Authors: Ram Chandra Choudhary*, Babu Lal Fagodia and Kumara Swamy R.V.
*Corresponding author: ramchandra026@gmail.com
Rajasthan College of Agriculture, Maharana Pratap University of Agriculture and Technology, Udaipur-313001.
For crop improvement several genetic traits have been introduced in plants and thereby plants efficiency has been increased several times as compared to the normal plants. Herbicides are those chemicals which are used to eliminate unwanted weeds that are two types
(1) selective herbicide which kills only some specific weeds and
(2) non-selective herbicide or broad spectrum, kills all types of weeds.
Generally crop plants have lack of tolerance to the chemical by one or more of the major world crops, e.g. rice, maize, soybean, wheat and rapeseed. Use of multiple types of herbicides to broaden the spectrum of the affected weeds, which in turn increases the possibility that the crop is injured also and lack of high toxicity to weeds while crops are not affected. An ideal herbicide must have some characteristics features like it should control all types of weeds or all types of species except the crop of interest, a high degree of environmental safety i.e. it should be not harmful for the environment, minimum persistence in soil i.e. it should be easily degrade by soil micro-organism. However it is difficult to find a herbicide having all these properties. The development of crop plants that are tolerant to herbicide is an important approach to control weed.
Three main approaches are
(1) Over expression of target gene
(2) Modification of target protein, and
(3) Detoxification of herbicide.
Resistance against glyphosate:
An active component of round herbicide is used as non-selective post emergence herbicide. It is suggested that glyphosate inhibits aromatic amino acids (phenyl alanine, tyrosine and tryptophan) biosynthesis by competitive inhibitor of EPSPS enzyme (5-enolpyruvyl shikimate-3 phosphate synthase) encoded by aroA gene from Agrobacterium spp. and Salmonella typhimurium.
Resistance against sulfonylurea and imidazolinone:
The herbicide action of sulfonylurea and imidazolinone is based on their ability to inhibit non-competitively an enzyme of branched chain amino acid biosynthesis, acetolactate synthase (ALS). ALS is the 1st enzyme in the biosynthesis of Valine, leucine and isoleucine.
Resistance against phosphinothricin:
L-phosphinothricin (PPT) is a naturally occurring amino acid with herbicidal activity and it is active component of herbicide "Basta". PPT is a potent inhibitor of glutamine synthase and thus causes rapid increase in ammonia concentration in plants and this leads to death of plant cell. Phosphinothricin acetyl transferase (PAT) enzyme (bar gene of Streptomyces hygroscopicus) that acetylates the free NH2 groups of phosphinothricin and renders it inactivates the herbicide Basta. This 'bar' gene with Camv 35S promoter has been transferred to tomato, tobacco and potato plants using Ti plasmid of agrobacterium.
Table 1: Types of herbicides by chemical families.
| Chemical Family | Affected System | Target Protein |
| Triazines (atrazine, ametryne, cyanazine, prometryn, simazine) | Photosystem II, electron transport from QA to QB | D-1 protein, product of psbA gene |
| Sulfonylureas, | Amino acid synthesis | Acetolactate synthetase (ALS) |
| Cyclohexanediones | Lipid synthesis | Acetyl coenzyme A carboxylase (ACCase) |
| Glyphosate | Amino acid synthesis | 5-enoylpyruvyl-shikimate-3-phosphats (EPSPS) |
| Bromoxynil | Photosystem II | D-1 protein |
| Phosphinothricin | Glutamine | Glutamine synthase |
Advantage of Herbicide Tolerant Crops:
Excellent weed control and hence higher crop yields, possible to control weeds later in the plant's growth, reduced numbers of sprays in a season, reduced fuel use, reduced soil compaction, use of low toxicity compounds which do not remain active in the soil and the ability to use conservation-till systems, with consequent benefits to soil structure and organisms.
REFERENCES
1 Mulwa, M. S. Richard, and Mwanza, M. Lydia. 2006. Biotechnology approaches to developing herbicide tolerence/selectivity in crops. African journal of Biotechnology 5(5): 396-403.
2 Bhat S. R. and V. L Chopra. 2006. Choice of technology for herbicide-resistant transgenic crop in India: Examination of issues. Current science 91(4): 435-438.
3 Macias, F. A., J. M. G. Molinillo, A. Oliverous-Bestidas, D. Martin, and D. Chinchilla. 2004. A natural strategy for weed control. Allelopathy 69(3): 13-19.
About Author / Additional Info:
I am PhD research scholar of Agricultural Biotechnology and Molecular Biology at MPUAT, Udaipur.