable levels of free auxin, which average 2.5 times higher than wil d-type seedling levels, and fass seedlings evolve 3 times as much ethy lene as wild-type seedlings on a per-plant basis over a 24-h period. T he length-to-width ratios of fass seedlings can be changed by several compounds that affect their endogenous ethylene levels, but fass is ep istatic to etr1, an ethylene-insensitive mutant. fass's high levels of free auxin may be inducing its high levels of ethylene, which may, in turn, result in the fass phenotype. We postulate that FASS may be act ing as a negative regulator to maintain wild-type auxin levels and tha t the mutation may be in an auxin-conjugating enzyme.