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Plant hormones: they are not just for horticulture anymore

Grain crop performance may benefit from hormone manipulation.

Plant hormones have long been used in horticultural applications for plant propagation and fruiting control, as well as in agriculture for weed and fungus control. Researchers are now looking to plant hormones to provide the next advances in improved crop growth and yields.

Research is attempting to identify the use of plant hormones to limit lodging potential in wheat.

Plant hormones are signalling molecules that are produced within the plant to control plant growth and responses to the environment. The better-known classes of plant hormones are abscisic acid, auxins, cytokinins, ethylene and gibberellins, which are involved in various processes such as cell division and growth, stress responses, dormancy, flowering, fruiting and senescence. Plant hormones occur naturally in all plants, algae and some fungi, and elicit responses at extremely low concentrations.

Current research on plant hormones for use in crops such as wheat, corn, soybean, and others, falls into three general categories related to different mechanisms of plant growth: the application of hormone treatments to improve growth, genetic modifications to protect plants against stress, and genetic modifications to improve seed filling. These methods can affect final yields in different ways.

Hormone treatments can improve specific aspects of growth
Dr. Alexander Pavlista, professor of agronomy and horticulture at the University of Nebraska, studies the use of hormone treatments to improve the early growth of winter wheat. In Nebraska, as in Ontario and other similar areas, the optimal planting date for winter wheat is early to mid-September to allow enough growth before winter. However, fall crops such as potato, sugar beet and soybean in Ontario, are not harvested until early October. To achieve increases in wheat seedling height, Pavlista is testing the use of gibberellins, particularly gibberellic acid, or GA3, which is the most active of the gibberellins and promotes longitudinal cell growth.  By applying small amounts of GA3 (125B1000 ppm) to wheat seeds, Pavlista found that seedlings from treated seeds grew taller faster than their untreated counterparts for both a semi-dwarf and a standard variety of wheat. He is currently in the third year of field trials. “What we discovered is that we promote the height growth of the plant and that we gain two weeks. In other words, the GA-treated plants that are planted in early October are the same size as the untreated plants that are planted in mid-September. We still find a little of that effect in height in March, but it disappears by May.” He notes that by May, there are no differences in height, biomass, yield or lodging between plants from treated and untreated seeds. “We figure that the GA3 goes away after three to four weeks. The plants have their own natural mechanism for controlling the amount of hormone that is in the plant, and all of a sudden, you’ve thrown at it a whole bunch of hormone, and so the feedback mechanism kicks in to destroy it. So it’s a short window of effect,” says Pavlista. In addition, there is no effect on non-target species because it is only the seeds that are treated prior to planting.

In a sort of reverse process, Pavlista also examines whether compounds that inhibit the synthesis of gibberellins can reduce wheat lodging. These are applied in May to reduce the height of the plant, which is especially important for irrigated wheat. “Just before the wheat really shoots up, you want to add something that will block the natural gibberellin from acting at full force.” As a result, the plants are shorter and less susceptible to being knocked over by the wind. Pavlista notes that there is also a potential for yield increases, but whether they are related to more complete harvests of standing than of lodged wheat, to a shift in resources from stem growth to grain filling, or to a combination of these, is still in question.

Because of the short window of effect of plant hormone treatments, their use to improve crop yields in the field has limited applications. In some cases, the hormones must be applied to a very specific site on the plant to achieve the desired result. The timing and rate of application is critical, especially since what you are doing is manipulating what’s inside the plant,” says Pavlista.

But what if there were a way to temporarily extend the life of a plant hormone so that it is resistant to natural breakdown by the plant, but still maintains its biological activity? This is the research focus of Dr. Sue Abrams, chemist with the National Research Council of Canada. Abrams works with abscisic acid (ABA), a hormone that is involved in seed germination and plant responses to environmental stress such as drought. “ABA is involved in closing the stomata, or pores, on the leaves of plants so that water isn’t lost. “We’ve been making chemical versions of the plant hormone that can be applied to plants to make the plant close the stomata for longer, making it drought tolerant,” says Abrams.
 
Normally, plant enzymes inactivate ABA by adding an oxygen atom to a specific site on the ABA molecule, explains Abrams. The chemical versions, or ABA analogues, are built like a normal ABA molecule and are recognized by the plant, but have a minor alteration that prevents the addition of oxygen.
Treatment with such an ABA analogue could allow the plant to conserve water temporarily and survive a drought. However, this also temporarily prevents the plant from absorbing the carbon dioxide it needs to grow. Says Abrams, “It’s not going to grow, but it’s not going to die, either.”
 
Gene modification can alter plant responses to stress hormones
Another way to achieve the effect produced by external hormone application is to genetically alter how plants perceive their own hormones in response to external cues such as environmental stress. For example, in the late 1990s, a gene mutation was discovered in the plant Arabidopsis that made it much more sensitive to an increase in its ABA and much more drought tolerant. The mutation completely prevented the expression of the gene, but unfortunately had other negative effects on the plant. “What we found in subsequent research is that you don’t have to completely knock out the function of that one gene,” says Dr. Malcolm Devine, vice president of food crops and commercialization for Performance Plants in Saskatoon. “If you can go in there and just reduce the expression of that gene so that it’s only operating at maybe 20 percent capacity, the plant will grow and behave normally, but will have the one desired effect of that mutation, that is, it will be more tolerant of drought.”
 
Researchers at Performance Plants have modified the expression of this same gene in canola and other species. Several years of field tests showed that the mutated canola produced better yields under drought conditions than did normal canola.
 

One advantage of this method is that it involves the manipulation of a gene that is normally in the plant, rather than the introduction of a foreign gene. “We’re not putting in a bacterial gene or even a gene from another plant species,” explains Devine. Rather than waiting for a natural mutation to occur and then breeding to emphasize that trait, as in standard plant breeding programs, the initial mutation is engineered using specific methods. “The plant isn’t making any new protein at all. In fact, it’s making less of a protein that’s already in it,” says Devine.
 
The trait is now licensed to seed companies to develop it for commercial release. The first release is expected in 2011 or 2012 for corn.

Gene modification may alter seed filling directly
The alteration of hormonal responses to stress ultimately has only an indirect effect on yield.  Another approach is to affect yield directly at the seed filling stage by altering the plant’s hormonal response to regular developmental cues. Since 1994, Dr. Neil Emery, a professor at Trent University, has been doing leading-edge research on seed filling and cytokinins, which promote cell division. Emery says that the objective is to get the plant to produce the hormone at the right time and the right place. “The overall way of looking at how a cytokinin acts is that it increases the ability of a cell to draw assimilates into it,” explains Emery. “If you dump it on the entire plant, it’s increasing the ability of every cell, so they’re all fighting amongst each other to get the same stuff.” The timing is also specific. “There’s a cell division phase during embryo development and there’s a cell filling stage when the seeds fill. You want to get them when they’re making their cells so that the more cells they make, the more potential they have for a large seed.”
 
This stage is very short, approximately four to 10 days after fertilization. This combination of factors makes external hormone application difficult. Emery is looking to identify a gene promoter that is normally activated at the cell division stage and to attach it to a cytokinin gene. The promoter would then turn on cytokinin production at the desired stage of embryo development. This research is not yet been applied, although Emery is working with industry partners to develop potential applications.

Plant hormone uses  expected
The plant hormone approach to improving crop performance and yield seems a fresh perspective on an old problem.  One might presume that the consumer response to plant hormone-related genetic modifications would be more accepting than that to previous types of modification because they do not involve the introduction of genes from other species, but rather the alteration of genes that are already in the plant. But this remains to be seen.

Despite the challenges involved, the benefits could be great. For one, plant hormones may be an alternative to high additional nutrient inputs because they seem to redistribute the resources that the plant already has to different areas of the plant; for example, from height growth to seed filling, or to allow better retention of resources like stomatal closure to conserve water. In terms of the effects of plant hormones in the diet, “You eat them every time you eat a salad,” says Pavlista. “They are ubiquitous.”