The sigma 54 genes rpoN1 and rpoN2 of Xanthomonas citri subsp. citri play different roles in virulence, nutrient utilization and cell motility

The sigma factor 54（σ^（54）） controls the expression of many genes in response to nutritional and environmental conditions. There are two σ^（54） genes, rpo N1（XAC1969） and rpo N2（XAC2972）, in Xanthomonas citri subsp. citri. To investigate their functions, the deletion mutants ΔrpoN1, ΔrpoN2 and ΔrpoN1N2 were constructed in this study. All the mutants delayed canker development in low concentration inoculation in citrus plants. The bacterial growth of mutants was retarded in the medium supplemented with nitrogen and carbon resources. Under either condition, the influence degree caused by deletion of rpoN 2 was larger than the deletion of rpoN 1. Remarkably, the mutant ΔrpoN 1 showed a reduction in cell motility, while the mutant Δrpo N2 increased cell motility. Our data suggested that the rpoN 1 and rpoN 2 play diverse roles in X. citri subsp. citri.

Rice ferredoxin OsFd4 contributes to oxidative stress tolerance but compromises defense against blight bacteria

Ferredoxins(Fds)in plastids are the most upstream stromal electron receptors shuttling electrons to downstream metabolic systems and function in various physiological processes of dicots,but their roles in monocots’response to stresses are still unclear.In this study,the functions of OsFd4,the major non-photosynthetic type Fd in rice,were characterized under oxidative stress and Xanthomonas oryzae pv.oryzae(Xoo)infection.OsFd4-knockout mutants displayed no defects in key agronomic traits and blast resistance,but were more sensitive to hydrogen peroxide(H2O2)treatment than the wild type.Transient expression of OsFd4 alleviated H2O2-induced rice cell death,suggesting that OsFd4 contributes to rice tolerance to exogenous oxidative stress.Deletion of OsFd4 enhanced rice immune responses against Xoo.OsFd4 formed a complex in vivo with itself and OsFd1,the major photosynthetic Fd in rice,and OsFd1 transcripts were increased in leaf and root tissues of the OsFd4-knockout mutants.These results indicate that OsFd4 is involved in regulating rice defense against stresses and interplays with OsFd1.

Recent advances and emerging trends in antiviral defense networking in rice

Plants and viruses coexist in the natural ecosystem for extended periods of time,interacting with each other and even coevolving,maintaining a dynamic balance between plant disease resistance and virus pathogenicity.During virus–host interactions,plants often exhibit abnormal growth and development.However,plants do not passively withstand virus attacks but have evolved sophisticated and effective defense mechanisms to resist,limit,or undermine virus infections.It is widely believed that the initial stage of infection features the most intense interactions between the virus and the host and the greatest variety of activated signal transduction pathways.This review describes the most recent findings in rice antiviral research and discusses a variety of rice antiviral molecular mechanisms,including those based on R genes and recessive resistance,RNA silencing,phytohormone signaling,autophagy and WUSmediated antiviral immunity.Finally,we discuss the challenges and future prospects of breeding rice for enhanced virus resistance.

Leafhopper salivary carboxylesterase suppresses JA-Ile synthesis to facilitate initial arbovirus transmission in rice phloem

Plant jasmonoyl-L-isoleucine(JA-Ile)is a major defense signal against insect feeding,but whether or how insect salivary effectors suppress JA-Ile synthesis and thus facilitate viral transmission in the plant phloem remains elusive.Insect carboxylesterases(CarEs)are the third major family of detoxification enzymes.Here,we identify a new leafhopper CarE,CarE10,that is specifically expressed in salivary glands and is secreted into the rice phloem as a saliva component.Leafhopper CarE10 directly binds to rice jasmonate resistant 1(JAR1)and promotes its degradation by the proteasome system.Moreover,the direct association of CarE10 with JAR1 clearly impairs JAR1 enzyme activity for conversion of JA to JA-Ile in an in vitro JAIle synthesis system.A devastating rice reovirus activates and promotes the co-secretion of virions and CarE10 via virus-induced vesicles into the saliva-storing salivary cavities of the leafhopper vector and ultimately into the rice phloem to establish initial infection.Furthermore,a virus-mediated increase in CarE10 secretion or overexpression of CarE10 in transgenic rice plants causes reduced levels of JAR1 and thus suppresses JA-Ile synthesis,promoting host attractiveness to insect vectors and facilitating initial viral transmission.Our findings provide insight into how the insect salivary protein CarE10 suppresses host JA-Ile synthesis to promote initial virus transmission in the rice phloem.

Suppression of Jasmonic Acid-Mediated Defense by Viral-Inducible MicroRNA319 Facilitates Virus Infection in Rice

MicroRNAs （miRNAs） are pivotal modulators of plant development and host-virus interactions. However, the roles and action modes of specific miRNAs involved in viral infection and host susceptibility remain largely unclear. In this study, we show that Rice ragged stunt virus （RRSV） infection caused increased accumulation of miR319 but decreased expression of miR319-regulated TCP （TEOSINTE BRANCHED/ CYCLOIDEA/PCF） genes, especially TCP21, in rice plants. Transgenic rice plants overexpressing miP,319 or downregulating TCP21 exhibited disease-like phenotypes and showed significantly higher susceptibility to RRSV in comparison with the wild-type plants. In contrast, only mild disease symptoms were observed in RRSV-infected lines overexpressing TCP21 and especially in the transgenic plants overexpressing miR319- resistant TCP21. Both RRSV infection and overexpression of miR319 caused the decreased endogenous jasmonic acid （JA） levels along with downregulated expression of JA biosynthesis and signaling-related genes in rice. However, treatment of rice plants with methyl jasmonate alleviated disease symptoms caused by RRSV and reduced virus accumulation. Taken together, our results suggest that the induction of miR319 by RRSV infection in rice suppresses JA-mediated defense to facilitate virus infection and symp- tom development.

Regulation of Rice Tillering by RNA-Directed DNA Methylation at Miniature Inverted-Repeat Transposable Elements

Tillering is a major determinant of rice plant architecture and grain yield.Here,we report that depletion of rice OsNRPD1a and OsNRPD1b,two orthologs of the largest subunit of RNA polymerase IV,leads to a high-tillering phenotype,in addition to dwarfism and smaller panicles.OsNRPD1a and OsNRPD1b are required for the production of 24-nt small interfering RNAs that direct DNA methylation at transposable elements(TEs)including miniature inverted-repeat TEs(MITEs).Interestingly,many genes are regulated either positively or negatively by TE methylation.Among them,OsMIR156d and OsMIR156j,which promote rice tillering,are repressed by CHH methylation at two MITEs in the promoters.By contrast,D14,which suppresses rice tillering,is activated by CHH methylation at an MITE in its downstream.Our findings reveal regulation of rice tillering by RNA-directed DNA methylation at MITEs and provide potential targets for agronomic trait enhancement through epigenome editing.

Oomycetes Seek Help from the Plant：Phytophthora infestans Effectors Target Host Susceptibility Factors

Plants have a sophisticated immune system to defend against a wide range of invaders,including insects,nematodes,bacteria,oomycetes,fungi,and viruses.Microbes may manipulate or suppress immunity by delivering effector proteins,either to the inside or outside of plant cells.Much attention has been focused on identifying the targets of effector proteins in the host and on characterizing how effector activities suppress immunity.

Polyamine-metabolizing enzymes are activated to promote the proper assembly of rice stripe mosaic virus in insect vectors

Both viruses and host cells compete for intracellular polyamines for efficient propagation.Currently,how the key polyamine-metabolizing enzymes,including ornithine decarboxylase 1(ODC1)and its antizyme 1(OAZ1),are activated to co-ordinate viral propagation and polyamine biosynthesis remains unknown.Here,we report that the matrix protein of rice stripe mosaic virus(RSMV),a cytorhabdovirus,directly hijacks OAZ1 to ensure the proper assembly of rigid bacilliform non-enveloped virions in leafhopper vector.Viral matrix protein effectively competes with ODC1 to bind to OAZ1,and thus,the ability of OAZ1 to target and mediate the degradation of ODC1 is significantly inhibited during viral propagation,which finally promotes polyamines production.Thus,OAZ1 and ODC1 are activated to synergistically promote viral persistent propagation and polyamine biosynthesis in viruliferous vectors.Our data suggest that it is a novel mechanism for rhabdovirus to exploit OAZ1 for facilitating viral assembly.