The Role Of Proteasome Maturation Factor OsUMP1 In Rice Resistance To False Smut Disease

Rice is one of the most important food crops in the world.However,rice can be attacked by various pathogens during its whole growth cycle,resulting in many diseases such as blast,sheath blight and bacterial blight.In recent years,with wide application of high-yield hybrid rice,the occurrence of rice smut is increasing,and has become one of the main diseases of rice.Rice false smut particularly damages flower organs of rice,not only reducing yield and quality,but also producing mycotoxins,which are harmful to human and animal health.However,there is a shortage of rice varieties and gene resources resistant to rice smut.It is urgent to explore and utilize rice smut resistance resources to provide theoretical and technical support for the prevention and control of rice false smut.We previously found that OsUMP1,encoding a proteasome maturation factor,regulates accumulation of hydrogen peroxide at pathogen infection sites by increasing the biosynthesis and activity of 26S proteasome,thus improving rice resistance to multiple diseases,including rice blast,sheath blight and bacterial blight.However,it is not clear whether this gene is expressed in floral organs of rice and whether it could improve rice resistance to false smut.This study focuses on the role of OsUMP1 in resistance to rice false smut.Main results are as follows:（1）Expression of OsUMP1 in rice flowers responds to the infection of U.virensAt the late stage of rice booting,the resistant rice Yahui 2115（R2115）and the susceptible rice Pujiang 6（Pu6）were inoculated with U.virens,and spikelets were collected at different time points for RT-q PCR analysis.The results showed that OsUMP1 was significantly induced in R2115,but less induced in Pu6,indicating that OsUMP1 may be involved in resistance of rice to false smut.（2）Overexpression of OsUMP1 enhances rice immunity against U.virensOsUMP1 gene region amplified from R2115 was ligated with its native promoter or 35S promoter and introduced into TP309.False smut assay showed that the expression level of OsUMP1 in the spikelets of UMP1^R2115 and UMP1^35S was significantly increased,leading to elevation of 26S proteasome abundance and activity.In turn,the abundance and activity of reactive oxygen species（ROS）scavenging enzymes POD and CAT were reduced,resulting in hydrogen peroxide accumulation and inhibition of U.virens growth.These results indicate that OsUMP1 enhances rice resistance to false smut through the 26S proteasome-ROS scavenging enzyme-hydrogen peroxide pathway.（3）Knockout of OsUMP1 improves false smut resistance probably through regulating panicle traits of riceKnockout osump1 mutants were made by CRISPR-Cas9 approach and subjected to false smut assay.The results showed that flower immunity was compromised in osump1 mutants,but changes of panicle traits led to improved resistance to false smut.Compared with wild type TP309,the panicle diameter of osump1 was significantly reduced,leaf angle of flag leaf/inverted leaf was increased,the heading time was earlier,and the compactness of panicle sheath was decreased.These changes may lead to reduction of humidity in the microenvironment of rice panicles,thus hindering disease development of false smut.（4）Overexpression of OsUMP1 maintains rice yield while knockout of OsUMP1 increases yieldUMP1^R2115 and osump1 plants were evaluated for yield traits.The results showed that UMP1^R2115had similar panicle number,filled grain number,1,000-seed weight,and yield per plant compared to TP309.By contrast,osump1 mutants showed increased panicle number,filled grain number,and yield per plant.In conclusion,this study reveals that OsUMP1 positively regulates rice immunity against U.virens.Overexpression of OsUMP1 improves rice resistance to false smut.Interestingly,knockout of OsUMP1 also promotes false smut resistance probably via altering panicle traits to make an adverse environment for disease development,although immunity of osump1 spikelets was weakened.Since manipulation of OsUMP1 does not penalize rice yield,it is of great potential to utilize OsUMP1 in rice resistance breeding.