New Development in the Fight Against Parasitic Plants

By studying the function of strigolactones (a plant hormone) in rice, scientists may have uncovered a genetic engineering and/or chemical approach to reduce parasitic plant destruction of crops

---

Parasite – a word we fear – because these are organisms that obtain some or all of their nutrients from another organism, usually slowly over time without killing their host. You may be familiar with parasites that feast on humans, such as the itch mite that causes scabies, but are you also aware that there are parasitic plants that feed on other plants? Many farmers and scientists are aware and are studying them diligently. This is because parasitic plants, like Striga hermonthica (a.k.a. ‘giant witchweed’), wreaks havoc on agricultural productivity by draining the life from crops. Witchweed is even considered a major threat for global food security, especially in Africa where it causes > $7 billion (USD) annual losses in cereal production.

So what are scientists doing about the problem? They are trying to develop Striga-resistant crops. One of the roads to achieving this goal is to understand the chemical signaling that occurs between the parasitic organism and the host. If this interaction can be understood, then scientists could develop ways to prevent the interaction.

One such cross-species chemical signal occurs through the group of plant hormones known as strigolactones (SLs). SLs are produced by plants and act as plant-developmental hormones and are also released into the soil to promote symbiotic relationships with microbes, such as mycorrhizal fungi (which help the plant uptake nutrients and water from the soil). In the soil, however, also resides the seeds of parasitic plants, which evolved to sense SLs as well. When a parasitic-plant seed encounters SLs, the seed ‘knows’ its close to a host and triggers germination (or the beginning of plant development). Then, the parasitic plant will grow intimately with the host plant – always stealing its share of nutrients and leaving behind a less hearty crop.

SLs were first discovered to trigger parasitic plant germination in a study from the 1960s, and since then, scientists have learned what genes and enzymes are involved in SL biosynthesis. So if we have that understanding, you may wonder why scientists have not yet genetically engineered crops to stop producing SLs, thereby preventing their release into the soil. Unfortunately, because SLs affect plant development, it is not that simple. Plants where scientists have altered SL biosynthesis often have developmental issues (e.g. shorter shoots and roots), which is not what a farmer wants for their crops. However, in a recent study, scientists may have uncovered a way to disrupt SL signaling to parasitic plants but have only minor, negligible effects on overall plant host development. This result came because of the understanding that not all SLs are the same.

SLs occur in two major varieties, canonical and non-canonical. Within these two categories there are over 30 known SLs that differ in their molecular structure and function. Previous studies show that plants release more than one type of SL into the soil and in different combinations depending on the plant species. When the types of SLs released into the soil were looked at more closely, scientists realized that canonical SLs are less commonly observed than non-canonical SLs across plant species. This observation (and others) led to the thinking that canonical SLs are probably not major regulators of plant development like non-canonical SLs. This is because hormones that evolved to be major regulators of plant development are expected to be widely spread across the plant kingdom.

That is why scientists in this recent study investigated the biological function of canonical SLs in rice, which have not been thoroughly characterized. The scientists hoped to find that disrupting canonical SL biosynthesis would lead to reduced parasitic plant germination, while not affecting plant development (size or morphology) or the colonization of roots by mycorrhizal fungi. What they found was pretty close.

The scientists studied the role of canonical SLs by genetically engineering plant mutants that have disrupted canonical SL biosynthesis. They then compared the mutants to plants with healthy SL biosynthesis (referred to as wild-type plants) in different categories. For parasitic-plant germination, the scientists found that disrupting canonical SL biosynthesis in the host led to > 50% decrease in the germination of two different parasitic-plant species. For plant size and morphology, the scientists found that disrupting canonical SL biosynthesis did not cause the usual developmental issues seen from disrupting the biosynthesis of other SLs, and instead only caused a subtle difference in plant morphology and size. For mycorrhizal colonization, the scientists found there was a significant delay in colonization in the first 10 days after inoculation with the fungus, however, this delay almost fully disappeared by 35 days.

Their findings suggest that canonical SLs play a major role in parasitic plant germination (although other SLs must as well), a minor role in plant growth – at least in rice (and tomato plants based on another study) – and a major role in the early establishment of mycorrhizal connections but not necessarily in their long-term development. So overall, the scientists concluded that genetically removing canonical SL biosynthesis from rice can significantly reduce damage caused by parasitic plants but not cause large, negative impacts on plant growth and mycorrhization.

However, successful genetic engineering can take years of development so the scientists also worked to identify a chemical inhibitor that would mimic genetic disruption of canonical SL biosynthesis. After diligent testing, they found an inhibitor (TIS108). This suggests that genetic engineering or chemical inhibition could be a technique to prevent parasitic plant destruction of crops in the future.

But, there is still work to be done, such as identifying ways to make the method more effective (e.g. completely remove parasitic-plant germination and/or have no effect on plant and mycorrhizal development). I wonder if there is a way to target the mechanism that releases canonical SLs into the soil instead of completely disrupting their biosynthesis. If this were possible, then perhaps the minor effects on plant development could be mitigated. There should also be extensive testing for any effects these chemicals may have on other organisms when entering the environment. Perhaps they would be disruptive to other native plants in the area of use.

Also, although the effects on mycorrhizal colonization seemed minor, I think a better understanding on the impact of these effects would be beneficial. For example, what would this mean when scaling up to an entire field of crops having delayed connections? What about farming areas where mycorrhizal connections are periodically disrupted? Would use of an inhibitor make it difficult for plants to consistently re-establish mycorrhizal connections? And what happens to mycorrhizal connections at timepoints past 35 days? The study hinted that TIS108 may have a negative effect on mycorrhizal connections at later time points.

However, overall, this study marks an important advancement in the battle against parasitic-plants. Scientists can use this data to help further develop a way to protect crops and increase crop yields. Researchers from the study also believe TIS108 will help uncover more genes involved in canonical SL biosynthesis. These future advances could lead to an immense reduction of wasted money in agriculture and perhaps get us closer to a world where there is enough food to go around.