Transgenic plants with herbicide resistance.
Due to increasing concern about contamination of environment due to herbicides, new herbicides are being developed that are safer and biodegradable. This has necessitated the development of resistance in crop plants against these new and safer herbicides. These newer herbicides affect processes like photosynthesis or biosynthesis of essential amino acids (Table 42.3). Transgenic plants resistant to these herbicides have been produced, utilizing one of the following two approaches : (i) Either the target protein in overexpressed (e.g. EPSPS)
or it should become insensitive to the herbicide (e.g. ALS).
(ii) A pathway is introduced, which will detoxify the herbicide. For instance, glutathione-S transferase (GST)
detoxifies atrazine, 'nitrilase'
coded by gene bxn,
and 'phosphinothricin acetyl transferase (PAT)'
coded by bar
gene, detoxifies 'L-phosphinothricin (PPT)'.
(For more details, see Table 42.3).
Transgenic plants with insect resistance.
Genes for insect resistance from three different sources are being transferred for developing insect resistance (i) bt2
gene encoding Bt toxin,
derived from Bacillus thuringiensis;
(ii) cowpea trypsin inhibitor gene
(CpTi) from cowpea (Vigna unguiculata
)and (iii) genes for other insecticidal secondary metabolites
derived from a number of legumes. In all these cases, successful attempts to transfer genes for insect resistance have already been made. Insect resistant transgenic plants derived thus are being field tested and may be released for commercial cultivation.
Transgenic plants with resistance against viruses.
Three different approaches were used for developing virus resistant transgenic plants : (i) Genes for virus coat protein
or capsid protein (CP)
were transferred to tomato, alfalfa, tobacco, potato, melon and rice for developing resistance against a variety of viruses (e.g. TMV, PVX and PVY, etc.). (ii) Gene for nucleocapsid protein (N)
was transferred from tomato spotted wilt virus (TSWV)
and provided resistance against this virus, which causes considerable damage to crops like tomato, tobacco, groundnut, pepper, etc. (iii) DNA fragments coding for satellite RNAs
(also called virusoids)
associated with viruses (e.g. 'cucumber mosaic virus'
and 'tobacco ringspot virus'
were transferred and provided resistance against these and other viruses.
Transgenic plants with resistance against bacterial and fungal pathogens.
Several examples are now available, where genes imparting resistance against bacterial and fungal pathogens have been successfully transferred to tomato (against Pseudomonas, Alternaria
)and potato (against Phytophthora
Transgenic tomato plants for hard skin and improved flavour.
In tomato using antisense RNA technology, transgenic plants have been produced which are either 'bruise resistant'
(suitable for transport and storage) or exhibit 'delayed ripening'
giving more time for ripening on the plant, thus permitting more time for sugar accumulation and also giving higher shelf life. These are given the name 'Flavr Savr'.
Transgenic plants for hybrid seed production.
During 1990-92, genes barnase
(encoding a ribonuclease)
(encoding a ribonuclease inhibitor)
derived from Bacillus amyloliquefaciens
were separately transferred to Brassica napus
to produce 'male sterile'
and 'fertility restorer'
plants. The gene constructs were prepared where barnase/barstar
was fused with TA29 promoter
form tobacco, permitting expression only in the anther causing male sterility or restoration. This system will prove to oe of immense value for hybrid seed production in crop plants in future.
Transgenic plants for molecular farming.
Transgenic plants are also proposed to be used as factories or bioreactors
for manufacturing speciality chemicals
Sugars, fatty acids, starch, cellulose, rubber, wax, etc. are obtained from plants and transgenic plants can be produced to increase their production. Following are some examples : (i) Transgenic tobacco plants with increased level of mannitol were produced using a gene for mannitol dehydrogenase
(this also gave resistance against salinity), (ii) Transgenic potato plants with increased level of cyclodextrins
were produced using a bacterial gene for cyclodextrin glucosyl transferase
(CDs are useful for pharmaceutical delivery systems, flavour and odour enhancement and for removing undesirable compounds like caffeine from food), (iii) Transgenic Arabidopsis
plants for production of pplyhydroxybutyrate
(a biodegradable thermoplastic polymer) were produced using two genes, phbB
(iv) Transgenic potato and tobacco plants for production of human serum albumin
(in leaf tissue) could be
Transgenic plants for study of regulated gene expression.
Transgenic plants have also been
produced for identifying, regulatory sequences
involved in the differential expression of gene
activity in time and space or due to a specific
stimulus like light, temperature, wound, etc. For
this purpose, different lengths of upstream flanking
sequences of a structural gene are fused with their
own structural gene or with
structural genes studied in the transgenic plants
thus produced. This allowed the identification of
regulatory elements e.g. light response elements (LRE), endosperm box, heat shock element
(HSE), etc. For instance when LRE was fused with
an unrelated gene, the latter started behaving like
a photosynthetic gene (expression in leaf and light,
not in roots and dark). A number of genes, thus
examined, are listed in Table 42.4.