Modern agricultural practices require the massive application of fertilizer to soils worldwide. Millions of tons of fertilizer are applied to agricultural soils each year. The adverse environmental impact and high cost of fertilizer use underscore the importance of improving the capability of plants to capture nutrients from soil. Therefore, engineering plants with improved nutrient uptake may help in sustainable agriculture in the twenty-first century. Advancements made in recent years in the area of recombinant DNA technology have provided an altogether new dimension to agricultural research. Now, it is possible to harness genes of economic importance form sexually incompatible wide and weedy relatives of crop plants and from related and unrelated species and phyla. This has created a situation where the whole biological world is now being considered as a 'single gene pool'. The expectation are that in near future, the availability of expanded genetic base will provide new and novel genes or gene combinations for accelerating the speed and quantum of all rounds growth in agriculture through the use of modern tools of biotechnology.
Molecular genetic tools have given the researcher the possibility to identify key regulatory steps in the acquisition of nutrients by plants. There are now convincing evidences that the genes coding for several target traits, mainly the transporters and other mechanisms for nutrients acquisition opens up several options to improve nutrient uptake and utilization in soils with low fertility. The transfer of the corresponding genes to agriculturally important crops might therefore allow to increase their nutrient uptake capacity.
Use of Transgenic Technology
Plants containing a gene or genes which have been artificially inserted instead of the plant acquiring them through pollination is known as transgenic plants or genetically modified or GM crops. The inserted gene sequence (known as the transgene) may come form another unrelated plant or from a completely different species. Hence, genetic engineering is a specific process in which gene from a species are modified or genes from unrelated species can be introduced into the crop species by transformation methods, followed by regeneration, which is the subsequent selection in tissue culture of transformed cell, under conditions where each cell will express its totipotency and finally, form a new viable plant. There are many methods for genetic transformation such as Agrobacterium-mediated, particle bombardment and protoplast fusion.
Plant transformation is generally accomplished using techniques developed from a naturally occurring bacterial disease called crown gall. The casual agent, Agrobacterium tumefaciens, transfers some of its own DNA into a plant cell nucleus. Subsequent expression of the bacterial genes by the 'transformed' plant cell causes it to proliferate into a gall structure and produces a food source for the bacteria. For genetic engineering purposes, the A. tumefaciens DNA has been modified so that the genes that cause gall formation have been deleted and in their place genes have added to enable selection of plant tissue that contains the transferred DNA (T-DNA) along with desired trait gene or genes. Once a desired gene has been isolated, the functional region identified and, if necessary, modification made to ensure expression of the gene in the new host, the genetic engineering process will result in transgenic plants that contain the desired trait. Although broad-host range A. tumefaciens strains exist, many of the major crop plant including most cereals and legumes have only successfully been transformed by alternative methods.
Genetically engineering technology has several advantages over conventional breeding methods for crop improvements such as the broadening of the germplasm base from which new character can be transferred, the ability to repeatedly transfer new genes directly into existing cultivars without many generations of additional crosses, the ability to transfer discrete gene without many unknown closely linked genes, and the ability to alter gene formulations that will produce new plant characteristics.
Transgenic crops are now being cultivated in as many as 15 countries including 5 developing nations with a total area of 44.2 million hectares. During the first generation of the transgenic technology, emphasis was laid on the agronomic advantages like resistance to pests and disease, and tolerance against herbicides, but in the second generation of transgenic technology, emphasis is on improvising the quality of plant produce such as the improvement of carbohydrates, proteins, oil quality, enrichment of crucial vitamins and minerals (Fe, Zn) composition of stable foods.
IRT1 gene from Arabidopsis is the first transporter gene to be isolated from plants and could be used for engineering plants to take up more iron. Two barley genes - ids2 and ids3 are also good candidate to increase the bioavailiability of iron in transgenic plants. Several other candidate genes (frohA, frohB, frohC, and frohD) that may encode Fe (III) reductase have been identified in Arabidopsis using degenerate Polymerase Chain Reaction (PCR) with primers designed against motifs common to the yeast Fe (III) reductase proteins (Fre1p, Fre2p, and Frp1) and therefore may be potential candidate gene for developing transgenic plants with high iron uptake.
The uptake of iron in transgenic tobacco was increased by constitutively expressing the yeast Fre2 gene encoding a ferric reductase. This was related to higher rate of Fe (III) reduction along the entire length of the roods and in the shoots. Transgenic plants were tolerant to iron deficiency and exhibited 50% higher Fe concentrations in younger leaves than nontransformed plants when cultivated in a iron-deficient medium. This suggests that the Fre2 gene may be used to improve iron uptake in crop plants.
Use of genes encoding S-adenosymethionine synthase, nicotianamine synthase (NAS) and nicotianamine aminotrasferase (NAAT) , and a through understanding of phytosiderophore (PS) biosynthesis and of the PS cation complex transport mechanism into the cytoplasm may allow the generation of dicot plants exhibiting strategies-II uptake and thus improved iron and zinc uptake. High iron-containing transgenic plants have been produced by expression of cDNA coding for ferritin under the control of either constitutive (CaMV 35S) or seed specific promoters. A three-fold greater iron content in rice (Oryza sativa) seeds were obtained.
Similarly, expression of Zn transporters may lead to increase zinc absorption in roots. Zinc transporters genes such as ZIP1, ZIP2, ZIP3, ZIP4, ZRT1, and ZAT could be good potential candidate genes to enhance zinc uptake and acquisition for the development of transgenic plants. This type of approach could also be evolved by increasing the level of methallothione for high zinc accumulation.
Nramp genes such as OsNramp1, OsNramp2, OsNramp3 (cloned from rice), AtNramp1, AtNramp2, AtNramp3 (cloned from Arabidopsis thaliana ) has significant potential for genetic enhancement of Mn extraction from soil. A COPT1 gene coding a putative copper transporter from Arabidopsis thaliana may be a potential candidate for copper uptake.
There are now convincing evidences that the genes coding for several transporters and other proteins for nutrient acquisition opens up many options to improve nutrient uptake from the soils. Genetic engineering and molecular biological techniques have advance our understanding of different transport processes in plants and provide adequate insight in the key steps in nutrients uptake and accumulation. Genes encoding diverse transporters have been identifies and isolated from a number of organisms. The functional significance species of high and low affinity transporters for nutrient have been cloned and characterized by studying the mutants and over-expression study. Most of the genes encoding transporters expressed in roots. But, several intracellular transporters have also been cloned and characterized. Improvement of nutrient acquisition in area where nutrients deficiency in soils limits crop productivity is probably the most challenging and rewarding areas of research to achieve the sustainable productivity of agricultural crops.
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