In order to promote healthy and vibrant crops, we have to deal with all the stresses and competition this entails for our burgeoning plants. Farmers do their best to help reduce limitations on growth by applying fertilizers, nutrients, water, and overall using mixed soils and crop rotation in order to create the best possible living space. The downside to this system is that it also gives the best possible growing conditions for the wrong kinds of plants. Weeds are one of the most direct banes to any agricultural system, as the undesired seedlings take nutrients away from the plants you want to receive them and create a negative, competitive environment for development and eventual yields.
The best way to deal with this has been to use herbicides of varying types in order to kill off the weeds and leave behind the wanted crops. Originally, spot application and hand weeding was the best one could hope for. But as farmers began growing for people besides themselves and their farms expanded to several acres or more, this became less and less feasible of a practice unless dozens of workers were available to do the work, driving costs up. So, farmers and agronomists sought alternatives.
The Problems of Success
The first herbicide resistant plant breed arrived on the scene quite some time ago, over a century past at this point in time. The plant breeders were using artificial selection and new methods of mutagenesis like the new-fangled property known as radiation. This helped speed things along until one lucky breeder created a plant with the first resistance to a used herbicide at the time. Lucky timing resulted in both this and new fertilization production techniques being made at around the same period, revolutionizing agriculture as a whole.
The scientific aspects of such cultivars has only expanded since, with ever better, more specified, and less generally toxic herbicides being developed as new understandings of biochemistry emerged. However, these discoveries also coincided with globalization and a greater availability and need of expanded agriculture around the world to feed a booming population. This, in turn, resulted in more farmers using those same developed herbicides, with the most commonly known today being glyphosate.
Unfortunately, a worldwide spread also entailed usage by those in countries without controls over best policies to prevent weed resistance. Thus, the past several decades has seen the rise of resistant species against practically all modern herbicides, creating a dilemma. Should we move back to using the more harmful older pesticides or should we look toward creating new alternatives? Scientists have been hard at work on the latter and have come up with options that go beyond simple herbicides and into the realm of influencing environmental plant growth as a whole.
A Sought After Alternative
The latest example of this trend comes from Texas A&M University, where its researchers have managed to make a transgenic cultivar of cotton with a unique fertilizer preference. Cotton plants were among the hardest hit by the formation of resistant weeds and yields have been slowly falling. In order to help improve the growth of these crops and combat weeds at the same time, the researchers borrowed a gene from the bacterium Pseudomonas stutzeri.
The gene, named ptxD, is a phosphite dehydrogenase, meaning that it confers the ability to break down phosphite (Phi) into orthophosphate (Pi), removing a hydrogen. When previously tested in rice and tobacco, it appeared to allow them to use phosphite as their sole source of phosphorus, not requiring any other forms from the soil or from fertilizer. As phosphate-reduced soil makes up nearly 2/3rds of all the arable land on the planet, this means that plants can be produced in this soil by adding phosphite. Meanwhile, weed species, as they lack the ability to break down phosphite, would be unable to grow to any meaningful degree in this same soil, dealing with the general problem of resistant weeds without the need for any form of herbicides.
Thus, the researchers transgenically transferred the gene into a cotton cultivar and confirmed its successful usage in that species as well. When tested against aggressive weeds like tall morning glory, purple false brome, and a glyphosate-resistant form of Palmer’s pigweed, the transgenic cotton quite easily outperformed all of them with no complications. So long as phosphite was the only available form of phosphorus, the cotton was able to thrive and leave the weeds in the dust. The capacity is even robust enough that the cotton still outperforms in acidic or alkaline soils or any other number of comparative conditions. The simple fact that the cotton could make and utilize a processable form of phosphorus proved a greater factor in competition with weeds than anything else.
In fact, it appeared that the weeds were growing even worse than they should have been, despite the lack of phosphorus. The researchers hypothesize that phosphite manages to “trick” the weeds into not even activating their soil nutrient response systems, exacerbating their lack of phosphorus state. Meaning that while phosphite isn’t outright a herbicide itself, it can still manage to indirectly inhibit the growth of weed species so long as usable phosphorus isn’t accessible.
And since phosphite isn’t harmful to microorganisms in general, supplying it in a specialized fertilizer mixture doesn’t affect the soil microbiome. The same is true if some of it is included in runoff into water systems. Algae would be unable to do anything with it, preventing the damaging algal blooms that have been occurring from excessible fertilizer runoff.
The Texas A&M scientists are now looking into determining strong root-specific promoters in the cotton genome that can be overexpressed to improve phosphite uptake by the transgenic cotton cultivars. Then they plan to test them against these originals to see if uptake overexpression proves better for plant development or not. They hope that this ongoing research will become a major solution to the issue of resistant weeds and concerns about herbicide usage in general.
Photo CCs: Cotton boll nearly ready for harvest from Wikimedia Commons