Hormone analysis
Jasmonates being the important signalling hormones induced in plants
after caterpillar herbivory, free JA, amino-acid conjugate of JA
(JA-Ile), and OPDA were measured in wild type and an ethylene
insensitive skl mutant of M. truncatula. Concentration of
free JA as well as JA-Ile was significantly higher in plants fed by
caterpillars with or without intact spinneret than control in both
genotypes. But the level of OPDA was variable across treatments and not
significantly different in caterpillar damaged plants from control. It
is observed that higher level of free JA and JA-Ile is accumulated in
wild type plants than in skl mutants after caterpillar herbivory
which indicates the effect of ethylene is JA biosynthesis or JA
signalling pathway.
ACC oxidase is an enzyme that catalyzes the last step in ethylene
biosynthesis pathway (Xu et al., 2008). It oxidizes ACC to
liberate ethylene. In a feed forward mechanism, ACC oxidase is also
responsive to hormone ethylene (Alexander and Grierson, 2002). Thus the
relative transcript level of ACC oxidase was compared between control
and caterpillar infested plants (both with and without intact labial
salivary gland) as an marker of both ethylene production and ethylene
responsive gene. But no significant difference in the transcript level
due to herbivory was seen in both genotypes which showed that the
production of ethylene is not affected by herbivory. However, it cannot
be ruled out that the level of hormone might have levelled off at the
time of sample collection which was done after 10 hours of herbivory.