10
Rice Brown Planthopper
of precocene II to penultimate instar nymphs of presumptive brachypterous adults.
Due to JH antagonizing effect of precocene II very high proportion of presumptive
brachypterous adults were transformed to macropterous ones (Ayoade et al., 1996).
Although the JH, what we considered so far as single chemical usually consists of
three similar compounds termed as JH I, JH II, and JH III. All these are sesquiterpenoid
compounds with small variation in their chemical structure. In BPH among the three
molecules, JH III is by far the most important one although the other two also have
been demonstrated to play some role in their function.
In case of BPH, ED consists basically of a steroid ring structure like in other insects.
It is well known that steroid ring structure cannot be synthesized by BPH similar to its
counterparts in Class Insecta. So it has to depend on external source for acquiring the
compounds containing basic steroid structure. The synthesis of ED in BPH involves
modification of the steroid compounds obtained from external source to ED presumably
through enzymatic action. Rice plant sap which is its food is the most likely external
source.
Mechanisms of JH titre maintenance in BPH:
In BPH maintenance of certain titre of JH in the insect body requires two types of
mechanisms, Mechanism for synthesis of the molecules at the time of necessity and
degradation of the compound when its concentration has to be brought down. Both the
functions require a set of enzyme systems and their corresponding genetic transcripts in
BPH genome.
Important enzymes playing key roles in the synthesis and regulation of JH are JH
methyltransferase, JH epoxidase, JH esterase (JHE) etc. JH methyl transferase and
JH epoxidase appear to be of primary importance in the synthesis process while JHE
appear to be responsible for degradation of JH. A negative correlation was observed
between JHE enzyme activities and JH titers, in the 5th instar macropterous nymphs
suggesting that the gene encoding for JHE is a key factor in regulating JH titer. Such
a negative correlation could not be observed in brachypterous nymphs where high JH
titre is present indicating that an increase in JH titer results due to decreased expression
of the gene encoding for JHE. (Liu et.al., 2008). JH methyltransferase or Farnesoic acid
O-methyltransferase (FAMeT) enzyme and the gene encoding for FAMeT could not be
found to have direct relationship with wing dimorphism in BPH indicating its minor or
