Coping Abiotic Stress with Plant Volatile Organic Chemicals (PVOCs): A Promising Approach - Crop Improvement: New Approaches and Modern Techniques - page 299

ARGONAUTE (AGO) protein complexes are guided by microRNAs (miRNAs) to
regulate expression of complementary RNAs. Brodersen et al. ( 2012 ) used mad3
and mad4 , the miRNA action deficient ( mad ) mutants, for the isolation of genes in-
volved in isoprenoid biosynthesis. The 3-hydroxy-3-methylglutaryl CoA reductase
(HMG1), acting in the initial C5 building block biogenesis that precedes isoprenoid
metabolism and acts as a key regulatory enzyme controlling the amounts of iso-
prenoid end products is encoded by MAD3 while, the sterol C-8 isomerase that acts
downstream in dedicated sterol biosynthesis is encoded by MAD4 . Complementa-
tion studies using yeast system and treatment in planta with an inhibitor of HMG1
(lovastatin), indicated that lack of catalytic activity in HMG1 is adequate to inhibit
miRNA activity. Further knockdown of HMG1/MAD3 reduced AGO1-membrane
interaction and specific hypomorphic mutant alleles of AGO1 displayed compro-
mised membrane association. The study has shown an interesting possibility that for
the activity of plant miRNAs, isoprenoid synthesis could be required and this could
unravel underlying mechanisms of microRNA function and regulation.
Conclusions and Future Perspectives
Abiotic stresses including salinity, drought and high temperature limit crop pro-
ductivity. In this regard, PVOCs either emitted or induced from different plant spe-
cies can be applied to confer better defense. Understanding of the biosynthesis of
volatile compounds and the genetic machinery involved has greatly contributed to
use this chemical repertoire for integrating biochemical, molecular and functional
data into stress alleviation. A complete picture of metabolic network of PVOC syn-
thesis and information on their regulation will necessitate further investigation. In
addition, screening and use of suitable compounds involved in the biosynthesis of
volatile-induced plant defenses will greatly facilitate fine tuning of plant responses
to stress factors. In the past decade, considerable progress has been made in the
metabolic engineering of the isoprenoid biosynthetic pathway in plants (Mahmoud
and Croteau 2001 ; Lucker et al. 2001 ; Nagegowda 2010 ). An increasing number of
successful attempts have raised hopes that their manipulation could offer a promis-
ing tool for increasing isoprenoid content for varied applications in stress tolerance
and protection from environmental damage.
Another direction in PVOCs is by using priming approach by which planting a
few transgenic plants that release defense volatiles in the field may contribute to
plant protection and provide an advantage to non-transgenic plants (Dudareva and
Pichersky 2008 ). In order to derive such benefits, it is imperative that we need to
investigate the molecular mechanisms underlying priming induced capacitance, the
detection of volatile signal components that activate the capacitance, species spe-
cific responses and molecular markers for the primed state in crop plants. It has also
been suggested that histone modifications that are operative during a primary event
might create memory associated reaction to a second stress exposure (Jaskiewicz
et al. 2011 ).