Coping Abiotic Stress with Plant Volatile Organic Chemicals (PVOCs): A Promising Approach - Crop Improvement: New Approaches and Modern Techniques - page 298

type (Sasaki et al. 2007 ). Similarly, the content in some plastidial isoprenoids has
also been successfully enhanced in plants through genetic engineering. Transgenic
mint over-expressing 1-deoxy-D-xylulose-5-phosphate synthase, one of the entry
enzymes into the MEP pathway (DXS), showed increased essential oil content
(Mahmoud and Croteau 2001 ). Arabidopsis plants over-expressing Brassica juncea
3-hydroxy-3-methylglutaryl-CoA synthase gene ( BjHMGS ), coding for the second
enzyme in the cytosolic isoprenoid biosynthesis pathway, have been shown to pro-
vide enhanced fungal and hydrogen peroxide-tolerance (Wang et al. 2011 ). The
Brassica gene was found to be down-regulated by abscisic acid, mannitol, and water
stress, but up-regulated by growth regulators like salicylic acid, methyl jasmonate,
and wounding, suggesting that it could have a role in plant stress resistance.
The genetic engineering of volatile compounds have also brought to light some
genetic changes on plant growth and development, and challenges to accomplish
efficient production of the suitable volatile terpenoid compounds in a spatial and
temporal mode (Dudareva and Pichersky 2008 ). For example in Arabidopsis, over-
expression of FaNES1 resulted in the diversion of carbon to linalool production,
without affecting the levels of chlorophylls, lutein and bcarotene, and resulting in a
growth-retardation phenotype that was stable through several generations (Aharoni
et al. 2003 ). Transgenic potato engineered for linalool production resulted in growth
retardation and leaf bleaching of plants when grown in the greenhouse (Aharoni
et al. 2006 ). Transgenic tobacco containing high levels of patchoulol as a result of
the expression of PTS coupled with FPP synthase , both targeted to the plastids, led
to plants with growth disturbances like leaf chlorosis, vein clearing, and reduced
stature (Wu et al. 2006 ). Such growth abnormalities are attributed to the conse-
quences of the reduction of isoprenoid precursors for other metabolites which are
otherwise are essential for plant growth and development, or that the newly intro-
duced terpenoids could become toxic to plant cells.
A number of plant species synthesize myriad of isoprenoid for plant growth,
development and for adaptation to environment (Leivara et al. 2011 ). The enzyme
3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) in the mevalonate pathway
is modulated by many endogenous and external stimuli. Two B′′ regulatory sub-
units (B′′α and B′′β) of protein phosphatase 2A (PP2A) interact with HMGR1S
and HMGR1L, the two major isoforms of Arabidopsis thaliana HMGR (Leivara
et al. 2011 ). Since B′′α and B′′β are Ca 2+ binding proteins of the EF-hand type, it
was found that PP2A modulates HMGR transcript. Under salt stress conditions, the
B′′α and PP2A mediated the decrease and subsequent increase of HMGR activity
in Arabidopsis seedlings, resulting from a steady rise of HMGR1-encoding tran-
script level and an early sharper reduction of HMGR protein level. In the non-stress
conditions, the PP2A operates as a posttranslational negative regulator of HMGR
activity with the involvement of B′′β. The authors suggested that PP2A can exert
multilevel regulation on HMGR through the five-member B′′ protein family in re-
sponse to stress conditions (Leivara et al. 2011 ).
The mevalonate pathway that mediates the production of isoprenoids has
been operative in higher eukaryotes. Brodersen et al. ( 2012 ) studied the necessi-
ty of isoprenoid biosynthesis for plant miRNA activity in Arabidopsis. In plants