The fact that a plant is strong against diseases or pests is expressed as “having strong resistance.” This “resistance” is not a single function but a combination of various functions, such as synthesizing chemical substances or changing the structure and surface of tissues. There is no universal resistance that is effective against all diseases and pests. For example, being resistant to diseases caused by fungi or viruses does not necessarily mean that the plant is resistant to insects. Because plants cannot move, they have developed various strategies to cope with diseases and pests.
Plant “resistance” is a weapon specialized for each enemy that tries to harm the plant, and sometimes it may miss the mark. There was an interesting study that gives us a glimpse into this.
The Existence of a Specific Mite that Infests Corn Varieties Resistant to Mites
Mites are troublesome pests that cause damage to many crops, but corn varieties resistant to mites have been developed, making them indispensable in industrial varieties. These varieties are known as B75 and B96. These two varieties have been confirmed to have resistance to “twospotted spider mite (Tetranychus urticae),” which causes damage to various crops, and to suppress its feeding and reproduction.
However, there exists another mite that is unaffected by the resistance of B75 and B96. That is the “Banks grass mite (Oligonychus pratensis).” The Banks grass mite can eat the leaves of resistant corn varieties, build nests, and reproduce. On the other hand, it has been found that the Banks grass mite only infests certain grasses, such as corn. It is considered a specific mite that has gained the ability to survive in environments where other mites show resistance by specializing in certain grasses.
Why Are Corn Varieties Resistant to Mites Vulnerable to “Specific Mites”?
The resistance factors of B75 and B96 lie in their ability to produce an antibiotic called “DIMBOA.” This antibiotic has been shown to suppress the growth of not only mites but also other pests such as moths. Because they can synthesize this antibiotic, B75 and B96 exhibit resistance to the twospotted spider mite.
However, it is believed that the Banks grass mite has evolved to avoid the effects of this antibiotic. Therefore, B75 and B96 are vulnerable to other pests with similar characteristics, including the Banks grass mite.
Although resistance was evolved to counter “generalist” pests like the twospotted spider mite, which harm many crops, there are no countermeasures against “specialist” pests like the Banks grass mite, which has honed its specificity to certain grasses.
The Importance of a “Library” that Can Always Provide New “Resistance”
To counter the Banks grass mite, a specialist of grasses, the antibiotic “DIMBOA,” possessed by B75 and B96, is not sufficient. To create corn resistant to the Banks grass mite, other forms of resistance must be considered. For example, making the plant’s surface less hospitable for mites or producing substances that mites dislike. In nature, such evolution requires an enormous amount of time, but humans can achieve this through breeding. For this reason, a library of varieties that already possess various resistances is crucial. For instance, varieties that are not tasty but strong against insects, or that do not bear large fruit but are resistant to diseases, are useful materials for breeding. Given the effects of climate change and globalization, it is uncertain what diseases and pest damages may occur in the future. Therefore, having a “resistance library” that can respond to various situations is of utmost importance.
Accumulating “Resistance” Through Breeding
The crops we commonly use could be considered a culmination of resistances. In addition to natural evolution, various resistances have been integrated into a single variety through human breeding. Such efficient evolution would be impossible in the natural world. On the other hand, there are varieties that require more water and fertilizers than wild species and can no longer grow without human intervention, which is also impressive as a successful business.
There are also diseases that no longer need to be addressed due to advances in agricultural technology, and examples where resistance has decreased compared to older varieties. For example, strawberries can now be protected from pathogens through soil sterilization, which has led to the development of current varieties with lower resistance.
Thinking of breeding as responding to the ever-changing “resistance needs,” it is similar to industrial trends. Breeding has no end, and it is important to engage in breeding with a forward-looking perspective.
It’s fascinating to think about what kinds of varieties with resistance will be created in the future.
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