Discovery of a Virulent Population of Meloidogyne graminicola Overcoming the Rice Resistance Gene MG1 and Population Genetic Analysis Based on mtCOI Sequences.

The rice root-knot nematode (Meloidogyne graminicola) is a major pathogen of rice. The resistance gene MG1 in the cultivar Zhonghua 11 (ZH11) confers effective resistance against this pathogen. To date, only one virulent population overcoming ZH11 has been reported, from Cambodia, but whether it overcomes MG1 remains unknown. Here, we collected and identified 31 M. graminicola populations from southern China. Ten of these populations were tested on ZH11. Two of the 10 tested populations were virulent, GZSD1 from Guangdong and HUNSD28 from Hunan, with reproduction factors (Rf) of 3.11 and 1.67, respectively. GZSD1 was also reproduced on NIL-MG1, a near-isogenic line developed by introgressing MG1 into the susceptible variety Huazhan, with an Rf of 2.78. Moreover, GZSD1 showed no significant differences in developmental progression or parasitic ability, either from an avirulent population HAINSD24 on the susceptible cultivar Nipponbare or between the resistant line ZH11 and the susceptible cultivar Nipponbare, suggesting that GZSD1 can overcome MG1-mediated resistance without a significant fitness cost under the tested conditions. Population genetic analysis based on mtCOI sequences revealed high haplotype diversity and low genetic differentiation with weak geographic structuring among the sampled populations. A signal of population expansion was detected. These findings identify the first virulent M. graminicola population overcoming MG1-mediated resistance in rice and suggest that virulence may have the potential to spread. This work highlights the threat to MG1-based resistance and calls for monitoring and mechanistic studies to support sustainable rice breeding.