Allelochemicals Biologica... - Name

ALLEOPATHIC ORGANISMS AND
MOLECULES
interfere with competing weeds. Transgenes for bromoxynil, glyphosate, and
glufosinate resistance are found in commercially available crops. Other herbicide
resistance genes are in development. Glyphosate-resistant crops have had a profound
effect on weed management practices in North America, reducing the cost of weed
management, while improving flexibility and efficacy. In general, transgenic, herbicideresistant
crops have reduced the environmental impact of weed management because
the herbicides with which they are used are generally more environmentally benign
and have increased the adoption of reduced-tillage agriculture. Crops could be given
an advantage over weeds by making them more competitive or altering their capacity
to produce phytotoxins (allelopathy). Strategies for producing allelopathic crops by
biotechnology are relatively complex and usually involve multiple genes. One can
choose to enhance production of allelochemicals already present in a crop or to impart
the production of new compounds. The first strategy involves identification of the
allelochemical(s), determination of their respective enzymes and the genes that encode
them, and, the use of genetic engineering to enhance production of the compound(s).
The latter strategy would alter existing biochemical pathways by inserting transgenes
to produce new allelochemicals (Duke et al., 2002c).(See controversy between organic
agriculture and biotechnology - Control of Weeds and Management of Agroecosystems,
Pag. 18, first paragraph)
More sophisticated techniques will be used to search for alternative to herbicides
in agroecosystems. The use of winter cover crops is beneficial to agriculture. Stanislaus
and Cheng (2002) tried to design a cover crop that self-destructs in response to an
environmental cue, thereby eliminating the use of herbicides and tillage to remove
the cover crop in late spring. Here, this novel concept is tested in a model system. The
onset of summer brings with it elevated temperatures. Using this as the environmental
cue, a self-destruction cassette was designed and tested in tobacco. A heat-shockresponsive
promoter was used to direct expression of the ribonuclease Barnase. Because
Barnase is extremely toxic to cells, it was necessary to coexpress its inhibitor, Barstar,
whose expression was under the control of the CaMV 35S promoter. The wild-type
and two mutated Barnase genes, one missense and one translation attenuated, were
tested. The results indicated that the translation-attenuated version of the Barnase
gene was most effective in causing heat-shock-regulated plant death. Analysis of the
T-2 progeny of a transgenic plant carrying this Barnase mutant showed that the Barnase
gene expression was sixfold higher in heat-shock-treated plants compared with
untreated plants. This level of Barnase gene expression was sufficient to kill transgenic
plants.
Many advances in disciplines such as chemistry, biochemistry, plant breeding,
genetics, engineering, and others have been applied in a positive manner to improve
knowledge in weed science. The emerging field of genomics is likely to have a similar
positive effect on our understanding of weeds and their management in various plant
agriculture systems. Genomics involves the large-scale use of molecular techniques
for identification and functional analysis of complete or nearly complete genomic
complements of genes. Commercial application of genomics has already occurred for
improvement in certain crop input and output traits, including improved quality
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