Broad-spectrum resistance (BSR) is an excellent trait, because it can produce resistance to more than one pathogen or most pathogen races of the same pathogen. This paper reports the identification and functional analysis of BSR genes in different species, and discusses the application of BSR in molecular breeding.
The diseases faced by crops are fungi, oomycetes, bacteria, viruses and nematodes.
Broad-spectrum resistance (BSR): Plants can resist two pathogens or multiple pathogen races of the same pathogen.
Resistance (R) genes: genes that are resistant to pathogenic bacteria, such as genes encoding receptor-like kinases and intracellular receptor NLRs (which can directly or indirectly detect homologous pathogenic effectors).
Quantitative trait locus (QTL): A specific chromosome region or gene locus, which is responsible for the variation of quantitative traits in the phenotype of biological population.
Species Nonspecific Broad-spectrum Resistance (SNS BSR): Plants are resistant to more than one pathogen.
Race Nonspecific Broad-spectrum Resistance (RNSBSR): Plants are resistant to several races of the same pathogen.
Previously, breeders used single dominant or recessive R genes because of their strong function and easy selection. Most genes are resistant to one or several specific kinds of pathogenic bacteria; However, the mutation of pathogen population and the transfer of virulence make these R genes have short resistance to specific races, and some QTL-controlled diseases are usually not race-specific. Although it is effective to combine a single R gene with QTL of the same genetic background for disease resistance, it is technically difficult and time-consuming. Therefore, choosing BSR is put on the agenda.
Ministry of Public Transport and ETI.
PAMPs is usually necessary for the survival of pathogenic bacteria and is evolutionarily conservative. The PRRs of plants are RLKs or RLPs located on the membrane. Five PRRs from Arabidopsis thaliana, rice and potato were reported as SNS BSR(T 1). The first RLK-PRR in Arabidopsis thaliana is FLS2, which has the SNSBSR of flagellin-carrying bacteria including Pseudomonas. Heterologous expression of FLS2 in other species enhances its resistance to some bacteria. Another PAMP of bacteria, elf 18, is an epitope at the EF-TU N terminal, which is recognized by EFR and also used as the protein of SNS BSR to regulate the resistance of Arabidopsis thaliana to bacterial diseases. Xa2 1 is the first RLK-PRR R gene in crops, which is resistant to most races of Xoo and Xoc. The heterologous expression of Xa2 1 in citrus, Arabidopsis and banana enhanced the resistance to various bacterial diseases. LYP4 and LYP6 are proteins containing lysin motifs in rice, and they are bifunctional PRRs, which can sense bacterial peptidoglycan and fungal chitin and activate resistance to bacteria and fungi. RLP-PRR RLP23 in Arabidopsis thaliana forms trimers with LRR receptor kinases SOBIR 1 and BAK 1 to regulate microbial protein necrosis and ethylene-induced peptide 1 protein-like (NLP) immune response. Therefore, PRRs, which can identify multiple microbial patterns, may be particularly suitable for designing crop immunity.
The SNS-BSR NLR proteins identified for the first time are RRS 1 (resistance to Ralstonia solanacearum 1) and RPS 4 (resistance to Pseudomonas syringae E4) related to Arabidopsis thaliana resistance. As a double R gene system, they are resistant to both bacteria and fungi. RPS4 acts in pairs with RRS 1, causing hypersensitivity (HR) and producing resistance to Pseudomonas syringae containing AvrRps4. In addition to AvrRps4, RRS 1/RPS4 can also recognize the effector protein PopP2 from Ralstonia solanacearum. In addition, RRS 1 and RPS4 are both necessary to resist fungal pathogen anthracnose, which may be achieved by identifying an unknown effector.
Cell wall-associated kinases (WAKs): A class of receptor kinases in plants, including extracellular polygalacturonic acid binding domain, transmembrane domain and intracellular Ser/Thr kinase domain.
Defense signal genes: Genes that play a role in signal transduction pathway are related to the recognition and defense activation of pathogenic bacteria.
Pathogenicity-related (PR) gene: The gene downstream of defense response is responsible for the production of antibacterial substances.
NHR (non-host resistance): the resistance of plants to all non-adaptive pathogens; The most common resistance of plants to most pathogenic microorganisms.
A total of ***42 defense signal genes are considered to be involved in the resistance of SNS BSR (supplementary table 1).
MAPKs are well-known defense signal proteins, which transmit defense signals from immune receptors to downstream proteins. For example, OsMAPK5 negatively regulates the resistance of rice to bacterial blight and fungal rice blast. OsMPK 15 negatively regulated PR gene expression and ROS accumulation, and osmpk 15 knockout mutant enhanced SNS BSR against Xoo and several rice blast races.
Besides MAPKs, other kinases, such as RLKs and RLCKs, also play a role in SNS-BSR. WAK, OsWAK25 and OsWAK9 1 of two rice varieties are important for the resistance of SNS BSR to rice blast and bacterial blight.
Ubiquitin-mediated protein degradation also plays an important role in SNS BSR. Spl 1 1 (spotted leaf11) of rice U-box E3 gene encodes a negative regulator of cell death, while the mutant spl11increases the SNSBR of rice blast and Xoo. Knockout of Spin 6(SPL 1 1- interacting protein 6) also enhanced the resistance of plants to these two pathogens. Another multi-subunit E3 ubiquitin ligase, OsCUL3a (Cullin3a), negatively regulates cell death and SNS BSR against rice blast and bacterial blight by targeting and degrading OSNPR 1 (a non-inhibitory factor related to pathology). OsBAG4 is a homologue of human BAG(Bcl2-2 related athanogene) in rice, and it forms a module with E3 ubiquitin ligase EBR 1 (enhanced blink and blast) in the loop domain to control programmed cell death and the resistance of SNS BSR to rice blast and bacterial blight.
Superficial supervision of SNS BSR. For example, the resistance to rice blast and bacterial blight was enhanced by silencing HDT 70 1 (histone H4 deacetylase gene 70 1) in rice.
Transcription factors are key components in plant immune signals and play an important role in regulating the expression of defense genes. For example, overexpression of WRKY transcription factor, OsWRKY45- 1 or OsWRKY45-2 activates resistance to rice blast, but inhibits resistance to sheath blight. In addition, these two transcription factors play opposite roles in regulating rice resistance to bacteria: OsWRKY45- 1 negatively regulates rice resistance to Xoo and Xoc, while OsWRKY45-2 positively regulates rice resistance to Xoo and Xoc. In Arabidopsis thaliana, the overexpression of NPR 1 enhanced the SNS BSR against Pseudomonas syringae and oomycetes, and this resistance was dose-dependent. It is worth noting that over-expression of NPR 1 will lead to spontaneous immunity and pleiotropic phenotype.
The production of antibacterial substances (defense enzymes, defensins, secondary metabolites such as plant antitoxin, reactive oxygen species, callose deposition, cell wall modification and programmed cell death) is usually regulated by PR gene, which is unique in plants and effective against many pathogenic bacteria.
Snssbsr of these PR genes is usually achieved by overexpression. For example, the overexpression of caAMP 1 (pepper antimicrobial protein 1) in Arabidopsis thaliana enhanced its resistance to many pathogens.
Protein related to plant hormone synthesis also plays an important role in BSR, such as OsACS2 (ethylene synthase). Overexpression of OsACS2 enhanced ethylene production, defense gene expression and resistance to sheath blight and most rice blast races. However, the overexpression of OsACS2 has no effect on agronomic traits.
Susceptibility gene: any plant gene that promotes the infection process or supports susceptibility to pathogenic bacteria.
S gene is usually targeted or induced by pathogenic bacteria, which negatively regulates host disease resistance. Xa5, the γ subunit encoding TF IIA, is the first S gene identified in rice and SNS BSR, and several races that negatively regulate Xoo and Xoc were found. Xa13/ossweet11encodes a sugar transporter, which promotes bacterial and fungal infection and enhances resistance to Xoo and sheath blight after inactivation.
Bsr-k 1 (kitaake- 1) was cloned in rice, and it was found that it encoded a peptide repeat domain RNA binding protein, which negatively regulated SNS BSR. The Bsr-k 1 knockout resulted in the up-regulation of phenylalanine ammonia-lyase (OsPALs) gene expression in rice, which enhanced the resistance of rice to rice blast and Xoo.
Compared with resistance mediated by major genes, quantitative resistance controlled by QTL is generally considered to be non-species specific and more persistent.
Lr34/Yr 18/Pm38 encodes an ATP binding cassette transporter, which can partially resist wheat leaf rust, stripe rust and powdery mildew.
NHR is the most common form of plant resistance to most potential pathogenic microorganisms. The first NHR gene isolated is the NHO 1(NONHOST 1) of Arabidopsis thaliana, which regulates the SNS BSR of several non-host pathogens such as Pseudomonas syringae and botrytis cinerea.
The Pi2/Pi9 locus on rice chromosome 6 contains many RNS-BSR genes, including Pi2, Pi9, Pi50, piz-t and Pigm.
Nine RNS-BSR R genes encode non-NLR proteins (Supplementary Table 2); For example, rice gene Xa4 encodes WAK protein and provides persistent RNS BSR for Xoo without affecting grain yield. In uninfected plants, XA4 activates the transcription of cellulose synthase gene CesA, promotes cellulose biosynthesis, inhibits the expression of expansin, increases the mechanical strength of plant cell wall and inhibits Xoo infection.
Ubiquitin-mediated signaling pathway plays an important role in RNS BSR by activating NLRs and downstream immune signals. Rice E3 OSBBI 1 (induced by rice blast and BTH 1) produces RNS BSR to rice blast by changing the host cell wall. Overexpression of OsBBI 1 increased the accumulation of reactive oxygen species, such as H 2 O 2. Another rice variety, E3 OsPUB 15, interacts with R protein Pid2 of rice blast, thus positively regulating cell death and basic resistance, so it has RNS BSR against rice blast.
Protein kinase gene is also involved in RNS BSR. Resistance of OsBRR 1 to rice blast: Lecrk-V (L-type lectin receptor kinase V) cloned from hexaploid wheat was resistant to powdery mildew at seedling stage and mature stage.
Polymerization: the process of combining two or more genes through genetic strategy to form an excellent strain or variety.
Marker-assisted selection (MAS): This is a supplementary tool for traditional breeding. In traditional breeding, individual selection depends on the relationship between polymorphic molecular markers and traits.
So far, five S genes that transfer RNS BSR have been found. Mlo is the first S gene found in barley, and it was later found to exist in almost all higher plants. MLO is located on the membrane and contains a conserved transmembrane domain and a C-terminal calmodulin binding domain.
The S gene Pi2 1(QTL) in rice encodes a proline-rich protein with a heavy metal binding domain and a protein interaction domain. The recessive allele of pi2 1 (mutated on a proline-rich motif) has RNS BSR of some physiological races of Magnaporthe grisea. Another rice RNS-BSR S gene Bsr-d 1 (broad-spectrum resistance digu 1) encodes C2H2 TF. Single nucleotide mutation in the promoter region of Bsr-d 1 enhanced the binding with MYB transcription factor MYBS 1 and inhibited Bsr-d65438+. Some S genes also play a role in the pathological system of rice -Xoo, including Xa25/OsSWEET 13 and Xa41(t)/Ossweet14, which encode sugar transporters that promote bacterial infection and reduce RNS BSR to Xoo.
Three RNS-BSR·QTL have been cloned in wheat, corn and potato. Fb 1 in wheat, ZmWAK-RLK in corn and R8 in potato.
Rice with multiple R genes usually has a wider resistance spectrum than rice with a single R gene. For example, rice lines containing Pi2/Pi 1, Pigm/Pi54, PI2/PI54 and PIZ-T/PI54 have better resistance than rice lines containing only a single R gene. The excellent rice varieties Xa4, Xa2 1, Xa7, Xa23 and Xa27 polymerized by MAS have wider resistance spectrum and higher resistance level than those with only one gene.
When plants are not attacked by pathogens, the expression of plant genes is usually strictly controlled to avoid autoimmunity; However, the over-expression of a few R genes can activate the immune response and produce BSR against a variety of pathogens without causing a high level of cell death. If different promoters are used to increase the expression of rice R gene Xa3/Xa26, including natural WRKY 13 promoter and maize ubi promoter, the resistance spectrum to Xoo can be increased. Overexpression of PRRs OsLYP4 and OsLYP6 in rice resulted in BSR Xoo and rice blast.
It is possible to design BSR by using defense signals and PR genes, because they usually act downstream of immune receptors.
Using TALEN/CRISPR to target wheat Mlo locus to make plants resistant to powdery mildew. In tomato, using CRISPR to knock out the homologous gene SIMlo 1 of Mlo leads to powdery mildew resistance. In rice, CRISPR induced knocking out proline-rich motifs of Pi2 1 to provide RNSBR for rice blast, and editing the promoter regions of three sweet genes resulted in BSR of all tested indica rice Xoo lines.
In rice, planting disease-resistant and susceptible varieties in many places for two years can greatly reduce the severity of rice blast of the two varieties.
pigm，bsr-d 1，IPA 1 .
Overexpression of immune receptors, defense signals, PR and NHR genes usually leads to cell death and dwarfing phenotype. The upstream open reading frame is located in the 5'UTR region, which is a powerful homeopathic regulatory element for the rich translation process and mRNA turnover in angiosperm genome.
Widespread and long-term planting of BSR varieties may increase the selection pressure of pathogens and increase the emergence of drug-resistant groups. The establishment of natural disease nursery to evaluate the disease resistance of different varieties is also helpful to test the validity of BSR gene.
Combining PRR with NLRs or QTL can improve the resistance level and spectrum of transgenic plants.
Previous studies have shown that in a pyramid, one R gene may mask the influence of other genes, so some R gene combinations provide lower disease resistance than others. The disease resistance of rice containing piz5 and Pita is lower than that of rice containing piz5 alone.
Living pathogens and dead pathogens use different strategies: dead pathogens kill host tissues because they settle and thrive on the contents of dead or dying cells, while living pathogens rely on living host cells to complete their life cycle. In many cases, plants that are resistant to living pathogens are easily infected by dead pathogens, and vice versa.
1. Breeding of new variety BSR is an important goal of crop breeding.
2.BSR gene encodes PRRs, NLRs and other defense-related proteins.
3. Genes based on QTL, susceptibility loss and non-host resistance also involve BSR.
4. The long-term BSR of crops can be achieved by different breeding strategies.
5. Low-cost localization strategies, such as RenSeq, can be applied to the rapid isolation of wild variety BSR gene.
6. Genome editing technology, such as CRISPR, plays an important role in BSR design and breeding.
Paper link: https://www.annualreviews.org/doi/ABS/10.1146/annurev-arplant-010720-02221.