Evasion of wheat resistance gene Lr15 recognition by the leaf rust fungus is attributed to the coincidence of natural mutations and deletion in AvrLr15 gene.

Employing race-specific resistance genes remains an effective strategy to protect wheat from leaf rust caused by Puccinia triticina (Pt) worldwide, while the newly emerged Pt races, owing to rapid genetic evolution, frequently overcome the immune response delivered by race-specific resistance genes. The molecular mechanisms underlying the newly evolved virulence Pt pathogen remain unknown. Here, we identified an avirulence protein AvrLr15 from Pt that induced Lr15-dependent immune responses. Heterologously produced AvrLr15 triggered pronounced cell death in Lr15-isogenic wheat leaves. AvrLr15 contains a functional signal peptide, localized to the plant nucleus and cytosol and can suppress BAX-induced cell death. Evasion of Lr15-mediated resistance in wheat was associated with a deletion and point mutations of amino acids in AvrLr15 rather than AvrLr15 gene loss in the Lr15-breaking Pt races, implying that AvrLr15 is required for the virulence function of Pt. Our findings identified the first molecular determinant of wheat race-specific immunity and facilitated the identification of the first AVR/R gene pair in the Pt-wheat pathosystem, which will provide a molecular marker to monitor natural Pt populations and guide the deployment of Lr15-resistant wheat cultivars in the field.

Transcriptome analysis of Lr19-virulent mutants provides clues for the AvrLr19 of Puccinia triticina.

Introduction: Wheat leaf rust caused by Puccinia triticina (Pt) remains one of the most destructive diseases of common wheat worldwide. Understanding the pathogenicity mechanisms of Pt is important to control wheat leaf rust. Methods: The urediniospores of Pt race PHNT (wheat leaf rust resistance gene Lr19-avirulent isolate) were mutagenized with ethyl methanesulfonate (EMS), and two Lr19-virulent mutants named M1 and M2 were isolated. RNA sequencing was performed on samples collected from wheat cultivars Chinese Spring and TcLr19 infected with wild-type (WT) PHNT, M1, and M2 isolates at 14 days post-inoculation (dpi), respectively. Screening AvrLr19 candidates by quantitative reverse transcription PCR (qPCR) and Agrobacterium-mediated transient assays in Nicotiana benthamiana. Results: 560 genes with single nucleotide polymorphisms (SNPs) and insertions or deletions (Indels) from non-differentially expressed genes were identified. Among them, 10 secreted proteins were screened based on their fragments per kilobase of exon model per million mapped reads (FPKM) values in the database. qPCR results showed that the expression profiles of 7 secreted proteins including PTTG_27471, PTTG_12441, PTTG_28324, PTTG_26499, PTTG_06910, PTTG_26516, and PTTG_03570 among 10 secreted proteins in mutants were significantly different with that in wild-type isolate after infection wheat TcLr19 and might be related to the recognition between Lr19 and AvrLr19. In addition, a total of 216 differentially expressed genes (DEGs) were obtained from three different sample comparisons including M1-vs-WT, M2-vs-WT, and M1-vs-M2. Among 216 DEGs, 15 were predicted to be secreted proteins. One secreted protein named PTTG_04779 could inhibit programmed progress of cell death (PCD) induced by apoptosis-controlling genes B-cell lymphoma-2 associated X protein (BAX) on Nicotiana benthamiana, indicating that it might play a virulence function in plant. Taken together, total 8 secreted proteins, PTTG_04779, PTTG_27471, PTTG_12441, PTTG_28324, PTTG_26499, PTTG_06910, PTTG_26516, PTTG_03570 are identified as AvrLr19 candidates. Discussion: Our results showed that a large number of genes participate in the interaction between Pt and TcLr19, which will provide valuable resources for the identification of AvrLr19 candidates and pathogenesis-related genes.

First Report of Rust Caused by Tranzschelia discolor on Peach Leaves in Shenzhou, China.

Peach (Prunus persica [L.] Batsch) as an economically important fruit tree is widely cultivated in Shenzhou, China. In September 2021, peach rust was observed in the peach tree in Shenzhou City, Hebei Province (lat. 38°02'56'' N, long. 115°54'57'' E, altitude 22 m). We investigated a peach orchard with a planting area of 1334 m2, where a total of 162 peach trees were planted, and found that about 10% of peach trees exhibited severe disease symptoms. The leaves of infected plant developed 100% disease symptoms, in which 50% of the infected leaves showed about 10 small pale-yellow spots on the front of leaves and reddish-brown pustules on the corresponding abaxial surface of leaves. Urediniospores varied from obovoid to clavate in shape, sometimes in irregular shape. They were orange-brown, echinulate near base with spines smaller towards apex and often smooth at apex, with germ pores 3-4 at equator, size ranging from 25.4 to 38.6 × 10.1 to 18.7 µm (n=100), and with wall 1 to 1.5µm thick at sides and 5-7 µm thick at apex. Golden capitate paraphyses were present, ranging from 25 to 40 µm in length, with a head in diameter of 12 to 14 µm and a tail in width of 5.2 to 6.5 µm. Based on the rust morphological characters, this pathogen was primarily identified as Tranzschelia discolor (Fuckel) Tranzschel & Litv. (Hiratsuka et al. 1992). For molecular identification, total DNA was extracted from 2 isolates, respectively, and the internal transcribed spacer (ITS) region was PCR-amplified using the primer set ITS5-u and ITS4-u (Pfunder et al. 2001). Obtained sequences were compared with sequences in the GenBank repository using BLAST algorithm. BLAST showed a 100% sequence identify to T. discolor (accession nos. AB097449、MT786217、KU712078、KY764179、MH599069). The sequence has been deposited in GenBank with (accession NO. ON950745 and ON950747). Thus, combining morphological observations and molecular identification, the isolate was identified as T. discolor. The pathogenicity was verified by inoculating the abaxial surface of peach leaves with a suspension of 1 × 106 urediniospores/ml. Peach leaves sprayed with sterile water were used as controls. The inoculated peach trees were placed in a greenhouse at 20°C under dark for 24 hours and maintained at 100% relative humidity to promote disease development. Next, the peach trees were grown in a greenhouse at 20°C with a 12 h day length and symptoms were observed on the leaves 14 days after inoculation. In contrast, the control leaves were asymptomatic. Previous studies reported that peach rust occurred in Oman, Korea and Brazil was caused by T. discolor. (Deadman M L, et al.2007, Shin, H D, et al. 2019, Vidal G S, et al. 2021). To our knowledge, this is the first report of T. discolor as a causal agent causing peach leaf rust in Northern China, which will enable us to rapidly diagnose this disease, identify the occurrence of this disease and develop adequate management strategies to control it in China.

TaPR1 Interacts With TaTLP1 via the αIV Helix to Be Involved in Wheat Defense to Puccinia triticina Through the CAPE1 Motif.

Pathogenesis-related (PR) proteins play important roles in plant defense response and systemic acquired resistance (SAR). PR1 has antifungal activity against many plant pathogens. In our previous study, RNA sequencing (RNA-seq) was conducted on resistant wheat line TcLr19 and sensitive wheat cultivar Chinese Spring inoculated with Puccinia triticina (Pt) race PHNT. In this study, seven salicylic acid (SA)-induced TaPR1 genes involved in plant disease resistance were found in the RNA-seq library. Quantitative PCR (qPCR) results showed that TaPR1-4 was most induced by Pt among these seven TaPR1 genes in the incompatible interaction. Yeast two-hybrid (Y2H) results showed that TaPR1-4 interacted with TaTLP1 via the αIV helix. Protein-mediated phenotyping assays in vivo and antifungal activity in vitro demonstrated that wheat leaves infiltrated with pure TaPR1-4 protein developed significantly less disease compared to control leaves. This effect was correlated with a strong increase in defense gene expression, and resistance activity was dependent on the CAPE1 motif located in the C-terminal region of TaPR1-4. These findings increase current knowledge regarding the interaction of TaPR1 and TaTLP1 and provide new insights on the role of TaPR1 protein in the resistance of wheat to Pt.

Functional Analysis of Wheat NAC Transcription Factor, TaNAC069, in Regulating Resistance of Wheat to Leaf Rust Fungus.

NAC transcription factors are one of the largest transcription factor families having functions in a variety of stress responses. Few NACs have been reported for interactions between wheat and the wheat rust fungus Puccinia triticina (Pt). In this study, based on analysis of RNA-seq data from wheat line TcLr19 inoculated by Pt, the NAC transcription factor TaNAC069 was cloned from wheat, and its transcriptional activity and homologous dimer formation were verified. Quantitative real-time PCR analysis showed that the expression of TaNAC069 was induced by Pt and associated signaling molecules. To further characterize the function of the TaNAC069 gene in wheat resistance to Pt, virus-induced gene silencing (VIGS) was utilized, and it revealed that Pt resistance in TaNAC069-silenced plants was significantly reduced. Potential interaction targets of TaNAC069 from wheat and Pt were screened and identified by yeast two-hybrid technology. Eukaryotic elongation factor eEF1A, CBSX3 protein, and cold acclimation protein WCOR410c were screened by yeast one-hybrid technology. The results indicate that the TaNAC069 gene plays a positive regulatory role in wheat resistance to Pt, laying a good foundation to analyze the molecular mechanisms of TaNAC069 and its functional role in wheat resistance to Pt.

TaTLP1 interacts with TaPR1 to contribute to wheat defense responses to leaf rust fungus.

Thaumatin-like proteins (TLPs), which are defined as pathogenesis-related protein family 5 (PR5) members, are common plant proteins involved in defense responses and confer antifungal activity against many plant pathogens. Our earlier studies have reported that the TaTLP1 gene was isolated from wheat and proved to be involved in wheat defense in response to leaf rust attack. The present study aims to identify the interacting proteins of TaTLP1 and characterize the role of the interaction between wheat and Puccinia triticina (Pt). Pull-down experiments designed to isolate the molecular target of TaTLP1 in tobacco resulted in the identification of TaPR1, a pathogenesis-related protein of family 1, and the interaction between TaTLP1 and TaPR1 was confirmed by yeast two-hybrid experiments (Y2H), bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation (Co-IP). In vitro, TaTLP1 and TaPR1 together increased antifungal activity against Pt. In vivo, the disease resistance phenotype, histological observations of fungal growth and host responses, and accumulation of H2O2 in TaTLP1-TaPR1 in co-silenced plants indicated that co-silencing significantly enhanced wheat susceptibility compared to single knockdown TaTLP1 or TaPR1 plants. The accumulation of reactive oxygen species (ROS) was significantly reduced in co-silenced plants compared to controls during Pt infection, which suggested that the TaTLP1-TaPR1 interaction positively modulates wheat resistance to Pt in an ROS-dependent manner. Our findings provide new insights for understanding the roles of two different PRs, TaTLP1 and TaPR1, in wheat resistance to leaf rust.