ARGONAUTE 2 increases rice susceptibility to rice black-streaked dwarf virus infection by epigenetically regulating HEXOKINASE 1 expression

ARGONAUTE (AGO) proteins play crucial roles in plant defence against virus invasion. To date, the role of OsAGO2 in rice antiviral defence remains largely unknown. In this study, we determined that the expression of OsAGO2 in rice was induced upon rice black-streaked dwarf virus (RBSDV) infection. Using transgenic rice plants overexpressing OsAGO2 and Osago2 mutants generated through transposon-insertion or CRISPR/Cas9 technology, we found that overexpression of OsAGO2 enhanced rice susceptibility to RBSDV infection. Osago2 mutant lines exhibited strong resistance to RBSDV infection through the elicitation of an early defence response, including reprogramming defence gene expression and production of reactive oxygen species (ROS). Compared to Nipponbare control, the expression level of OsHXK1 (HEXOKINASE 1) increased significantly, and the methylation levels of its promoter decreased in the Osago2 mutant on RBSDV infection. The expression profile of OsHXK1 was the opposite to that of OsAGO2 during RBSDV infection. Overexpression of OsHXK1 in rice also induced ROS production and enhanced rice resistance to RBSDV infection. These results indicate that OsHXK1 controls ROS accumulation and is regulated by OsAGO2 through epigenetic regulation. It is noteworthy that the Osago2 mutant plants are also resistant to southern rice black-streaked dwarf virus infection, another member of the genus Fijivirus. Based on the results presented in this paper, we conclude that OsAGO2 modulates rice susceptibility to fijivirus infection by suppressing OsHXK1 expression, leading to the onset of ROS-mediated resistance. This discovery may benefit future rice breeding programmes for virus resistance.     

Rice stripe virus coat protein induces the accumulation of jasmonic acid,activating plant defence against the virus while also attracting its vector to feed

The jasmonic acid (JA) pathway plays crucial roles in plant defence against pathogens and herbivores. Rice stripe virus (RSV) is the type member of the genus Tenuivirus. It is transmitted by the small brown planthopper (SBPH) and causes damaging epidemics in East Asia. The role(s) that JA may play in the tripartite interaction against RSV, its host, and vector are poorly understood. Here, we found that the JA pathway was induced by RSV infection and played a defence role against RSV. The coat protein (CP) was the major viral component responsible for inducing the JA pathway. Methyl jasmonate treatment attracted SBPHs to feed on rice plants while a JA-deficient mutant was less attractive than wild-type rice. SBPHs showed an obvious preference for feeding on transgenic rice lines expressing RSV CP. Our results demonstrate that CP is an inducer of the JA pathway that activates plant defence against RSV while also attracting SBPHs to feed and benefitting viral transmission. This is the first report of the function of JA in the tripartite interaction between RSV, its host, and its vector.     

Rice Dwarf Virus P2 Protein Hijacks Auxin Signaling by Directly Targeting the Rice OsIAA10 Protein,Enhancing Viral Infection and Disease Development

The phytohormone auxin plays critical roles in regulating myriads of plant growth and developmental processes. Microbe infection can disturb auxin signaling resulting in defects in these processes, but the underlying mechanisms are poorly understood. Auxin signaling begins with perception of auxin by a transient co-receptor complex consisting of an F-box transport inhibitor response 1/auxin signaling F-box (TIR1/AFB) protein and an auxin/indole-3-acetic acid (Aux/IAA) protein. Auxin binding to the co-receptor triggers ubiquitination and 26S proteasome degradation of the Aux/IAA proteins, leading to subsequent events, including expression of auxin-responsive genes. Here we report that Rice dwarf virus (RDV), a devastating pathogen of rice, causes disease symptoms including dwarfing, increased tiller number and short crown roots in infected rice as a result of reduced sensitivity to auxin signaling. The RDV capsid protein P2 binds OsIAA10, blocking the interaction between OsIAA10 and OsTIR1 and inhibiting 26S proteasome-mediated OsIAA10 degradation. Transgenic rice plants overexpressing wild-type or a dominant-negative (degradation-resistant) mutant of OsIAA10 phenocopy RDV symptoms are more susceptible to RDV infection; however, knockdown of OsIAA10 enhances the resistance of rice to RDV infection. Our findings reveal a previously unknown mechanism of viral protein reprogramming of a key step in auxin signaling initiation that enhances viral infection and pathogenesis.     

Auxin response factors (ARFs) differentially regulate rice antiviral immune response against rice dwarf virus

There are 25 auxin response factors (ARFs) in the rice genome, which play critical roles in regulating myriad aspects of plant development, but their role (s) in host antiviral immune defense and the underneath mechanism remain largely unknown. By using the rice-rice dwarf virus (RDV) model system, here we report that auxin signaling enhances rice defense against RDV infection. In turn, RDV infection triggers increased auxin biosynthesis and accumulation in rice, and that treatment with exogenous auxin reduces OsIAA10 protein level, thereby unleashing a group of OsIAA10-interacting OsARFs to mediate downstream antiviral responses. Strikingly, our genetic data showed that loss-of-function mutants of osarf12 or osarf16 exhibit reduced resistance whereas osarf11 mutants display enhanced resistance to RDV. In turn, OsARF12 activates the down-stream OsWRKY13 expression through direct binding to its promoter, loss-of-function mutants of oswrky13 exhibit reduced resistance. These results demonstrated that OsARF 11, 12 and 16 differentially regulate rice antiviral defense. Together with our previous discovery that the viral P2 protein stabilizes OsIAA10 protein via thwarting its interaction with OsTIR1 to enhance viral infection and pathogenesis, our results reveal a novel auxin-IAA10-ARFs-mediated signaling mechanism employed by rice and RDV for defense and counter defense responses.     

Abscisic acid negatively modulates plant defence against rice black‐streaked dwarf virus infection by suppressing the jasmonate pathway and regulating reactive oxygen species levels in rice

Abscisic acid (ABA) plays a multifaceted role in plant immunity and can either increase resistance or increase susceptibility to some bacterial and fungal pathogens depending on the pathosystem. ABA is also known to mediate plant defence to some viruses. In this study, the relationship between the ABA pathway and rice black‐streaked dwarf virus (RBSDV) was investigated in rice. The expression of ABA pathway genes was significantly reduced upon RBSDV infection. Application of exogenous hormones and various ABA pathway mutants revealed that the ABA pathway plays a negative role in rice defence against RBSDV. Exogenous hormone treatment and virus inoculation showed that ABA inhibits the jasmonate‐mediated resistance to RBSDV. ABA treatment also suppressed accumulation of reactive oxygen species by inducing the expression of superoxidase dismutases and catalases. Thus, ABA modulates the rice–RBSDV interaction by suppressing the jasmonate pathway and regulating reactive oxygen species levels. This is the first example of ABA increasing susceptibility to a plant virus.     

The jasmonic acid-amino acid conjugates JA-Val and JA-Leu are involved in rice resistance to herbivores

The phytohormone jasmonic acid (JA) plays a core role in plant defence against herbivores. When attacked by herbivores, JA and its bioactive derivatives are accumulated at the damage site, and subsequently perceived by the jasmonate co-receptors COI1 and JAZ proteins. The (+)-7-iso-jasmonoyl-L-isoleucine (JA-Ile) is known to be the main active JA derivative controlling vascular plant responses to herbivores as well as other JA-regulated processes. However, whether other endogenous JA-amino acid conjugates (JA-AAs) are involved in herbivore-induced defence responses remain unknown. Here, we investigated the role of herbivore-elicited JA-AAs in the crop plant rice. The levels of five JA-AAs were significantly increased under the armyworm, leaf folder and brown planthopper attack. Of the elicited JA derivatives, JA-Ile, JA-Val and JA-Leu could serve as ligands to promote the interaction between rice COI1 and JAZs, inducing OsJAZ4 degradation in vivo. JA-Val or JA-Leu treatment increased the expression of JA- and defence-related pathway genes but not JA-Ile levels, suggesting that these JA-AAs may directly function in JA signalling. Furthermore, the application of JA-Val or JA-Leu resulted in JA-mediated plant growth inhibition, while enhancing plant resistance to herbivore attack. This study uncovers that JA-Val and JA-Leu also play a role in rice defence against herbivores.     

ANGUSTIFOLIA negatively regulates resistance to Sclerotinia sclerotiorum via modulation of PTI and JA signalling pathways in Arabidopsis thaliana

Sclerotinia sclerotiorum is a devastating pathogen that infects a broad range of host plants. The mechanism underlying plant defence against fungal invasion is still not well characterized. Here, we report that ANGUSTIFOLIA (AN), a CtBP family member, plays a role in the defence against S. sclerotiorum attack. Arabidopsis an mutants exhibited stronger resistance to S. sclerotiorum at the early stage of infection than wild-type plants. Accordingly, an mutants exhibited stronger activation of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) responses, including mitogen-activated protein kinase activation, reactive oxygen species accumulation, callose deposition, and the expression of PTI-responsive genes, upon treatment with PAMPs/microbe-associated molecular patterns. Moreover, Arabidopsis lines overexpressing AN were more susceptible to S. sclerotiorum and showed defective PTI responses. Our luminometry, bimolecular fluorescence complementation, coimmunoprecipitation, and in vitro pull-down assays indicate that AN interacts with allene oxide cyclases (AOC), essential enzymes involved in jasmonic acid (JA) biosynthesis, negatively regulating JA biosynthesis in response to S. sclerotiorum infection. This work reveals AN is a negative regulator of the AOC-mediated JA signalling pathway and PTI activation.     

Rice siR109944 suppresses plant immunity to sheath blight and impacts multiple agronomic traits by affecting auxin homeostasis

Plant small RNAs (sRNAs) play significant roles in regulating various developmental processes and hormone signalling pathways involved in plant responses to a wide range of biotic and abiotic stresses. However, the functions of sRNAs in response to rice sheath blight remain unclear. We screened rice (Oryza sativa) sRNA expression patterns against Rhizoctonia solani and found that Tourist‐miniature inverted‐repeat transposable element (MITE)‐derived small interfering RNA (siRNA) (here referred to as siR109944) expression was clearly suppressed upon R. solani infection. One potential target of siR109944 is the F‐Box domain and LRR‐containing protein 55 (FBL55), which encode the transport inhibitor response 1 (TIR1)‐like protein. We found that rice had significantly enhanced susceptibility when siR109944 was overexpressed, while FBL55 OE plants showed resistance to R. solani challenge. Additionally, multiple agronomic traits of rice, including root length and flag leaf inclination, were affected by siR109944 expression. Auxin metabolism‐related and signalling pathway‐related genes were differentially expressed in the siR109944 OE and FBL55 OE plants. Importantly, pre‐treatment with auxin enhanced sheath blight resistance by affecting endogenous auxin homeostasis in rice. Furthermore, transgenic Arabidopsis overexpressing siR109944 exhibited early flowering, increased tiller numbers, and increased susceptibility to R. solani. Our results demonstrate that siR109944 has a conserved function in interfering with plant immunity, growth, and development by affecting auxin homeostasis in planta. Thus, siR109944 provides a genetic target for plant breeding in the future. Furthermore, exogenous application of indole‐3‐acetic acid (IAA) or auxin analogues might effectively protect field crops against diseases.     

Signal transduction downstream of salicylic and jasmonic acid in herbivory-induced parasitoid attraction by Arabidopsis is independent of JAR1 and NPR1

Plants can defend themselves indirectly against herbivores by emitting a volatile blend upon herbivory that attracts the natural enemies of these herbivores, either predators or parasitoids. Although signal transduction in plants from herbivory to induced volatile production depends on jasmonic acid (JA) and salicylic acid (SA), the pathways downstream of JA and SA are unknown. Use of Arabidopsis provides a unique possibility to study signal transduction by use of signalling mutants, which so far has not been exploited in studies on indirect plant defence. In the present study it was demonstrated that jar1‐1 and npr1‐1 mutants are not affected in caterpillar (Pieris rapae)‐induced attraction of the parasitoid Cotesia rubecula. Both JAR1 and NPR1 (also known as NIM1) are involved in signalling downstream of JA in induced defence against pathogens such as induced systemic resistance (ISR). NPR1 is also involved in signalling downstream of SA in defence against pathogens such as systemic acquired resistance (SAR). These results demonstrate that signalling downstream of JA and SA differs between induced indirect defence against herbivores and defence against pathogens such as SAR and ISR. Furthermore, it was demonstrated that herbivore‐derived elicitors are involved in induced attraction of the parasitoid Cotesia rubecula     

Pathological hormone imbalances

Plant hormones play important roles in regulating developmental processes and signalling networks involved in plant responses to a wide range of biotic and abiotic stresses. Salicylic acid (SA), jasmonates (JA) and ethylene (ET) are well known to play crucial roles in plant disease and pest resistance. However, the roles of other hormones such as abscisic acid (ABA), auxin, gibberellin (GA), cytokinin (CK) and brassinosteroid (BL) in plant defence are less well known. Much progress has been made in understanding plant hormone signalling and plant disease resistance. However, these studies have mostly proceeded independently of each other, and there is limited knowledge regarding interactions between plant hormone-mediated signalling and responses to various pathogens. Here, we review the roles of hormones other than SA, JA and ET in plant defence and the interactions between hormone-mediated signalling, plant defence and pathogen virulence. We propose that these hormones may influence disease outcomes through their effect on SA or JA signalling.     

Mechanisms of resistance in the rice cultivar Manikpukha to the rice stem nematode Ditylenchus angustus

The incompatible interaction between the rice cultivar Manikpukha and the rice stem nematode Ditylenchus angustus has been reported recently. This research focuses on the underlying mechanisms of resistance in Manikpukha. Invasion, post‐infection development and reproduction of D. angustus were compared in compatible and incompatible interactions to identify the stage in which resistance occurs. The results indicate that resistance in Manikpukha is associated with reduced development and reproduction, implying that resistance acts post‐invasion. We studied the possible involvement of three classical defence hormones, salicylic acid (SA), jasmonic acid (JA) and ethylene (ET), in response to infection in a compatible interaction using biosynthesis/signalling‐deficient transgenic rice lines. All three hormones appear to have an influence on the basal defence of Nipponbare against the stem nematode. Although hormone application increases basal defences, expression studies and hormone analyses after nematode infection in Manikpukha did not show a clear involvement of the hormone defense pathways for SA, ET and JA. However, it seems that OsPAL1 plays a pivotal role in resistance, indicating that the phenylpropanoid pathway and its products might be key players in the incompatible interaction. Lignin measurement showed that, although basal levels are similar, Manikpukha had a significantly higher lignin content on nematode infection, whereas it was decreased in the susceptible cultivar. The results presented here show that SA, ET and JA are involved in basal defences, but the resistance of Manikpukha against D. angustus probably relies on products of the phenylpropanoid pathway.