Does GMO make the plant produce different chemicals? If so, then if you wanted the plant to grow bigger, what chemical would it produce?

Expert response from Peter J. Davies

Professor of Plant Physiology and International Professor of Plant Biology, Cornell University, Ithaca New York, USA

Friday, 17/08/2018 16:03

A GMO plant can be made to produce different chemicals. At the initial level the products of added genes are proteins, but proteins can also function as enzymes i.e., they cause chemical reactions and these chemicals can affect growth.

A bigger plant could be one that produces more tissue in a given time, or it could be one that is taller; these are different situations.

• More mass: For a plant to grow bigger it has to make better use of the available resources, such as light and CO₂ for photosynthesis, water, or minerals. There are currently no such crops in production, but research has been progressing on several fronts.
• A “Holy grail” of plant production would be to change the way plants photosynthesize: there are two main biochemical pathways of CO₂ fixation termed C3 and C4: wheat and rice are C3, corn (maize) C4; C4 is favored in warm climates so if the genes for C4 could be transferred to rice there could, theoretically be a bump in rice yields. However, this is easier said than done as C4 is a multistep process also involving a different leaf anatomy, but it is being researched at several institutions. Interestingly some of the increase in corn yield has been by a conventional breeding change in plant architecture, enabling more plants to be grown per acre.
• Another sought-for change would be to add the ability of non-legume (pea family) crops to gain the ability to fix nitrogen, a required ingredient for proteins, as occurs in legumes via the hosting of Rhizobium bacteria in root nodules. Again, this would involve inserting multiple genes for a complex process, but it may be possible in the future. One way around this is to enhance the uptake of nitrogen from the soil via nitrogen-ion uptake-proteins, and this has been achieved in rice allowing larger plants to be produced at low levels of nitrogen fertilizer. For example, Arcadia Biosciences recently reported that genetically-engineered rice carrying the nitrogen use efficiency, water use efficiency, and salinity tolerance traits outperformed non-modified lines by an average of 25 percent under limiting nitrogen applications, and up to 50 percent under drought conditions. The caveat here is that these traits only display advantages under appropriate nutrient-poor/adverse conditions, but as that is the case in many places, especially in developing countries, these traits can provide desirable yield increases.
• Increased height: Changes in plant height are regulated by plant hormones, specifically auxin and gibberellin. Increasing the general auxin level does not work as auxin regulates many developmental processes. Indeed, the original vector for genetic engineering is the bacterium Agrobacterium, which produces galls on plants by transferring multiple genes into the plant cells, including genes for auxin and cytokinin synthesis. As the hormone genes induced unregulated growth the molecular biologists stripped the hormone genes from the transferred DNA, thus removing the gall-forming characteristic, while retaining the gene-vector capacity for the insertion of other desired genes. Gibberellin has a more direct effect on stem elongation; for example, dwarf beans have a low gibberellin and pole beans a high gibberellin content, and you can convert a dwarf bean into a tall bean by spraying gibberellin or transferring a gene for the final step in gibberellin biosynthesis. But would you want to? A problem with older varieties of wheat is that they were too tall and frequently lodged (fell over) in the rain and wind.  Dwarf varieties could be produced by blocking the production of gibberellin using genetic engineering.  However modern high-yielding, stiff-strawed wheats are the result of a block in the gibberellin response system. Within the plant there is a natural brake on the growth system and what gibberellin does is take the brake off. A mutation in the brake gene means that the plant grows tall without gibberellin.  However, if the gibberellin receptor is modified so it doesn’t respond to gibberellin then the resulting plant is dwarf. As these genes are known other tall or dwarf crops could be produced by genetic editing using CRISPR-Cas9, which simply makes gene modifications but does not add any genetic material.  Interestingly the current dwarf wheats are the result of a long-ago spontaneous mutations and most derive from dwarf wheats from Japan that have genes for the blocked gibberellin-signaling system.