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expression in transgenic plants regarding salt stress
and similar expression for drought and cold tolerance.
Efforts have been made to engineer
LEA
genes in rice
which resulted in improved tolerance to drought (Xu
et al., 1996; Cheng et al., 2002; Xiao et al., 2007).
However deciphering the role of LEA proteins still
require a lot of work which is to be done.
Among organic osmolytes, trehalose is a non reducing
sugar present in naturally drought tolerant plants.
There are reports of improved drought stress tolerance
due to engineering trehalose genes in rice (Garg et al.,
2002; Jang et al., 2003).
Most of these transgenic lines have been tested in
laboratory conditions. There full scale evaluation in
field would provide important information for the
further exploitation of transgenic work in breeding
programs. The ultimate aim of transgenic technology
is to identify and manipulate the genes in plants to
improve their performance in drought without
jeopardizing their physiological responses. Nevertheless
transgenic approaches offer new opportunities to
drought tolerance in rice by engineering genes from
different sources (Cattivelli et al., 2008).
6 Approaching drought tolerance through
functional genomics
The goal of functional genomics is to understand how
the genome functions to make a whole plant and also
deciphering the information conserved in genes
making up the genome. Second, utilizing that
information for crop genetic improvement is good
(Jiang et al., In Press). Full genome sequence of rice
(International Rice Genome Sequencing Project 2005;
Goff et al., 2002; Yu et al., 2002; Sasaki et al., 2002;
Feng et al., 2002) provides a basis for functional
genomic technologies (microarray, express sequence
tags (EST) etc.) by knowing the expression and
sequence of thousands of genes. These genes are then
screened by using RNA expression profiling to
identify potential candidate genes with their putative
functions related to drought stress. Using this
expression profiling, Kawasaki et al, 2001, identified
some putative genes in rice.
Rice genomics has greatly progressed in recent years.
Several functional genomic approaches like macro and
micro array have been applied in rice (Kawasaki et
al., 2001; Rabbani et al., 2003). Gorantla et al (2005)
used functional genomics and generated a large
number of ESTs from cDNA libraries and identified
589 genes involved in drought stress. These ESTs
were also helpful to dissect drought QTLs and
candidate genes. The availability of large number of
ESTs, microarrays in combination of bioinformatics,
will reveal the function of rice candidate genes
involved in drought tolerance (Shimamoto and
Kyozuka, 2002; Langridge et al., 2006).
Proteomics is a large and emerging field pertaining to
the study of proteins especially with reference to their
structure and function. Fortunately our knowledge of
rice proteome is relatively advanced as compared to
other crops (Komatsu and Tanaka, 2005). Even then
proteomics studies related to drought are at their
infancy (Ansuman et al., 2011). Salekdeh et al (2002)
identified more then 2000 proteins involved in
drought stress. After this they were able to identify 42
proteins in relation to their affect on drought to infer
their functions in detail. Another study conducted on
rice by Ali and Komatsu (2006) focused on a protein
actin depolymerizing factor (ADF). They observed a
marked increase in concentration of ADF in drought
tolerant plants suggesting the importance of this
protein with reference to drought stress. Rabello et al
2008 identified 22 proteins putatively associated with
drought tolerance using mass spectroscopy.
Micro RNAs are a newly identified class of small
single stranded non-coding RNAs playing their role in
post transcriptional gene regulation targeting mRNAs
for cleavage or translational repression (Zhao et al.,
2007). They used the oligonucleotide microarray in
rice to see the expression profile of micro RNA in
drought stress and identified two micro RNAs being
induced in drought stress.
7 Conclusion and future directions
Drought tolerance improvement is probably one of the
challenging tasks of rice breeders. This is due to its
complex and unpredictable nature. In past few years,
the world has witnessed a substantial progress in the
field of genomics making us to understand the