Overexpression of OsOSM1 Enhances Resistance to Rice Sheath Blight

Sheath blight (SB), caused by Rhizoctonia solani, is one of the most destructive rice diseases worldwide. It has been difficult to generate SB-resistant varieties through conventional breeding because of the quantitative nature of rice resistance to SB. In this study, we found that overexpression of the OsOSM1 gene, encoding an osmotin protein belonging to the pathogenesis-related protein 5 family, is able to improve rice resistance to SB in field tests. Although there are two osmotin genes in rice, OsOSM1 is the one mainly expressed in leaf sheath at the booting stage, coinciding with the critical stage of SB development in the field. In addition, OsOSM1 expression is strongly induced by R. solani in SB-resistant rice variety YSBR1 but not in susceptible varieties, suggesting its involvement in SB resistance. Overexpression of OsOSM1 (OsOSM1ox) in susceptible variety Xudao 3 significantly increases resistance to SB in transgenic rice. The OsOSM1 mRNA levels in different transgenic lines are found to be positively correlated with their SB resistance levels. Intriguingly, although extremely high levels of OsOSM1 were detrimental to rice development, appropriately elevated levels of OsSOM1 were obtained that enhanced rice SB resistance without affecting rice development or grain yield. The OsSOM1 protein is localized on plasma membrane. OsOSM1 is upregulated by jasmonic acid (JA); furthermore, JA-responsive marker genes are induced in OsOSM1ox lines. These results suggest that the activation of JA signaling pathway may account for the increased resistance in transgenic OsOSM1ox lines. Taken together, our results demonstrate that OsOSM1 plays an important role in defense against rice SB disease and provides a new target for engineering resistance to SB.

Overexpression of OsPGIP1 Enhances Rice Resistance to Sheath Blight

Rice sheath blight (SB), caused by necrotrophic pathogen Rhizoctonia solani, is one of the most destructive rice diseases, and no major resistance genes are available. Polygalacturonase-inhibiting proteins (PGIP) are extracellular leucine-rich repeat proteins and play important roles in plant defense against different pathogenic fungi by counteracting secreted fungal polygalacturonases (PG). However, the role of PGIP in conferring resistance to rice SB remains to be thoroughly investigated. Here, we showed that OsPGIP1 is capable of inhibiting PG derived from R. solani. Our real-time reverse-transcription polymerase chain reaction results indicated that resistant rice 'YSBR1' and 'Jasmine 85' express significantly higher levels of OsPGIP1 than susceptible 'Lemont'. Our results also show that OsPGIP1 is most highly expressed at the late tillering stage in the sheath of YSBR1, coinciding with the critical stage of SB development in field. More importantly, the OsPGIP1 level is highly elevated by inoculation with R. solani in resistant cultivars but not in susceptible Lemont. Overexpression of OsPGIP1 significantly increased rice resistance to SB and inhibited tissue degradation caused by R. solani-secreted PG. Furthermore, OsPGIP1 overexpression did not affect rice agronomic traits or yield components. Together, our results not only demonstrate the important role of OsPGIP1 in combatting the rice SB disease but also provide a new avenue to the improvement of rice SB resistance by manipulating an endogenous gene.

Comparison and Confirmation of Quantitative Trait Loci Conferring Partial Resistance to Rice Sheath Blight on Chromosome 9

Sheath blight (SB), caused by Rhizoctonia solani, is one of the worst rice (Orzya sativa) diseases worldwide. Resistance to the SB disease in rice is a complex trait controlled by quantitative trait loci (QTLs). Through map integration, we found several previously identified SB resistance (SBR) QTLs reported in inconsistent regions on the long arm of chromosome 9. Five of them were detected on 'Jasmine 85' (J85), 'Minghui 63' (MH63), and 'Lemont' (LMNT) rice and were designated qSB-9^J85-1, qSB-9^J85-2, qSB-9^MH63-1, qSB-9^MH63-2, and qSB-9^LMNT, respectively, in the present study. To further verify and physically map the five potential SBR QTLs, we introduced these SBR QTLs into a common susceptible variety (LMNT) and developed a few chromosomal segment substitution lines through marker-assisted selection. After artificial inoculation with the SB fungus, we were able to validate qSB-9^J85-2 but not the other four SBR QTLs; whereas, on MH63, an SBR QTL designated qSB-9^MH63-3 was confirmed in the region defined by markers Y83 and Y91.8 that included qSB-9^J85-2, covering approximately 1,235 kb. Both qSB-9^J85-2 and qSB-9^MH63-3 appeared to be dominant resistance genes and contributed to similar levels to SB resistance, reducing SB disease severity by approximately 1.0 on a 0-to-9 SB disease rating system. After comparing with another confirmed SBR QTL (qSB-9^TQ) from 'Teqing' rice (TQ), we conclude that qSB-9^J85-2, qSB-9^MH63-3, and qSB-9^TQ are probably controlled by the same allelic resistance genes. These results will accelerate the utilization of this major SBR QTL and its map-based cloning.