Research Progress in Molecular Mechanism of Broad-spectrum Disease Resistance in Rice

Jing SU Jinqi FENG Kailing CHEN Shen CHEN Xiaoyuan ZHU

Abstract Yield loss caused by crop diseases seriously restricts global food security. Breeding disease-resistant varieties with resistance （R） genes is acknowledged as the most economical and effective way to control diseases. Therefore， exploring broad-spectrum resistance （BSR） genes to pathogens and clarifying the underlying mechanism are the basis for effective utilization of BSR resources and finding new approaches for disease control. In the past 20 years， significant progress has also been made in plant immune mechanism and remarkable achievements have been made in researches on broad-spectrum disease resistance of major grain crops， and the potential molecular mechanisms of some BSR have been revealed. Here， the advances of broad-spectrum disease resistance in rice （including the representative cloned BSR genes and their molecular mechanisms） were reviewed. In addition， the problems， opportunities and challenges encountered by BSR were analyzed， and the development of BSR research and its application in rice production were prospected.

Key words Rice; Disease; Broad-spectrum resistance; Molecular mechanism

Received： May 5， 2021  Accepted： July 7， 2021

Supported by Guangzhou Science and Technology Planning Project （202002030001）; Fund for Modern Agro-Industry Technology Research System （CARS-01-3 2020KJ105）; "Fourteenth Five-Year" New Discipline Team Building Project of Guangdong Academy of Agricultural Sciences （202116TD）; Natural Science Foundation of Guangdong （2020A1515011213）.

Jing SU （1973-）， female， P. R. China， PhD， associate researcher， devoted to research about rice disease mechanism.

*Corresponding author. E-mail： zhuxy@gdppri.com.

With global climate change， population growth， and changes in the scale of farming， the production loss of major global crops caused by pests and diseases has seriously threatened food security. According to statistics， due to diseases and pests， global rice production loss is 25%-41%， corn loss is 20%-41%， and wheat loss is 10%-28%[1]. In order to reduce the occurrence of crop diseases and pests， the application of a large number of chemical pesticides has brought a huge burden to the environment and threatened human health. For food production， the use of resistant resources （disease resistance genes） to breed disease-resistant varieties is the most economical and effective way to deal with disease threats. In-depth study of plant immunity and disease resistance mechanisms is an important foundation for the development of green and efficient disease prevention and control technologies， as well as an important strategy to ensure stable production， increased production and high quality of crops.

In the long-term confrontation and interaction with pathogenic bacteria， the monitoring and defense systems of plants are constantly evolving， forming a multi-level defense mechanism， including extracellular immunity， such as stoma immunity， rhizosphere immunity and intercellular immunity， and immune recognition， signal transmission and coordination mediated by cell surface and intracellular receptors， as well as the heterogeneity of cell and tissue immunity and the cross-coordination between different types of immune levels. It is internationally recognized that plants have an innate immunity system similar to animals[2]. The first line of immune defense for plants is the defense response triggered by the recognition of the pattern-recognition receptors （PRRs） on the surface of the cell membrane to pathogen/microbeassociated molecular patterns （PAMPs/MAMPs） or damage-associated molecular patterns （DAMPs）. PAMPs are conserved components of pathogenic microorganisms including bacterial flagellin， peptidoglycan， lipopolysaccharide， and chitin; and DAMPs are mostly small peptide molecules produced by plants themselves after injury， such as AtPeps， Oligogalacturonides and Systemin[3]. PRRs are mainly composed of transmembrane receptor kinases （RKs） and transmembrane receptor proteins， such as FLS CERK1 and PEPR1/2. The immune response triggered by the recognition of PAMPs by PRRs is called PAMP-triggered immunity （PTI）. PRRs usually need to bind and interact with co-receptor proteins. They transmit immune signals through receptor-like cyto plasmic kinases （RLCKs） located in the cytoplasm， and activate the bodys system resistance through the mitogen-activated protein kinase （MAPK） cascade reaction pathway and Ca2+ signaling pathways， etc.， such as the opening and closing of stomata， the thickening of the cell wall of defense cells， the production of lytic enzymes to release immune-inducing factors， and the induction of pathogenesis-related （PR） gene expression， etc.， thereby limiting the invasion and proliferation of pathogens[ 4-6]. When pathogenic microorganisms break through the first line of defense and inject virulence factors or effectors into host plant cells to inhibit PTI in plants， they can be sensed by resistance genes （R genes） in plant cells， which initiates their effector-triggered immunity （ETI） against effector invasion， i.e.， the second line of defense of plants， which is mainly the immune response mediated by a type of nucleotide-binding domain and leucine-rich repeat receptors （NLRs）. NLR receptors can quickly recognize specific pathogenic effectors and activate a series of immune responses， that is to say， hypersensitive response （HR） occurs at the infection point of pathogens to kill pathogens in locally infected cells; and the response is extended by the signal conduction network to distant tissues， resulting in systemic acquired resistance （SAR）[2，4]. Due to the strong and obvious resistance response， NLRs are currently the most reported type of immune receptor protein. This type of disease resistance gene is the most useful and widely used type of resistance gene in disease resistance breeding. In addition， some quantitative trait loci （QTLs） also have the function of regulating plant resistance[7].

Broad spectrum resistance （BSR） refers to the resistance of a gene to different races of a certain pathogen or two or more pathogens. Since NLRs usually only recognize the effectors of one or a few specific pathogen races， typical ETI response is usually race-specific[8]， while PTI immunity usually causes a series of non-specific and common defensive responses in plants， and enhances the resistance of plants to other invading pathogens. Therefore， most of the key genes that mediate PTI resistance have a broad spectrum. In addition， some PRRs， NLRs or QTLs that can identify two or more pathogens， mediate broad-spectrum resistance; and some are involved in defense regulators （DRs） for defense signal regulation， because they are involved in gene transcription， protein translation and modification， intracellular transport and metabolic catalysis， etc.， and have the characteristics of broad spectrum and long-lasting resistance[9].

At all stages of rice growth， it will be invaded by more than 70 kinds of pathogenic microorganisms， which will seriously affect its production safety. Among these diseases， rice blast caused by Magnaporthe oryzae， sheath blight caused by Rhizoctonia solani， bacterial blight and streak disease induced by Xanthomonas oryzae pv. Oryzae （Xoo） and Xanthomonas oryzae pv. Oryzicola （Xoc）， and false smut caused by Ustilaginoidea virens are destructive rice diseases worldwide[4，7，9]. Rice blast is the most destructive rice fungal disease， which can cause a 30% reduction in global rice production， which is enough to feed 60 million people[8，10-11]. Bacterial blight and leaf spot are the main bacterial diseases on rice， which can reduce rice yield by 20% to 30%[12]. People have achieved improvements using disease-resistance resources. However， due to the diversity and variability of the distribution of rice pathogens， the problem of single resistance decline of rice varieties mediated by race-specific genes is prominent， and there is an urgent need to dig out broad-spectrum resistance genes， explain the mechanism of broad-spectrum disease resistance， and effectively apply them to the selection of new varieties of rice with high-quality disease resistance， which is the inevitable development direction of this field.

Research Progress and Current Status of Broad-spectrum Resistance to Main Rice Diseases

Since Flor proposed the "gene-to-gene" hypothesis of plant-pathogen interaction in 1955[13]， scientists have carried out extensive research on resistance genes and disease-related genes， and cloned a number of genes that regulate plant resistance. The number of papers published in the field of plant immunology in China has grown rapidly， and after 2015， the number of papers published has jumped to the first place. It has become an important force in promoting the development of plant immunology in the world[14]. As the demand for broad-spectrum disease resistance continues to increase， various countries have continuously strengthened research in the field of broad-spectrum disease resistance. According to statistics， the number of papers on broad-spectrum resistance research in the global agricultural field in the past 20 years has exceeded 2 500. Chinese scholars have made outstanding contributions in this field， and the published research papers accounted for 28.54%， ranking first in the world[15]. Reports on rice disease resistance genes mainly focus on resistance to rice blast and bacterial blight. Among the various genes， there are more than 100 major R genes and more than 500 QTLs that contribute to rice blast resistance. There are more than 40 major genes with bacterial blight resistance， and 11 genes have been cloned. In addition， some QTL genes that contribute to sheath blight， U. virens and virus resistance have also been found， but these genes have not been cloned yet[7]. Among them， there are about 10 major R genes and 4 QTL genes that confer broad-spectrum resistance in rice， and there are at least 5 DR genes that show broad-spectrum resistance to different pathogens （Table 1）.

Exploration of broad-spectrum resistance resources of rice blast

Among the 37 rice blast R genes that have been cloned， except for Pi-d2 which encodes β-lectin receptor kinase[16] and Ptr which encodes a proten with the domain harboring the ARM repeat，  other R genes are all dominant genes， and encode NLR protein[17]. The NLR genes generally specifically recognize their corresponding pathogenic effectors， and most of them do not have a broad spectrum. Most of resistant varieties that have been proven to show durable and broad-spectrum resistance to physiological races from various rice regions in the world at present are rice varieties that contain 3 to 5 disease resistance genes， such as the Vietnamese variety Tetep， the West African rice variety Moreberekan and the widely cultivated rice varieties IR64 and Sanhuangzhan 2[18]. However， there are only about 6 genes that have been proven to have broad-spectrum resistance. For example， Pi9 isolated from small wild rice IRBL9-W has a high level of resistance to at least 43 M. oryzae races originating from 13 countries[19]; Pi5 showed resistance to 32 rice blast fungus races from the Philippines and South Korea[20]; Pi50 showed durable resistance to 523 M. oryzae physiological races from major rice regions in China， and has been used for rice breeding for disease resistance[21]; and Pigm cloned from the disease-resistant variety Gumei No. 4 showed durable resistance to 50 M. oryzae physiological races originating from many countries in the world[22]. In addition， Pi1 and Pib identified from the IRBL1 and IR24 rice lines of the International Rice Institute， respectively， Pi7 cloned from the Thai rice species Jao Hom Nin， and Pi56 cloned from the South China rice species Sanhuangzhan 2 have also been reported to have broad-spectrum resistance to rice blast[23-26].

NBS-LRR， nucleotide-binding site-leucine-rich repeat; RLK， receptor-like kinase; LRR-RLK， leucine-rich repeat receptor-like kinase; TF， transcription factor; TM， transmembrane protein; TPR， Tetratrico peptide Repeats; WAK， wall-associated kinase; M. oryzae， Magnaporthe oryzae; Xoo， Xanthomonas oryzae pv. oryzae; Xoc， Xanthomonas oryzae pv. oryzicola. Superscript letters represent gene types， a： QTL， quantitative trait loci; b： PRR， pattern recognition receptor; c： DR， defense-responsive or defense-related.

Agricultural Biotechnology2021

Atypical NLR R genes also play an important role in broad-spectrum resistance to rice blast， but the resistance is often not as strong as that mediated by NLR R genes. Pi21 is a rice blast QTL gene， which encodes a protein with a proline-rich metal transport/detoxification domain， which confers non-race-specific resistance to rice blast and has a negative regulatory effect on rice blast resistance， and its loss-of-function allelic mutation pi21 produces broad-spectrum resistance to 10 widely distributed physiological races of M. oryzae[27]. ptr encodes a protein containing 4 Armadillo repeats， which has E3 ligase activity， which positively regulates the broad-spectrum resistance of rice to multiple M. oryzae races[28]. In addition， because DR genes resist the invasion of pathogens or participate in the regulation of defense signals through a certain way， they can stimulate crops to produce partial resistance （or incomplete resistance）， and have the characteristics of broad resistance spectrum and lasting resistance. For example， the ring finger protein OsBBI1 with E3 ligase activity can regulate resistance to various races of M. oryzae[29]， and through genome-wide association studies the bsr-d1 identified from the disease-resistant variety Digu is considered to be a new broad-spectrum resistance gene to rice blast[30]. Overexpression of a monocotyledon-specific S-domain receptor-like kinase SDS2 can enhance resistance to M. oryzae[31].

Molecular mechanism of broad-spectrum resistance to rice blast

As mentioned in the preface， typical ETI resistance is usually race-specific， while PTI immunity is mostly broad-spectrum. The research on the broad-spectrum disease resistance mechanism of rice mainly involves the coordination of PTI and ETI signals. Studies have found that in PTI and ETI signals， the stimulation of plant immune response usually causes some common downstream reactions， such as reactive oxygen species （ROS） burst， PR gene expression， antitoxin synthesis， and lignin thickening[4]. For example， the overexpression of OsBBI1 promotes the accumulation of high levels of H2O2 in the cells of rice plants， and accumulates high levels of phenolic compounds in the cell wall， resulting in thickening of the cell wall， thereby regulating the resistance to various physiological races of M. oryzae[29]; and receptor-like kinase SDS by interacting with two receptor-like cytoplasmic kinases OsRLCK118 and OsRLCK176， induces programmed cell death， accompanied by an ROS burst， and thereby enhances resistance to M. oryzae[31].

Since 1999 when Kawasaki[32] reported that rice OsRac1 is a regulator that stimulates ROS production and cell death and Ono[33] proved that constitutive activation of OsRac1 can increase rice resistance to rice blast and bacterial blight， the study of this small-molecule GTP-binding protein with GTPase activity gradually uncovered the information network of rice innate immunity. A series of studies have shown that OsRac1 is a key downstream signal switch for the two types of immune receptors， pattern recognition receptors and resistance proteins， and plays an important regulatory role in the broad-spectrum disease resistance signal transmission mediated by R genes and DR genes. OsRac1 can be activated by the combination of the R protein Pit and OsSPK1 （a guanylate exchange factor） to initiate immunity[34]， and it also plays an important role in the anti-disease response mediated by Pia and Pid3[35]. OsRac1 plays an important regulatory role in the broad-spectrum resistance signal transduction mediated by DR genes. It can form a disease-resistant complex （Defensome） with OsRAR RACK HSP90， and HSP70. The important regulatory element of the complex， OsRac1GEF， can interact with OsFLS2 （bacterial flagella recognition protein） through cytoplasmic domains， and can also interact with OsCERK1 （fungal chitin recognition protein）， indicating that OsRac1 is related to the immune signal pathways induced and regulated by bacterial and fungal diseases[36]. OsRac1 is involved in immune regulation mediated by E3 ubiquitin ligase. SPL11 can promote the degradation of SDS2 protein and a small GTPase-activating protein SPIN6， and is a switch that coordinates OsRac1 from an active， GTP-bound state to an inactive， GDP-bound state[31，37]. Therefore， as an important signal node for R and DR gene-mediated immunity， OsRac1 may be the center of plant immune signal transmission， and manipulating the activity of OsRac1 may obtain new rice germplasm with broad-spectrum disease resistance.

In recent years， major breakthroughs have been made in the study of new broad-spectrum disease resistance mechanisms mediated by transcription factors. Bsr-d1 encodes a C2H2 zinc finger transcription factor， which can directly bind to the promoters of two peroxidase genes to activate its transcription and reduce H2O2 accumulation， while the MYB transcription factor MYBS1 can specifically bind to the promoter of Bsr-d1 and inhibit its transcription. The results of genome-wide association analysis showed that the natural mutation of a base from A to G in the promoter region of Bsr-d1 produced the natural allele bsr-d which endowed it with a higher affinity with MYBS inhibited the transcription of BSR-D and reduced the expression of downstream peroxidase gene， making bsr-d1 plants accumulate a large amount of H2O2 and have non-race-specific and durable resistance[30]. The transcription factor ideal plant architecture 1 （IPA also known as OsSPL14） is the core factor for the establishment of an ideal plant type in rice. The latest research shows that when attacked by M. oryzae， the phosphorylation of IPA1 at S163 changes its DNA binding specificity， and it binds to the promoter of WRKY45， activates the transcription of WRKY45， and ultimately leads to enhanced immunity against a variety of rice blast fungi. When the resistance signal is activated， the phosphorylation of IPA1 is quickly released， and it continues to perform its function of regulating growth， realizing the coordination of plant resistance and yield of a single gene[38]. The NLR protein PigmR has strong resistance to rice blast. Usually， strong resistance selection will cause rapid mutation of the dominant populations of pathogens and loss of resistance persistence. Another NLR protein， PigmS， can balance immunity by interacting with PigmR. PigmS competitively weakens PigmR-mediated resistance by inhibiting PigmR-PigmR homodimerization instead of PigmR-PigmS heterodimerization， and thus reduces the selection pressure of PigmR on M. oryzae， so that rice can maintain a long-lasting broad-spectrum disease resistance[22]. Recently， it was discovered that the transcription factor PIBP1 with the RRM transactivation domain interacts with PigmR， and through the PIBP1 nuclear aggregation initiated by PigmR， it combines with the promoters of defense genes OsWAK14 and OsPAL1 to activate immunity and trigger rice blast resistance[39].

Exploration of broad-spectrum bacterial blight resistance resources

The structure of rice bacterial blight resistance genes is relatively diverse. At present， about 46 bacterial blight resistance genes have been identified in rice cultivars， wild varieties or mutant populations， of which 7 dominant and 4 recessive genes have been cloned or have been analyzed for their molecular mechanisms[7，40]. These genes encode multiple types of proteins， suggesting that bacterial blight R gene-mediated resistance has multiple mechanisms. Only Xa1 encodes the typical NLR protein， Xa21 and Xa3/Xa26 encode leucine-rich repeat receptor-like kinase （LRR-RLK） located on the plasma membrane， Xa4 encodes cell wall-associated kinase （WAK）， xa5 encodes transcription factors， and Xa10， Xa23， xa13， xa25， xa41 and Xa27 encode transmembrane protein （TM） or apoplast protein （apoplast protein）. Among them， Xa2 Xa23 and xa5 show high resistance to most strains of Xoo and are recognized as broad-spectrum resistance genes to bacterial blight[4，7，41-43].

Molecular mechanism of broad-spectrum resistance to bacterial blight

The complete resistance of bacterial blight resistance genes to Xoo is dependent on the transcriptional activation of transcription activator-like effectors （TALEs） of bacterial blight， and transcription activation occurs when Xoo TALE is binding with the promoter of the corresponding R gene[41-45]. So far， avrXa23 is present in all bacterial blight isolates tested in the field， and Xa23 has shown strong resistance to almost all of the strains[43]. Because xa5 encodes the basic transcription factor γ subunit， it has a broad spectrum and is widely used to improve rice resistance to bacterial blight. The molecular mechanism that has been revealed is that all the toxic TALEs of Xoo interact with dominant Xa5 instead of recessive xa5， resulting in toxic TALEs being capable of effectively activating transcripts of susceptible genes in the context of recessive xa5， thereby preventing rice bacterial blight[44-45]. Furthermore， xa5 is identified as the main QTL for Xoc resistance[46]. Similarly， all the toxic TALEs of Xoc only interact with the dominant Xa5， resulting in toxic TALEs of Xoc in rice varieties containing xa5 not being able to effectively activate corresponding susceptible genes， and making rice varieties carrying xa5 show broad-spectrum resistance to Xoc[45， 47]. Therefore， the use of xa5 gene in breeding has attracted more and more attention[48].

Xa21 and Xa3/Xa26 encode LRR receptor kinases located in the plasma membrane， which have a broad spectrum and basic mode to trigger immunity to most Xoo in the world[49]. Xa21 is the first rice bacterial blight resistance gene to be cloned， which can specifically recognize RAxX with sulfated tyrosine 14 （Y14） in Xoo， thereby triggering strong PTI immunity[50]. The Xa21-mediated broad-spectrum disease resistance signal network has been extensively studied. Several Xa21 binding proteins， including ATPase （XB24）， E3 ubiquitin ligase （XB3）， PP2C phosphatase （XB25）， WRKY transcription factor （XB10）， OsSERK2 and endoplasmic reticulum chaperone proteins， play an important role in the resistance triggered by Xa21 with positive or negative regulation mode of bacterial blight[4]. Although Xa3/Xa26 and its orthologues also show broad-spectrum resistance to different bacterial blight strains， they are limited by their pathogenic related molecular patterns in bacterial blight， which has not yet been determined， and corresponding molecular mechanisms of broad-spectrum resistance remains to be studied.

Similarly， some QTL and DR genes showed broad-spectrum resistance to X. oryzae pv. solani. The minimal QTL gene GH3-2 encodes indole-3-acetic acid （IAA）-amide synthase， which prevents the loosening of rice cell walls by inhibiting pathogen-induced IAA accumulation， triggers broad-spectrum basic resistance， and resists the invasion of bacteria Xoo， Xoc and M. oryzae[51]. Rice GDSL lipases OsGlip1 and OsGlip2 have similar functions. They can negatively affect rice immunity by regulating lipid homeostasis， and inhibition of OsGlip1 and OsGlip2 can enhance the basic resistance to bacteria Xoo and M. oryzae[52]. Rice bsr-k1 encodes a protein with an RNA binding function that harbors a triangular-shaped tetrapeptide repeats （TPR） domain. The binding of BSR-K1 protein to the RNA of the OsPAL gene promotes its digestion and ultimately leads to disease susceptibility. In bsr-k1 mutants， truncated bsr-k1 protein cannot bind and digest OsPAL gene mRNA， and the accumulation of OsPAL transcripts can greatly increase the synthesis and accumulation of lignin， and enhance the expression of PR genes， thereby realizing the characteristic of a broad spectrum of resistance to bacteria Xoo and M. oryzae[15，53]. WRKY45 plays a key role in benzothiadiazole-induced disease resistance by mediating salicylic acid signals. Overexpression of WRKY45 enhances the resistance to bacterial pathogens Xoo， Xoc and fungal pathogens， but rice plants overexpressing WRKY45 are vulnerable to the infection by R. solani， which limits its application in genetic improvement[54-55].

Some plants with activated or absent DR gene function will show a cell death phenotype due to continuous immune activation， and are also called lesion mimic mutants （lmm）. Due to the constant immune activation and cell death， the resistance of crops to different pathogens has been improved to varying degrees. In rice， spll1 spl28， Lrd6-6， oscul3a and ebr1 show broad-spectrum resistance to bacterial blight and rice blast[56].

Research on broad-spectrum resistance of other rice diseases

Except for the R genes for resistance to rice blast and bacterial blight， no R genes resistant to other rice diseases have been found， but many QTL genes against other rice diseases have been identified. Some of these QTL genes have been isolated and their molecular mechanisms have been studied. These QTL genes show moderate resistance to rice sheath blight， rice smut， rice stripe virus， rice black-streaked dwarf disease or other rice diseases. At present， more than 50 rice sheath blight resistance QTL genes have been detected. Among them， qSB-9TQ and qSB-11LE have been preliminarily located; and people can detect rice smut resistance QTL genes on at least 10 rice chromosomes except chromosomes 7 and 9， but no further mapping has been found[7]. At least 6 major QTLs for resistance to rice stripe virus have been identified. Among these mapped QTLs， only qSTV11KAS （named STV11）， which is resistant to sheath blight， has been cloned. It encodes a sulfotransferase that can catalyze the conversion of salicylic acid to sulfonated salicylic acid， and a large number of indica-indica varieties but not japonica-japonica varieties contain functional STV1 which is consistent with the finding that most japonica-japonica rice varieties are more susceptible to rice stripe virus[57]. Except for STV1 no other QTLs have been cloned， and there are few reports on the mechanism of disease resistance. At present， research on these diseases is more focused on the isolation and identification of pathogenic bacteria， the analysis of the pathogenicity of the strains， and the analysis of the resistance of different rice to virulent strains[58-61].

Opportunities and Challenges of Rice Broad-spectrum Disease Resistance Research

China is a populous country， and solving the problem of food safety production has always been a key issue in Chinas economy and agriculture. According to the National Sustainable Agriculture Development Plan （2015-2030）， the Ministry of Agriculture has issued an action plan centering on the concept of innovation， openness， and sharing of green development， and green production has become the future trend of Chinas agricultural development. In order to achieve green ecological regulation of crop diseases and ensure the sustainable development of green agriculture， the state continues to increase funding for plant protection and sustainable green disease prevention and control. Taking the National Natural Science Foundation of China as an example， in 2019， more than 360 projects were funded in the field of plant protection， with an amount of 170 million yuan[15].

Policy guidance and capital investment have promoted the development of research on broad-spectrum disease resistance of crops in China. International and domestic cooperation has established an excellent and advanced research platform， and by introducing technologies combined with our own innovation， China has made remarkable achievements in the research on the mechanism of plant disease resistance， such as the first analysis of the protein complex structure of the plant resistance some and the new immune activation mechanism mediated by it， the balance of disease resistance and yield， and regulation of rice resistance to virus[14-15]. Gene modification technologies， taking gene editing—clustered regularly interspaced short palindromic repeat （CRISPR） as a representative， are becoming more and more important in the creation of new rice resources. Editing the pi21 gene or the three genes of Pita， Pi2 and ERF922 at the same time can obtain new rice germplasms with significantly improved resistance to rice blast; and editing rice Xa13 and SWEET11/13/14 gene creates rice with enhanced bacterial blight resistance[62-63].

Based on the research on the mechanism of broad-spectrum disease resistance， it has been found that most QTL and DR genes exhibit broad-spectrum resistance without affecting plant growth or yield， and have potential value in rice variety improvement. In 2018， Ranf tried to transform PRRs such as EFR or XA21 into susceptible varieties. As a result， the disease resistance of the receptors， and even broad-spectrum resistance， could be improved， indicating that the downstream regulatory modules of PRRs are highly conservative. Based on the conservation of the downstream regulatory modules of PRRs， the transformation of PRRs identified in model plants into crops is expected to achieve the goal of improving crop resistance to broad-spectrum disease[64]. Field experiments have shown that bsr-d1 and bsr-k1 genes do not affect key agronomic traits or yields， and have become potential candidates for rice breeding[15].

Crop broad-spectrum disease resistance research faces both great opportunities and severe challenges. First， the efficiency and accuracy of the discovery of new broad-spectrum disease resistance genes is far from meeting the needs， and the development and utilization of new methods and technologies are urgently needed; second， pathogen field variation leads to frequent loss of crop resistance， and how to use the theory of plant immune mechanism research to guide the excavation of durable resistance genes， how to combine resistance genes and broad-spectrum resistance mechanisms to achieve the best prevention and control effects， and other issues have not been resolved; and third， the persistence of broad-spectrum resistance and the balance of resistance-yield-quality also restrict the breeding application of disease-resistant genes. In addition， although new research ideas， methods and technical means such as medicine， structural biology， and pan-genome are emerging in an endless stream， it is also a big challenge whether scientific and technological workers can integrate and innovatively use various scientific and technological forces to improve crop resistance in a broad spectrum.

Conclusions and Prospect

The occurrence rules of major crop diseases， pathogenicity mechanisms， complex interaction mechanisms during disease occurrence and development， and molecular immune mechanisms of plants in response to infringements are all key research directions in the future[1]. In response to the challenges faced by crop broad-spectrum disease resistance research， we must not only have a strong sense of crisis， but also have the confidence to actively respond. The first is to further deepen the research on crop germplasm resources with broad-spectrum disease resistance and discover new types of disease-resistant genes; a variety of new methods and analysis methods can be applied to quickly identify and clone new broad-spectrum disease resistance genes， and comprehensively analyze their regulatory mechanisms; and advanced and efficient methods can be applied to obtain new materials with enhanced broad-spectrum disease resistance to improve and enhance crop disease resistance. Specifically， ① Using high-throughput genomics， transcriptomics and pan-genomics analysis methods to rapidly identify new broad-spectrum disease resistance genes， susceptibility genes， and new MAMPs and new immune receptors through a comprehensive analysis of plant resistance and pathogenicity mechanisms; ② Interpretation the regulation mechanism of immune stimulation， enhancement and maintenance of immune receptors through structural biology analysis; ③ Analysis of the information flow of crop-pathogen-environment interaction， and revealing the fine-tune mechanism of crop immunity and its interaction with pathogens; ④ the universal mechanism of susceptible genes or small RNAs inducing silencing of pathogenic target genes across borders can be analyzed， and new ways for disease resistance can be designed by gene editing， small RNA-induced silencing and other negative regulation strategies; ⑤ the coordinated regulation and control mechanism of crop immunity with other agronomic traits can be analyzed; and ⑥ based on the above research， ideal crop immune systems can be artificially reconstructed.

On the basis of in-depth understanding of the mechanism of broad-spectrum disease resistance， we should focus on the application and evaluation of newly identified broad-spectrum disease resistance genes in breeding practice， and rationally use genes that take into account resistance， yield and quality， so as to improve resistance while taking into account the balance between yield and quality， as well as the relationship with other ecological adaptability such as stress resistance. Only by comprehensively improving research in this field can it help meet the major needs for food security and ecological security.

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