Whether through natural selection or human artificial selection from breeding, developing resistances to pests and other plant-damaging organisms isn’t always a straightforward path. There won’t always be a simple single gene and trait that can be manifested that will fix all the problems. For many traits, there is a required tradeoff where the organisms may have to take a negative outcome just because it gets rid of or inhibits the far bigger negative of a pathogen or parasite. 

Low Resistance Sustainability

Those sorts of traits aren’t an overall positive growth and development result, but so long as the overall line on their well-being is raised, the choice can be considered good enough for the trait to stick around and be selected for thanks to fitness benefits. However, there can easily be other factors that tip things back over that grey line, such as a region with poor soil nutrition. 

So you can end up with the same species across a broad region that has variable population uptake of a gene and trait based around those additional conditions and cause the change to have a low adoption rate that doesn’t trend toward fixation, but also never completely goes away. Since it is still at least an overall good for a portion of the local population, even if not for the greater population in general. 

A more precise example of this comes from the case of sorghum growth in Africa, specifically sorghum bicolor, and its dealings with the parasitic weed striga hermonthica, more commonly known as witchweed. There has been difficulty in the past in studying these small population impacts, especially those that are mainly found among smallholder farmers, so the amount of genetic mapping and overall diversity of plant and pathogen isn’t well known. 

Hormonal Fungal Alliance

Researchers at Penn State University wanted to see what exactly was giving some of these localized sorghum crops their resistance to witchweed and figure out why was the trait so rare overall and not spreading to fixation. Their first step was to set up genotype-environment association (GEA) maps to find out if there is a correlation between where the trait was popping up and some factor in those same environments. 

This is especially relevant to sorghum because of its ability to grow in many locales, including tolerating being in what would be an incredibly harsh landscape for other species. And there is an at least partially understood plant hormone that helps control for these resistances called strigolactones. 

Under conditions of water or nutrient deprivation, these hormones are released from the roots to interact with the soil microbiome and the surrounding community of arbuscular mycorrhizal fungi. These are the type of fungi that use their mycorrhiza tendrils to extend into the plant root cells themselves and form a hollow known as an arbuscule. Thankfully for these plants, the fungi in this case are fully beneficial and symbiotic with the plants, using these signals to find their friends and to even help overall germination speeds of the plants themselves. 

But it is this very use of signaling hormones that is the problem, as it leads the witchweed parasite to them as well and acts as a germination signal to the parasitizing roots they attach to the roots of their host plant. The researchers have been thus looking into some populations of sorghum where the witchweed is no longer able to find their hosts to invade for nutrients. 

A Back And Forth

What they found is a particular gene variation in the gene Low Germination Stimulant 1 (LGS1) where they had a loss of function, causing produced strigolactones to have a different molecular composition and structure than normal. Though it was to another known strigolactone, from 5-Deoxystrigol to Orobanchol through the loss of a hydroxyl (oxygen with hydrogen) group. 

This very minute alteration, among other similar changes, seemed to confer almost perfect resistance to witchweed and the scientists wanted to know why. Other mutations in strigolactone biosynthesis and signaling genes did not seem to confer such a trait, as far as could be seen, only LGS1 did. Deeper investigation was conducted by using CRISPR-Cas9 to knockout the LGS1 genes with different kinds of mutations, to see how each kind of change affected the overall resistance pattern, if at all. 

These deletion lines saw a massive reduction in witchweed germination rates with reductions even reaching less than 1% total parasitic germination of the plants and wild type seeing 40% of the control group being infected. Overall gene expression of over 2000 genes was observed in the LGS1 knockout lines as compared to the controls, implying a massive change in general regulation and production of strigolactones. Surprisingly, they also saw several other impacts, including reduced photosynthesis gene expression and smaller overall leaf area.

Their experiments concluded that there is a tradeoff for obtaining resistance to witchweed. Altering strigolactones in order to prevent parasitic germination also reduced other genes due to lacking those proper phytohormones. And the fact that this trait change has only been adopted in a small number of locations tells us that it is a difficult tradeoff to make. 

Complicated Relationships

Not only do the difference in selective pressure in various regions change the setup for such a tradeoff, but also the variant of witchweed and what strigolactones they respond to alter how useful and effective LGS1 mutations are to a population of sorghum. Negatively affecting the plants’ ability to contact their fungal microbiome for symbiotic assistance can additionally be a bane to such a selection. 

What all of this highlights for us is that there is indeed a much more complex interactive web going on between plants, their symbionts, and their parasites. And that they can happen in such a localized area that one can’t be certain that any particular trait is consistent for the global population and one can’t know if there are further variations out there that exist only in small regions of the world that are unknown to everyone else. 

Evolution, natural selection, and the pressures of ecosystems can cause an almost infinite number of effects on organisms and their growth and if scientists want to know this information for the betterment of everyone, then our work is always ongoing. There will always be more to learn and see and we’ll just have to put our heads together to figure them out.

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Image CCs: Germinated Striga hermonthica Delle (Benth.) from Wikimedia Commons

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