Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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B. Rathinasabapathi and R. Kaur
or complete shutting down of various cell functions and maintenances, such as cell
division (Guy, 1999; Taiz and Zeigler, 2002). Plant breeders and geneticists have utilized
natural variability for stress tolerance within germplasm collections for selecting cultivars
with improved stress tolerance (Bolaños et. al., 1993; Ceccarelli et. al., 1991, Chang
and Loresto, 1986). However, this approach is limited by the availability of variability
within the germplasm and crossability of the species.
Metabolic engineering can be defined as the manipulation of cellular, enzymatic,
regulatory and transport processes using recombinant-DNA technology for the purpose
of enhancing specific product yield (Bailey, 1991). Development of methods to
transfer genes across species has led to research aiming to produce stress-tolerant
transgenic crops using metabolic engineering. Today, scientific community is dedicated
to identify potential target genes for metabolic engineering of crops for enhanced
biotic and abiotic stress tolerance. Advancement in revealing the human, yeast,
Arabidopsis and bacterial genomes, and in addition, quickly growing bioinformatics
databases are playing a major role in this goal.
Although the principles of rational metabolic engineering have been well developed
in microorganisms, its application in plants has progressed at a slower pace. One
of the major limitations has been finding out which one of a multistep pathway can be
manipulated to achieve a desired stress tolerance phenotype. In many cases, the
connections between a pathway (or its end product or a specific gene) and stress
tolerance have not been well established. Therefore, most research in model species
aims to provide that connection between a gene and a stress tolerance phenotype. For
the application of metabolic engineering in major crops, thorough field-level assessments
of stress tolerance are required. Such assessments have not been realized in
many of the studies and this has been articulated in Flowers et. al. (1997). Despite the
complexity and lack of field studies, the past two decades have seen some impressive
progress especially on understanding how plants cope with and adapt to various environmental
stress factors. The objective of this chapter is to highlight some of the
modern approaches to engineer transgenic crops tolerant to abiotic stress factors.
Since the literature contains a large number of stress factors and a larger number of
tolerance mechanisms, the scope of this chapter is restricted to two aspects. Firstly,
generalities in applying metabolic engineering for plant stress tolerance will be outlined.
Secondly, select recent, metabolic engineering studies on manipulating plants
for known stress-related pathways and processes will be reviewed. In this section,
three sets of studies will be considered: (a) pathway manipulations to increase metabolites
for specific stress factors, (b) manipulations of transport processes and (c) improving
stress tolerance of plants by modulating regulatory proteins.
2. TOOLS FOR METABOLIC ENGINEERING OF CROPS
2.1. Gene Transfer Technology
Worldwide, the area under transgenic crop cultivation was increased from 4.2 to 104.7
million acres between 1996 and 2000 (Cockburn, 2002). Currently, scientists are enriched