Introduction
Host resistance (R) gene to RKN is a gene which directly or in directly restrict reproduction (eggs) and root galling of parasitic RKN. Host resistance genes encode intracellular immune receptors are called resistance (R) proteins. The vast majority of plant R genes are encode protein belongs to NB-LRR receptor family, which consist of a central nucleotide-binding (NB) domain and C-terminal Leucine-Rich Repeat (LRR) domain that structurally and functionally resemble mammalian nucleotide-oligomerization domain-like (NOD-like) receptors (Meyers, et al., 1999).NBS-LRRs (NL) plant immune receptors can be divided into two subclasses on the basis of their amino-terminal sequence. One encode an N-terminal domain with Toll/Interleukin-1 Receptor homology (TIR-NB-LRR, TNL), and the other one encode an N-terminal coiled-coil motif (CC-NB-LRR, CNL) (Dang, et al., 2016). CNL and TNL genes act as receptors or coreceptors of pathogen-derived elicitors. Genome organization of many resistance genes encodes both classes of NBS-LRR genes (TIR-type and CC-type/non-TIR) are commonly present in multigene clusters in plant genomes and canoccur as true alleles across naturally variant geneticbackgrounds. These R-proteins are generally constitutively expressed and act as modular and presumed to undergo intra- and inter-molecular reconfiguration upon effector recognition in order to activate plant immune defense (Dangl, et al., 2013).
Molecular basis of resistance to parasitic is often determined by variation at single genetic loci encoding either factors mediating active recognition of, or susceptibility to the parasite. Recent advances in molecular genetics have lead to insights into the mechanisms by which plants either prevent pathogens from infecting them in the first place, or actively recognize and eliminate pathogens. Several independent single dominant R genes from different plants confer resistance for numerous species of root-knot nematode have been identified and mapped in different chromosomes. This review included the molecular and cellular basis of resistance mechanism mediated by R-genes includes, Mi, Ma,Ma,PsoRPM2,SacMi and RMja genes (Milligan, et al., 1998, Claverie, et al., 2011, Zhu, et al., 2017, Zhou, et al.,2018),confer resistance to RKN in Tomato (L. peruvianum), Prunus species (Myrobalan plum = Prunus cerasifera), woody fruit tree (Prunus sogdiana and transgenic tobaccos) and Eggplants (S. aculeatissimum) respectively.
3.1. Root-knot Nematode Host Mi-Resistance Genes Family
Gene confer resistance to root-knot nematode has been identified in numerous crop plants such as wild tomato, sweet potato and pepper (Milligan, et al., 1998, Chen, et al., 2007, Niu, 2007). The numerous tomato verities carries single dominant gene called Mi-1, that confers resistance to root-knot nematode species. In the early 1940, the Mi gene was first discovered in wild tomato (L. peruvianum) and then introduced into cultivated tomato (L. ycopersicon esculentumm) (Smith, 1944). The Mi-1 gene provide resistance to the three most damaging root-knot nematode species, M. incognita, M. arenaria and M. javanica (Milligan, et al., 1998, Williamson, 1998, Nombela, et al., 2003). Recently, numerous tomato cultivars have received Mi-R gene with help of isozyme marker and DNAlinked markeraps-1 and Rex-1 respectively (Medina-Filho and Tanksley, 1983; Williamson, et al., 1994a). The Mi gene controls RKN at soil temperatures below 28°C and has commonly been used in tomatoes grown throughout the world.
However, the resistance conferred by the Mi gene has some critical limitations. Those are, Mi gene is not effective at soil temperatures above 28°C (Dropkin, 1969). Besides, repeating cultivation for long time and high numbers of pathogen inoculation produced some nematode population those have overcome the resistance conferred by Mi has also been reported. (Roberts, et al., 1990, Castagnone-Sereno, 1994, Kaloshian, et al., 1996). In addition, the presence of naturally occurring resistance-breaking populations such as M. incognita, M. javanica and M. arenaria has been reported (Maleita, et al., 2011). Another constraint of Mi gene is the susceptibility of Mi gene for some damaging RKN species, M. hapla or M. enterolobi (Williamson, 2006). The effect of resistance outcomes of Mi gene is also affected by gene dosage, depending on whether the resistant cultivars are heterozygous (Mimi) or homozygous (MiMi) as shown by (Tzortzakakis, et al., 1998).
Structure and Functions Mi-Resistance Genes Family (Mi-1.1 and Mi-1.2)
Molecular studies of root-knot resistance gene, Mi-1 were carried out in the 1980's with the aims of both isolation of DNA markers for indirect selection for resistance and cloning the sequence. Although, efforts to localize the Mi gene have been hampered for many years because of the severe repression of recombination near this gene in L. esculentum lines carrying the introgressed L. peruvianum DNA (Messeguer, et al., 1991, Ho, et al., 1992, Liharska, et al., 1996). Recently, genetic mapping using molecular markers, and screening large populations of tomato for recombinants in progeny from controlled genetic crosses of L. peruvianum plants with and without resistance (L. esculetum), resulted in localization of the gene to a small region of the genome of 65 kb short arm of chromosome 6 (Kaloshian, et al., 1998). Subsequently, DNA sequence analysis of this region (by sequencing 52 kb contiguous DNA) of the genome identified three closely related candidate genes: Mi-1.1, Mi-1.2and Mi-1.3 (pseudo gene). Mi-1.3 is an apparent pseudogene because it lacks both the N- and C-terminal coding sequences and contains a deletion and several nonsense codons relative to Mi-1.1 and Mi-1.2. The structures of Mi-1.1 and Mi-1.2 genes were studied through comparison of the cDNA and genomic sequences showed that each of Mi-1.1 and Mi-1.2 genecontains two introns at conserved positions near their 59 end. Intron one of Mi-1.1 and Mi-1.2 are 1306 and 556 bp respectively. Beside, intron one of both genes interrupts the untranslated region, whereas intron 2 interrupts the coding region. The position of the initiating ATG codon is conserved between the two genes and begins 42 bp 59 of intron 2. The 5-UTRs region of both genes were predicted 86 nucleotides. The lengths of the 3-UTR of Mi-1.1 and Mi-1.2 are 132 and 108 nucleotides, respectively. The two genes have an identical TATA box sequence (TATATTT) at 230 bp from the putative transcript start. In addition, Mi-1.1 has a CAAT box sequence at 276 bp (Milligan, et al., 1998).
Functional analysis of Mi-1.2 gene using nematode-susceptible tomato line (Money maker) transformed with full construct of Mi-1.2 mediated by agrobacterium revealed that 14.7-kb DNA insert carrying Mi-1.2 is sufficient to confer resistance to RKN species, M. Javanica and M. incognita that show similar of specificity of Mi resistance gene. Whereas similar experiment performed for Mi-1.2 failed to produce resistance phenotype of Mi-gene. All of the Mi-1 family members in both susceptible and resistant tomato that appear to be intact genes are transcribed (Seah, et al., 2007). The predicted proteins encoded by Mi- 1.1 and Mi-1.2 were 1255 and 1255 amino acids respectively. The highest similarity of Mi-1.2 to a gene whose product has known function is that to Rpi-blb2 (82% identity), which is located on chromosome 6 in the corresponding genomic position to Mi-1 in the wild potato Solanum bulbocastanum and confers broad resistance against the oomycete Phytophthora infestans (van der Vossen, et al., 2005). Mi-1.1 and Mi-1.2 each contain a predicted leucine zipper motif. A second region containing seven isoleucine/leucine heptad repeats not present in the other R genes spans residues 460 to 502 of Mi-1.2. These heptad repeats contain two proline residues, which would be predicted to cause a bend in the structure. The highest similarity among the leucine zipper–nucleotide binding–LRR proteins is in the 260–amino acid central conserved region suggestive of a conserved function for this part of the protein. This region contains two motifs, kinase-1a (P-loop) and kinase-2 consensus sequencesthat conform in sequence and spacing to those found in known ATP and GTP binding proteins (Traut, 1994).
A potential kinase-3a motif that differs somewhat from the published consensus (Traut, 1994) but is highly conserved among nucleotide binding–LRR genes. Additional conserved regions include a hydrophobic domain containing the sequence GLPL, which is almost invariant among nucleotide binding– LRR genes described to date (Hammond-Kosackand Jones, 1997). The C-terminal region of Mi-1.1 and Mi-1.2 can be arranged into z14 LRRs of z24 amino acids. This framework is most similar to that of Prf. The consensus sequence of the LRR of Mi-1.2 is aXXLXXLXXLXa(X)12 (where a indicates an aliphatic amino acid residue and X indicates any amino acid; a consensus is assigned if the amino acid is present in 50% of the residues at a particular position in the repeat. This consensus most strongly resembles that for the cytoplasmic class of nucleotide binding–LRR proteins (Jones and Jones, 1996).
Mi-9 gene confer resistance to the same spectrum of RkN species (M. javanica, M. arenaria and M. incognita) as Mi-1, but which provide stable resistance in higher temperature, has been mapped on short arm of chromosomes 6 as Mi-1 in S. arcanum (Ammiraju, et al., 2003). Molecular studies using RNA interference (RNAi) to silence genes in the Mi-1 family indicate that Mi-9 is a homologue of Mi-1 (Jablonska, et al., 2007).
Molecular and Cellular Mechanism of Resistance Mediated by Mi-Genes
In tomato, the R gene locus Mi-1 contains two genes that encode two proteins, Mi-1.1 and Mi-1.2, which have high sequence similarity and contain NBS-LRR motifs (Milligan, et al., 1998). Mi-1.1does not function in pest resistance; Mi-1.2 confers race-specific resistance against RKNs, potato aphids (Macrosiphum euphorbiae), and whiteflies (Bemisia tabaci) (Milligan, et al., 1998, Nombela, et al., 2003). Mechanism of resistance mediated by Mi-1 genes has been proposed to be involved in specific recognition of pathogen products, effectors. Based on the absence of a signal peptide, it is probably that Mi-1.2 is cytoplasmically localized. The Mi-1.2 gene resistance response to RKN infection, cellular hypersensitive response (HR) occurs near the anterior end of the nematode at 12 hours after inoculation. This corresponds roughly to the time when the nematode would be expected to inject effectors initiate giant cell. This coinciding of time of effectors injection and HR is consistent with the hypothesis that Mi-1.2 resistance mediated through gene-to-gene model. In other word, Mi-recognizes something (effectors) that the nematode injects into the plant cell that triggered resistance response, localized hypersensitive response (HR) at and surrounding areas of giant cell initiation (Ho, et al. 1992, Milligan, et al. 1998, Dangl and Jones 2001). Despite extensive efforts during the past two decades, molecular mechanisms connecting Mi-1.2 R-mediated effector recognition with regulatory processes involved in basal defense and PTI are largely elusive.
However, the molecular, histological and histo-chemical studies of mechanism of Mi-1.2 resistance response have reported that NBS domain of the Mi-1.2 R protein can be autoactivated to trigger defense signaling in tomato (Tameling, et al., 2002, Lukasik-Shreepaathy, et al., 2012). The LRR domain of the Mi-1.2 protein has many roles in the regulation of RKN recognition and HR signaling (Hwang and Williamson, 2003). Resistance to Meloidogyneis speculated to be regulated by a protein kinase acting either early in Mi-1.2 signal transduction or upstream of Mi-1.2 and is required for Mi-1.2-mediated RKN resistance (de Ilarduya, et al., 2001; Martinez de Ilarduya, et al., 2004). Notably, the chaperones Hsp90-1 and Sgt1 are involved in the formation of the Mi-1.2 signaling complex (Bhattarai, et al., 2007) (Figure). Virus inducing gene silencing, the plant receptor-like kinase somatic embryogenesis receptor kinase 3 (SERK3)/brassinosteroid insensitive 1-associated kinase 1 (BAK1), (SlSERK1) revealed a role for SlSERK1 in Mi-1-mediated resistance to potato aphids, but not to RKNs (Peng, et al., 2011).
Transcript analysis has revealed that several genes encoding heat shock transcription factors (Hsfs) and heat shock proteins (Hsps) (Hsp90) are essential for Mi-1.2-mediated resistance to RKNs by functioning as a chaperone of the R protein signaling complex during pathogen attack (Bhattarai, et al., 2007). Using, virus-induced gene silencing (VIGS) in tomato, the heat shock protein HSP90-1 and the co-chaperone SGT1 were shown to be required for resistance mediated by the R gene Mi-1 (Meloidogyne spp.) and potato aphids (Macrosiphumeuphorbiae). In addition, VIGS also identified the involvement of a mitogen-activated protein kinase (MAPK) cascade including the MAPK kinase (MAPKK) LeMKK2 and the MAPKs LeMAPK1, LeMAPK2 and LeMAPK3 in Mi-1 resistance against potato aphids (Li, et al., 2006b). Furthermore, recent studies have reported that Mi.1.2 R-proteins directly interfere with transcriptional regulators (SlWRKY72a and SlWRKY72b) to activate the transcriptional network controlling Mi-1.2-triggered ETI immunity (Bhattarai, et al., 2010). The functional link between Mi-1 and WRKY72-like TFs appears a step of direct defense activation. Mechanism of Mi-1-triggered ETI may require transcriptional up-regulation of SlWRKY72a and SlWRKY72b, which in turn boosts basal defense responses controlled by these TFs. A role of WRKY72-type TFs in Mi-1-mediated resistance is consistent with the fact that both Mi-1 and AtWRKY72 appear to utilize SA-independent defense mechanisms (Bhattarai, et al., 2008).
In sammary, the mechanisms mediated by Mi-resistance response to RKN infection are characterized by early hypersensitive response (HR), a highly strong and effective defense reaction in tomato (Solanumlycopersicum) and similar to the Me3 R gene in pepper (Capsicum annuum), in contrast to late necrosis of imperfect giant cells, as for the Me1 R gene of pepper and the Rk R gene of cowpea (Williamson, 1998, Pegard, et al., 2005, Das, et al., 2008). The Mi-1.2 NBS domain of the Mi-1.2 R protein can be autoactivated to trigger defense signaling in tomato (Tameling, et al., 2002, Lukasik-Shreepaathy, et al., 2012). The LRR domain of the Mi-1.2 protein has many roles in the regulation of RKN recognition and HR signaling (Hwang and Williamson, 2003). The chaperones Hsp90-1 and Sgt1 are involved in the formation of the Mi-1.2 signaling complex (Bhattarai, et al., 2007). The involvement of a mitogen-activated protein kinase (MAPK) cascade including LeMAPK1, LeMAPK2 and LeMAPK3 in Mi-1 resistance against potato aphids (Li, et al., 2006b). The Mi.1.2 R-proteins directly interfere with transcriptional regulators (SlWRKY72a and SlWRKY72b) to activate the transcriptional network controlling Mi-1.2-triggered ETI immunity.
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