Different Hypotheses for Resistance Loss of Rice Varieties to Magnaporthe oryzae

Letter

Different Hypotheses for Resistance Loss of Rice Varieties to

It is widely believed that a rice variety loses its resistance to rice blast after three to five years in commercial production due to the emergence of newpathotypes (or physiological races) (Ou, 1985). However, we believe that a resistant rice variety loses its resistance tonot due to the emergence of new physiological races. We propose that pathogenic physiological races already exist in the natural environment, serving as potential physiological races. It is only because of the lack of appropriate environmental conditions, particularly the lack of a suitable rice host, on which such a race is unable to propagate widely and exists as an inferior race. However, when there is an appropriate rice host, the potential pathogenic physiological race can quickly proliferate to become the dominant physiological race, thus leading to the loss of blast resistance in resistant rice varieties.

Typically, the occurrence of variation in organisms, particularly in higher plants and animals, is an exceedingly long and slow process. The occurrence rate of genetic variation in reproductive cells is 10-5–10-8. Therefore, centuries or even tens of thousands of years of gradual evolution are required for a new species to emerge.

Based on Flor’s gene for gene theory, the rice variety possesses resistance to blast because it harbors a gene for resistance to, whileharbors an avirulence gene that corresponds to the resistance gene of rice variety. In contrast, the rice variety is susceptible to blast because it harbors a gene that is susceptible to, whileharbors a virulence (pathogenic) gene that corresponds to the susceptible gene of rice variety (Silué et al, 1992).

The resistance genes of rice varieties lose their effectiveness after a short period in commercial production, typically 3–5 years (Feng et al, 1995; Zeng et al, 2011; Shi et al, 2014; Zhu et al, 2014). The general consensus so far is that the loss of resistance is due to the emergence of newpathotypes, also known as physiological races, which can overcome resistance of host (Ou, 1985). Therefore, the emergence of new physiological race ofenables the virulent pathogens to overcome the resistance of the rice variety to the previous physiological races and leads to the loss of blast resistance.However, there is no experimental evidence from studies conducted under strictly controlled conditions to show that a new physiological race ofemerges after a new rice variety has been planted for 3–5 years. Therefore, this hypothesis remains unproven and may only be speculation.

Our hypothesis is that the resistance loss tois not because the emergence of a new physiological race after 3–5 years, but because various microorganisms already exist in the natural environment. Many types of physiological races exist in nature; the only difference is the existence of different races ofthat are dominant and physiologically inferior.

When new rice varieties with blast resistance are grown, the physiological races ofthat harbor an avirulence gene comprise the dominant population, whereas thephysiological races ofthat harbor the virulence gene are the inferior races. Perhaps the rice varieties that were previously planted in the local area are resistant to the physiological races of, that is, the rice varieties harbor the resistance gene to the physiological races. This leads the physiological races () exist in a starvation mode, in which it can merely survive and is unable to multiply in quantity.

When new rice varieties are resistant to the previous local dominant race but are susceptible to the inferior population or race, the previouslyinferior population (or physiological race) possesses pathogenicity (or harbors the virulence gene) to the new resistant rice varieties. The new variety provides an abundant ‘source of nutrients’ to the inferior population (or physiological race). After several years of thriving and proliferating on a mass scale, the physiological race that had previously been an inferior population (or physiological race) becomes the dominant population (race). This result leads to the conclusion that the blast resistant rice variety has lost its effectiveness after 3–5 years of growth, and the belief that a new physiological race has emerged. In actuality, no new physiological race has emerged, and the physiological race has previously existed. However, the lack of a suitable rice variety that provides sustenance constrains its development (mass proliferation) and results in its status as a marginal group (inferior population or very few numbers). Once there is a host that is suitable for its parasitism and provides abundant source of nutrients, the conditions exist for quick mass proliferation that enables the race to become the dominant one. This change in conditions leads to the belief that a new physiological race ofhas emerged.

There is no experiment that verifies the hypothesis described above. Do you feel that our hypothesis addresses the appropriate scientific principles?

The most important project for future studies on this pathosystem is to identify strong evidence to prove whether the newpathotypes emerges or the existing physiological races become the dominant one from members of an inferior population.

Acknowledgements

This work was funded by the Key Research and Development Project of Zhejiang Province, China (Grant No. 2019C02018), the National Key Research and Development Projects of China (Grant Nos. 2016YFD0200801 and 2018YFD0200304), and Innovation Project of Chinese Academy of Agricultural Sciences (Grant No. CAAS-ASTIP-2013-CNRRI).

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Ou S H. 1985. Rice Diseases. 2nd edn. Kew, UK: Commonwealth Mycological Institute: 109–201.

Shi M L. 2014. Relationship ofdiversity and rice varieties and geographical distribution. [Master Thesis]. Nanjing, China: Nanjing Agricultural University. (in Chinese with English abstract)

Silué D, Notteghem J L, Tharreau D. 1992. Evidence of a gene for gene relationship in the-pathosystem., 82(5): 577–580.

Zeng F S, Xiang L B, Yang L J, Yang X L, Yang J S, Yu D Z. 2011. Diversity analysis for resistance of 251 rice (L.) varieties (lines) to rice blast disease., 4: 399–410. (in Chinese with English abstract)

Zhu D. 2014. Analysis of avirulence gene of, genotype of rice resistance to blast and resistance identification in fields in Hubei Province. [Master Thesis]. Wuhan, China: Huazhong Agricultural University. (in Chinese with English abstract)

Zhao Kehan, Huang Shiwen

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This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer review under responsibility of China National Rice Research Institute

http://dx.doi.org/10.1016/j.rsci.2020.11.003

Huang Shiwen (Huangshiwen@caas.cn)

16 March 2020;

22 April 2020