Improved Harvest Index, Shade Avoidance Syndrome, Ecological Sense, Elongation of the Stem, Population of Plants

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Home >> Plant Biotechnology and Genomics >> Genetically Modified-GM Crops and Floricultural Plants >> Improved Harvest Index

	Improved harvest index (overexpression of phytochrome A or PHY A gene) The plant architecture in any crop is dependent on responses of the plant to signals from light. When plants in a population grow in close proximity to each other, the shade avoidance syndrome is invoked. In other words, many crop plants respond to crowding by growing taller. Although this response has competitive advantage in an ecological sense, this leads to reduced allocation of assimilates to leaf growth, storage organ production and reproductive development. Thus shade avoidance carries a penalty. Although it has adaptive significance, it leads to reduction in crop plant productivity due to non productive elongation of the stem.
In a crowded population of plants, photosynthetic pigments absorb most of the light in the visible region (i.e. 400-700 nm) but reflect or transmit radiation in the far red (FR) region (700-800 nm). FR reflection signals are perceived by the phytochromes {photoreceptors, whose two forms Pr and Pfr are interconvertible . The ratio of the two forms of phytochrome (Pr:Pfr) is a direct function of the ratio of red and far red light absorbed. In Arabidopsis., five phytochrome genes (PHY A, PHY B, PHY C, PHY D, PHY E) are known and the products of PHY A and PHY B, i.e. Phy A and Phy B exert mutually antagonistic effect on stem elongation as a shade avoidance response to far red.
This suggested that artificial maintenance of Phy A at a high level in light grown plants might suppress shade avoidance response and thus may avoid the resource allocation for this wasteful growth from the point of view of crop productivity. Transgenic tobacco plants when produced in 1996 for overexpression of Phy A, actually exhibited suppression of shade avoidance response to light that was artificially enriched in far-red light. For production of transgenic plants, the gene PHY A, cDNA, driven by CaMV-35S promoter was utilized.
The product Phy A (which is labile to light and is quickly replaced by Phy B in wild plants) is stable in these transgenic plants. It can accumulate to high concentration, thus permitting high density crop, yet keeping dwarf. This led to increase in harvest index. This successful technology can now be used for other crop plants exhibiting shade avoidance response. In such crop plants (e.g., rice), the use of phytochrome gene for an increase of harvest index may lead to increased crop productivity.

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