Abstract
Arabidopsis (Arabidopsis thaliana) hexokinase-like1 (HKL1) lacks Glc phosphorylation
activity and has been shown to act as a negative regulator of plant growth. Interestingly,
the protein has a largely conserved Glc binding domain and protein overexpression was
shown previously to promote seedling tolerance to exogenous 6% (w/v) Glc. Since these
phenotypes occur independently of cellular Glc signaling activities, we have tested
whether HKL1 might promote crosstalk between the normal antagonists Glc and
ethylene. We show that repression by 1-aminocyclopropane-1-carboxylic acid (ACC) of
the Glc-dependent developmental arrest of wild-type Arabidopsis seedlings requires the
HKL1 protein. We also describe an unusual root hair phenotype associated with growth
on high Glc media that occurs prominently in HKL1 overexpression lines and in gin2-1, a
null mutant of hexokinase1 (HXK1). Seedlings of these lines produce bulbous root hairs
with an enlarged base, after transfer from agar plates with normal media to plates with
6% Glc. Seedling transfer to plates with 2% Glc plus ACC mimics the high Glc affect in
the HKL1 overexpression line, but not in gin2-1. A similar ACC-stimulated, bulbous
root hair phenotype also was observed in wild-type seedlings transferred to plates with
9% Glc. From transcript expression analyses, we found that HKL1 and HXK1 have
differential roles in Glc-dependent repression of some ethylene biosynthesis genes. Since
we show by co-immunoprecipitation assays that HKL1 and HXK1 can interact, these two
proteins likely form a critical node in Glc signaling that mediates overlapping, but also
distinct cellular responses to Glc and ethylene treatments.