Genetic analysis and fine mapping of the gall midge resistance gene Gm5 in rice (Oryza sativa L.).

KEY MESSAGE: The rice gall midge resistance gene, Gm5, confers remarkable antibiosis and is located in the same region on chromosome 12 in three different rice varieties. Fine mapping narrowed this region to a 49-kb segment and identified two candidate genes showing remarkable response to GM infestation. The Asian rice gall midge (GM; Orseolia oryzae; Diptera: Cecidomyiidae) invades rice shoots and forms galls, adversely affecting plant growth and yield production. Thus, the development of resistant varieties through the identification, mapping, and application of GM resistance genes is considered the most efficient strategy for managing this insect. Here, a GM resistance survey of F2 populations derived from intercrosses between resistant rice varieties 'ARC5984,' '570011,' and 'ARC5833' indicated that the resistance gene Gm5 was located on the same chromosomal region in the three varieties. For the initial mapping, three independent F2 mapping populations were developed for the three resistant varieties, and the Gm5 gene was consistently mapped to the same chromosomal region near marker 12M22.6. Fine mapping, which was conducted using the BC1F2 and BC2F2 populations derived from the 9311/ARC5984 cross, narrowed the Gm5 gene region to a 49-kb segment flanked by the markers Z57 and Z64. In the final mapped region, we detected 10 candidate genes, of which six were analyzed for their relative expression. Consequently, two of these genes, Os12g36830 and Os12g36880, showed significantly higher expression in GM-resistant plants than in GM-susceptible plants at 24 and 72 h after GM infestation. Finally, the PCR amplification of markers 12M22.5 and 12M22.6 yielded clear single bands, and these markers were effectively applied for the marker-assisted selection (MAS) of the Gm5 gene. With the developed MAS markers, the fine mapping of this resistance gene will facilitate its map-based cloning and incorporation into insect-resistant rice varieties through breeding.

Virulence and DNA fingerprinting analysis of Xanthomonas oryzae pv. oryzae identify a new pathotype in Guangxi, South China.

Bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most destructive diseases affecting rice worldwide. However, little is known about the population structure of this organism in Guangxi Zhuang Autonomous Region, South China. Here, pathotypic and DNA fingerprint analyses were conducted to characterize the isolates of Xoo collected from rice leaves in five districts of the region from 2013 to 2016. Their pathogenicity was tested by leaf clipping, and the DNA fingerprints were analyzed by repetitive sequence-based polymerase chain reaction and endogenous insertion sequence element-based polymerase chain reaction assays using the repetitive extragenic palindromic sequence and enterobacterial repetitive intergenic consensus primers, respectively. Pathogenicity assays of 70 representative isolates were conducted using a series of near-isogenic lines and two new pathotypes were identified. All the pathotypes were found to be incompatible with xa5 and Xa7. One pathotype was virulent to Xa14, Xa21, and Xa23, whereas another virulent to Xa21 and Xa23, but incompatible with Xa14. A dendrogram generated for the data sets obtained from DNA fingerprinting suggested the prevalence of high genetic diversity of Xoo throughout Guangxi, and no association between the molecular haplotypes and pathotypes was identified.

Development of molecular marker and introgression of Bph3 into elite rice cultivars by marker-assisted selection.

The brown planthopper (BPH) is a serious insect pest of rice and a substantial threat to rice production. Identification of new BPH resistance genes and their transfer into modern rice cultivars are effective breeding approaches to reduce the damage caused by BPH. In this study, we mapped a BPH resistance gene to a 50-kb genomic interval between two InDel markers 4M03980 and 4M04041 on the short arm of chromosome 4 in indica rice cultivar BP60, where the BPH resistance gene was mapped in Rathu Heenati by Liu et al. (2015) and named "Bph3". This region contains two annotated genes Os04g0201900 and Os04g0202300, which encode lectin receptor kinases responsible for BPH resistance. We also developed a molecular marker "MM28T" for Bph3, and introgression Bph3 into susceptible rice restorer lines Guihui582 and Gui7571 by the marker-assisted selection (MAS) approach. The BPH resistance level is significantly enhanced in the Bph3-introgression lines, the resistance scores decrease from 8.2 to 3.6 for Guihui582 and decrease from 8.7 to around 3.8 for Gui7571. Therefore, developing molecular markers for the BPH resistance gene Bph3 and using them for molecular breeding will facilitate the creation of BPH-resistance rice cultivars to reduce damage caused by BPH.

Map-based cloning and characterization of BPH29, a B3 domain-containing recessive gene conferring brown planthopper resistance in rice.

Rice (Oryza sativa L.) production, essential for global food security, is threatened by the brown planthopper (BPH). The breeding of host-resistant crops is an economical and environmentally friendly strategy for pest control, but few resistance gene resources have thus far been cloned. An indica rice introgression line RBPH54, derived from wild rice Oryza rufipogon, has been identified with sustainable resistance to BPH, which is governed by recessive alleles at two loci. In this study, a map-based cloning approach was used to fine-map one resistance gene locus to a 24kb region on the short arm of chromosome 6. Through genetic analysis and transgenic experiments, BPH29, a resistance gene containing a B3 DNA-binding domain, was cloned. The tissue specificity of BPH29 is restricted to vascular tissue, the location of BPH attack. In response to BPH infestation, RBPH54 activates the salicylic acid signalling pathway and suppresses the jasmonic acid/ethylene-dependent pathway, similar to plant defence responses to biotrophic pathogens. The cloning and characterization of BPH29 provides insights into molecular mechanisms of plant-insect interactions and should facilitate the breeding of rice host-resistant varieties.

[Geographical populations of brown planthopper in Nanning of South China and in Vietnam: a comparative study].

A comparison was made on the differences of the geographical populations of brown planthopper (BPH), Nilaparvata lugens (Stål), collected from Nanning of South China and from Ha Noi, Ha Tinh, Hue, TP. Ho Chi Minh, and Song Cuu Long of Vietnam. The BPH populations from Nanning, Ha Noi, Ha Tinh and Hue could damage the rice varieties with Bph1 and bph2 genes, whereas the BPH populations from TP. Ho Chi Minh and Song Cuu Long showed stronger virulence to the rice varieties with Bph1, bph2, Bph3 and bph4 genes than the other populations. The survival rates of the nymphs from Nanning, Ha Noi, Ha Tinh and Hue populations on the rice varieties TN1, Mudgo and ASD7 were all above 52%, but those on the rice varieties Rathu Heenati, Babawee and Ptb33 were below 44%. The nymphs of the BPH populations from TP. Ho Chi Minh and Song Cuu Long on the TN1, Mudgo, ASD7, Babawee and Ptb33 had a survival rate of above 50%, but those on Rathu Heenati had a survival rate of below 50%. The nymphal development durations of the BPH populations from Nanning, Ha Noi and Hue were significantly shorter on TN1, Mudgo and ASD7 than on Rathu Heenati, Babawee and Ptb33, but for the BPH population from Ha Tinh, the nymphal development duration on TN1, Mudgo and ASD7, and on Rathu Heenati and Ptb33 had no significant difference. The nymphal development durations of the BPH population from TP. Ho Chi Minh on TN1 and ASD7 were significantly shorter than those on Rathu Heenati, Babawee and Ptb33, but for the BPH population from Song Cuu Long, the nymphal development durations on TN1, Mudgo, ASD7, Rathu Heenati, Babawee and Ptb33 had no significant difference. After infested for 192 h, the nymph density of the BPH populations from Nanning and Ha Noi was significantly lower on Rathu Heenati, Babawee and Ptb33 than on TN1, Mudgo and ASD7, that of the BPH populations from Ha Tinh was significantly lower on Babawee than on TN1, Mudgo, the BPH populations from Hue and TP. Ho Chi Minh had a significantly lower nymph density on Babawee than on TN1, Mudgo, ASD7 and Rathu Heenati, and the BPH population from Song Cuu Long had a significantly lower nymph density on Babawee and Ptb33 than on TN1 and ASD7. The virulence of the BPH population from Nanning was similar with that of the BPH populations from central and northern Vietnam, which was a new proof that the BPH in South China was migrated from the central and northern parts of Vietnam.

[Resistance stability of rice varieties to different biotypes of brown planthopper].

In order to develop an effective analytical and monitoring method in breeding rice varieties with higher resistance stability to brown planthopper (BPH), Tai's method (1971) was employed to estimate the resistance stability of twelve rice varieties to different biotypes of BPH. It was shown that light intensity, seedling age, and nitrogen application rate had significant effects on the resistance performance and stability of rice varieties to different BPH biotypes. Among the varieties with resistance to BPH biotype II, the RHT, RP1976-18-6-4-2, and Ptb33 showed stable, IR56 showed unstable, while IR36 and ASD7 showed the most unstable resistance; whereas among the varieties susceptible to BPH biotype II, the TN1 showed stable, Guihuazhan, Foshanyouzhan and IR26 showed relatively stable, while Guojing No. 4 and Mudgo showed unstable susceptibility. For the rice varieties resistant to BPH biotype Bangladesh, the RHT, RP1976-18-64-2 and Ptb33 presented unstable, and IR56 presented very unstable resistance; whereas for the varieties susceptible to BPH biotype Bangladesh, the Guihuazhan, Foshanyouzhan and IR26 performed stable, TN1 and IR36 performed unstable, and Guojing No. 4, Mudgo and ASD7 performed the most unstable susceptibility.

[Secondary compounds in rice varieties resistant to Nilaparvata lugens].

By the method of high-performance liquid chromatography (HPLC), this paper studied the contents of 13 secondary compounds from 130 samples of 26 rice varieties resistant to brown planthopper (BPH) Nilaparvata lugens biotype II. A prediction model was established through principal component analysis and multiple regression analysis: Y = 3.4593 - 0.02491 X1 + 0.08475 X2 - 0.04227 X8 + 0.1174 X12. The relationships between the BPH-resistant level of rice varieties and the area values of chromatographic peaks were significant (r2 = 0.84, P < 0.01). The results demonstrated that Peak 1, Peak 2, Peak 8 and Peak 12 were the major secondary compounds affecting the resistance to BPH biotype II, which suggested that the BPH resistance of the varieties came from the action of several secondary compounds that varied in contribution to the resistance.