- Auxin and ethylene regulate many important aspects of plant growth and development and are
essential to the plants survival. They help regulate such diverse processes as cell elongation,
growth of adventitious roots, fruit growth, germination, and a variety of other processes. Few
studies have examined how the hormones auxin and ethylene interact. An attempt to understand
how these two hormones interact is presented. To try and observe and interaction, two mutants,
diageotropica (dgt) and Never Ripe (Nr), both near isolines of the tomato (Lycopersicon
esculentum) parent Alisa Craig (AC). dgt is an auxin insensitive, single gene, recessive mutant.
Nr is a semi-dominant mutation in the AC background as characterized by its fruit phenotype in
which some fruit ripens and others remain orange. Nr has a mutation in an ethylene receptor
(ETR3) but it is not impaired in its ethylene biosynthesis capabilities, only its ability to perceive
ethylene. These two mutants (dgt and Nr) were cross-pollinated using standard pollination
techniques to generate an Fl population. The Fl was backcrossed to homozygous dgt plants to
obtain an F2 population. The four resultant populations (double mutant phenotype, dgt
phenotype, Nrphenotype, and WT phenotype) each have a 25% change for being observed. For
each cross completed, a reciprocal was also conducted. To try and characterize an interaction at
the seedling level, WT, Nr, dgt, F1, and F2 seed were gravistimulated under red light and the
curvature of growth was measured. In addition the number of lateral roots present was counted.
The F2 had a significant proportion of seedlings lacking lateral roots, so the F2 populations were
segregated based on the presence or absence of lateral roots. The Fl population, which was
expected to have a gravitropic response similar to Nr, exhibited an intermediate response, while
all F2 populations, regardless of lateral root presence, had responses similar to dgt. It was found
that dgt may have be semi-dominant because the F1 response was lower than that of the expected
and the all the F2 populations exhibited a similar response to gravity. Nr may also help to
partially repair the dgt lesion in the FI as indicated by the intermediate response.