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Soybean is the second most important crop for the United States with an annual production output of over 100 million metric tons. This crop is native to Asia and was first brought to the United States to feed animals. However, it has slowly become one of the most important crops for humans too. Almost all of the vegetable oil sold in the supermarkets are actually soybean oil. All the tofu that we eat are made from soybeans, and of course the most popular soy beverage or soy"milk" is also made from soybeans. Not just food, soybeans are used to make biodiesel that can run powerful trucks and engines. Talk about vegan machines! Furthermore, plastics, inks, candles and paints are also made from soybeans. No wonder we grow so much soybeans everywhere. If you took a road trip anywhere countryside in the midwest United States, especially the upper midwest, you are definitely going to see fields of soybeans alternating with fields of corn.
When soybeans were first introduced, they did not grow well in the United States. It was suspected that the soil did not have the beneficial bacteria that help soybeans grow well. Hence, decision was made to bring some soil from soybean's origination, as inoculum for the beneficial nitrogen-fixing bacteria. We believe that the soybean cyst nematode (SCN) caught a piggy-back ride on that trip. Not much came through, but within a few years, the nematodes managed to reproduce to an extent of being an issue for the poor soybeans.
Did you know?
Did you know?
Soybeans are not native to the Americas
Soybeans are not native to the Americas
"3"
"3"
Three is a magical number in the soybean world
Three is a magical number in the soybean world
Although Asian, India is not a country that produces or uses a lot of soybeans, not at least in the south where I am from. Although I must acknowledge that it is getting more visibility and popularity these days; the last decade. So, my introduction to soybeans was not until I came to the United States and started working with SCN. Like any other plant, soybeans are interesting to me. I also think that they are a little fussy. They do not grow well outside of a narrow pH range of 6 and 7 and they are very sensitive to watering. If you are growing soybeans, please note that they are very dramatic and want to die on you all the time. They have trifoliate leaves, meaning that their leaves grow in threes and the beans are borne in green pods. Soybeans take about 120 days to go from seed to seed and require a lot of sun and temperatures above 25 degrees C (77 degrees F). In my opinion, they have fluid growth and can take up the space that is available around them. If there is more space between two plants, they would grow bushy and if there isn't much space, they grow like a hybrid between woody shrubs and climbers creepers. Light availability and quality also affects the shape of soybean plant growth (bushy vs stringy). Depending on the variety of soybeans, their flowers can be purplish-pink or white.
Soybeans and SCN
Soybeans and SCN
There are so many diseases soybeans can suffer from. However, the SCN is the most devastating of all of them. No fungus, bacterium or oomycete or other microbes are as successful as SCN is. This nematode is also very sneaky. Whenever soybean suffers a bacterial, fungal or oomycete disease, the symptoms are soon evident. With SCN infection, above-ground symptoms are too late to appear. By the time you see short and yellow plants that is struggling to set any pods or seeds, it is too late and the field is well entrenched by the nematodes. The yield loss could be anywhere between 10 - 40%.
The SCN is more like a leech on soybean roots. They attach to one spot and start draining all the nutrients. When all the food soybeans make through photosynthesis is directed to feed and nourish SCN, there is nothing for the plants to use to grow or make more seeds. Moreover, this means that SCN needs to keep its host alive as long as possible - this type of relationship is called a biotrophic relationship. This also means that the SCN is in constant tussle with the soybean plant and its immune system (yes, plants have immune systems too! plants get sick too!) to exploit its host's ability to photosynthesise. In the above picture (courtesy of University of Minnesota), there are soybeans in the field that are sick because of SCN infestation on the left and the ones that are not sick on the right. However, the reason for that is because the soybean plants on the right have genetic resistance to SCN. So, that begs the question...
Do all soybeans get sick with SCN?
Do all soybeans get sick with SCN?
Obviously, there is more than one variety of soybean. There are different soybeans for different uses. There are those which are great for milking, those that are great to be consumed as edamame and those that are grown for oils. In extension, not all soybeans get sick the same way when they are attacked by SCN. There is genetic diversity in all life forms. But when it comes to agriculture, we select a few genetic varieties that we like, based on how they grow, look, taste or even how much oil or protein they have. Most importantly, when we see disease, we also take into account naturally disease resistant varieties, to include in our breeding programs. Plant breeding has its own challenges and some gene editing technologies and genetic modification techniques can help overcome them. For example, what might take decades to bring characteristics from two different genetic varieties together (say protein content and disease resistance) can be achieved in a few years with these tools for genetic modification. If you think about it, breeding is in fact genetic modification, albeit slow and old-fashioned. For more of my thoughts on GMOs, check this page out on my website.
Picture from Signature Soy
There is more than one variety of soybeans
There is more than one variety of soybeans
as there are many uses for this crop
as there are many uses for this crop
Picture from Pioneer Canada
Two main sources of genetic resistance in soybean towards SCN
Two main sources of genetic resistance in soybean towards SCN
Often times, there is more than one variety of a plant that resists the same disease causing organism in completely different ways. In that case, we would want to combine as many different ways of resisting the pathogen or the parasite, into one variety. This will give more resistance to the pathogens/ parasites and hence dampen the rate at which the parasite/pathogen can evolve to overcome those resistances. Unfortunately, when we try to breed those resistance traits together, we lose out on yield and other agronomic characteristics. This is the main reason why we do not have sustainable resistance in the fields. We often end up with one type of resistance and using that resistance for a long time in the fields will incentivise the nematodes to evolve against that resistance within a few decades. This is in fact the case with soybeans right now.
The genetic variety PI88788 has been the most successful in integrating its traits into other varieties with good agronomic traits (yield, protein and oil content, size, colour, etc.). The varieties with PI88788 as the resistant parent has a field share of 95%, meaning that 95% of resistant soybeans in the field use the same type of resistance derived from the PI88788 variety. This has been going on for decades and we are already seeing some SCN populations in the field that have overcome this resistance. Another resistant variety called "Peking" has been the most of the other 5% of resistance in the market and the fields. However, historically, this "Peking" resistance is associated with lower yields and that explains why it has not been successful thus far. It is noteworthy that that is not true anymore. There are high-yielding "Peking" derived soybean varieties these days. Thank you, SCIENCE!
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