Vital Fungal Resistance Gene of Tomato : Identified Genes in the Wild Source of Tomato

Tomato ( Solanum lycopersicum L.) is the second most important and consumable vegetable crop after potato in the world. Though tomato is very important consumable vegetable and a model plant for genetic studies, its quality, and yield have been immensely declined by different biotic and abiotic factors and among the biotic factors fungal disease are the most devastating, could cause 100% yield loss in sever condition. Major fungal diseases of tomato posing a threat in tomato production are late blight caused by Phytophthora infestans , early blight caused by Alternaria solanii , septoria leaf spot caused by Septoria lycopersici , fusarium wilt Fusarium oxysporium fsp. Oxysporium and verticilium wilt caused by Verticilium dahlea . Other fungal diseases of tomato include powdery mildew caused by Oidium lycopersicum and leaf mold caused by Cladosporium fulvum. To overcome this using resistance gene from the wild source is fundamental and the aim of this review was to appraise and discuss the major identified fungal resistance genes of tomato. There are 16 wild relatives of the cultivated tomato which are reach id different disease resistance genes and currently, among approximately 35,000 encoding genes of tomato twenty two genes have been reported that they are fungal resistance genes which are found in almost all chromosomes except chromosome two and five with available molecular markers. Using this as an opportunity and use different conventional and molecular techniques for gene pyramiding is indispensable.


Introduction
Tomato (Solanum lycopersicum L.) is the second most important vegetable crop after potato in the world. It is estimated that 4.6 million hectare of tomato are grown worldwide annually producing more than 126 million metric ton (http://faostat.fao.org). It is also second most consumed vegetable next to Potato in the world which is used as salad, paste, whole peeled tomatoes, diced products, and various forms of juice, sauces, and soups. It is a significant source of vitamin A and C as well red tomato is the major component of lycopene which has an antioxidant property to protect human against cancer and heart disease ( Elcio P. et.al., 2007). In addition it is being used as a model plant species for genetic studies of qualitative and quantitative traits related to fruit quality, biotic and abiotic factors. Because of its economical contribution to agricultural industry there is an abundant interest to use genomic tools in improving the genomic composition of tomato to meet the interest of the demand in quality, disease resistance as well as to increase yield per acre (Panthee & Chen, 2009). The problem of plant diseases is a worldwide issue also related to food security (Park, 2017). According to the World Food Program (WFP), about 795 million people in the world do not have access to get a proper food (WFP, 2016 andPark, 2017). Transboundary plant pests and diseases affect food crops, causing significant losses to farmers and threatening food security (http://faostat.fao.org). Nowadays, no matter boundaries, media or technology, the effect of diseases in plants are becoming a challenging approach, and deserves to be treating with special attention (Park, 2017. In spite of the fact that results have been obtained via conventional breeding and selection in decades, there are still a large number of fungal diseases that make tomato production challenging in various parts of the world. This is because of limited wild resistance cultivars and the ability of the pathogen becoming virulent against the resistance gene of the cultivars (Richard et al 1998). This trigger the breeders towards another mechanisms and advanced tools like functional and structural genomics to overcome tomato yield problem. As a result, this review initiates to appraise and discuss identified genes of tomato resistance to fungi diseases. Major fungal diseases of tomato posing a threat in tomato production are late blight caused by Phytophthora infestans, early blight caused by Alternaria solanii, septoria leaf spot caused by Septoria lycopersici, fusarium wilt Fusarium oxysporium fsp. oxysporium and verticilium wilt caused by Verticilium dahlea. Other fungal diseases of tomato include powdery mildew caused by Oidium lycopersicum and leaf mold caused by Cladosporium fulvum (Panthee, and Chen, 2009 Another species of tomato powdery mildew, Leveillula taurica (Lev.) Arm., has been reported first in USA to occur in subtropical regions and may cause losses of up to 40% of tomato crop yields (Jones and Thomson, 1987) . The mycelium of L. taurica grows into the leaf and is visible on the lower side of the leaf. It is different from O. neolycopersici that grows mainly on the upper epidermis and usually does not penetrate the leaf (Lindhout et al. 1994a).

Vital Vascular Fungal Diseases
A. Fusarium wilt (FW) Fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici is a destructive disease of tomato crops worldwide and it may causes up to 80% yield reduction (Akbar et al., 2016) There are two distinct forms of the pathogen Fusarium wilt, F. oxysporum f. sp. lycopersici W. C. Snyder & H. N. Hans which causes vascular wilt, and F. oxysporum f. sp. radicis-lycopersici W. R. Jarvis & Shoemaker which is a necrotrophic pathogen, causal agent of tomato crown and root rot (FORL), which results in severe losses in the greenhouse, field crops and hydroponic cultures (McGovern RJ, 2015). Although various methods have been employed to control this pathogen, the use of resistant cultivars is the most acceptable and economic system of control (Szczechura W. et al, 2013). Both of these pathogens are soil borne and occur throughout most tomato growing areas (Agrios, G.N. 2005& Panthee & Chen, 2009). Infected leaves start drooping, curve downwards and turn yellow. Disease symptoms are apparent during flowering and fruiting stages, and leaflets on one side of the plants typically show more severe symptoms than leaves on the other side because of the specific vascular tissue affected by the pathogen. Subsequently, plants start wilting during hot days and eventually die (Jones et al, 1991& Panthee & Chen, 2009. Fusarium oxysporum f. sp. lycopersici have three races, race 1(Avr1), 2 (Avr2) and 3 (Avr3), of which race 3 is the most devastating.
B. Verticilium wilt (VW) Verticilium wilt (VW) caused by Verticilium dahliae is also a soil borne Ascomycete and like FW causes significant losses in tomato. V. dahliae has a wide host range and is distributed throughout the world. The fungus overwinters in plant debris and alternate hosts. Relatively cool temperatures, high humidity and high soil moisture are conducive to the spread of this disease (Agrios, 2005& Panthee & Chen 2009). Disease symptoms appear on the lower leaves as yellow blotches, wilting and eventually dropping off. There are two races of this fungus that are active in tomato, Ve-1 and Ve-2.

Wild genetic Resources of Tomato
Tomato, the genus Solanum; formerly was nested in the genus Lycopersicon (and the cultivated Solanum lycopersicum L., formerly Lycopersicon esculentum Miller) contains seventeen wild species (including the cultivated S. lycopersicum) has a diploid genome size of 950 Mb with 12 chromosome pairs encoding approximately 35,000 genes and most intensively investigated Solanaceous species for genetic studies (Bai, 2004;Foolad, 2007;Barone et al., 2008 andBitew,2018). All those species; S. cheesmaniae, S. galapagense, S. chilense, S. chmielewskii, S. habrochaites, S. neorickii, S. pennellii, S. arcanum, S. corneliomulleri, S. huaylasense, S. peruvianum, S. pimpinellifolium, S. juglandifolium, S. lycopersicoides, S. ochranthum, S. sitiens and the cultivated S. lycopersicum exhibits great difference in morphological characters such as matting system, for biotic and abiotic resistance, and other agronomic traits important for breeding Bitew, (2018). Though hybridization barriers to make crossign was told by Bai, (2004) between the two Solanum complexes; the "esculentum complex which consists of fourteen species, and the "peruvianum complex which is comprised of two extremely diverse species, S. chilense and S. peruvianum" (Figure 1), gene transfer is possible using Molecular and various embryo rescue techniques between the wild species and to the cultivated tomato (Rick, 1982;Rick andYoder, 1988 andFoolad, 2007).
Hence, the cultivated tomato has a narrow genetic diversity that resulted from its intense selection and inbreeding during evolution and domestication (Zhang et al., 2002 andAdhikari, Oh, &Panthee, 2017); thus, these species are more prone to disease epidemics. Because of many traits of economic importance like almost all the major disease resistances including insects like Tuta basoluta (Bitew, 2018) originate from wild Lycopersicon species as well for abiotic factors tolerance genes than cultivated tomato species. Therefore, the hindrance caused by several diseases can be tackled through the development of resistant cultivars by plant breeding approaches utilizing resistance in the wild species. To identify the resistance gene knowing the wild species of the domesticated crop have crucial role for the breeders. As a result, there is a great diversification to improve cultivated tomato for yield and to combat pathogens that minimizes the quality and quantity of desired tomato yield. Though there were this much informative and sequenced genome data available from the cultivated tomato and there are still huge gaps and possibilities to incorporate resistance gene and pyramiding from those wild relatives.

Figure 3: Wild genetic resources of the cultivated tomato, S. lycopersicum
The prodigious thing is that there is an ample Molecular and conventional technique to introgress fungal resistance gene of wild species to the cultivated tomato.

Major Fungal Resistance genes of tomato
Based on the concept of gene to gene model that for every fungal avirulence gene (Avr ) there is a corresponding tomato resistance gene that mediates recognition of the fungal pathogen by the host in which the defence responses are activated culminating in a hypersensitive response (a type of programmed cell death) that limits further growth of this biotrophic pathogen.
For late blight disease there are five resistance genes in tomato, Ph-1, Ph-2, Ph-3, Ph-4 & Ph-5 (Table 1) which is mainly derived from Solanum pimpinellifolium, the wild relative of tomato. Ph-5 is the newly resistance gen which confers resistance to several pathogen isolates including those overcoming the previous resistance genes ( (Foolad et al, 2008;Truong et.al 2013& Akbar et.al 2016. Four resistance gene of tomato against Fusarium wilt; for the F. oxysporum f. sp. lycopersici four genes, i.e., I-1, I-2, I-3 & I-7, and one gene for the F. oxysporum f. sp. radicis-lycopersici; Fr1 have been developed and other fungal disease resistance genes also shown ( Table 1). The presence of two or more gene for a given pathogen indicates that the pathogen suppressed the previous gene and then tomato developed new gene for resistance and it continues in such away as gene-to-gene concept.
The most important thing is designing techniques and pyramiding those genes to one single cultivar for the sec of getting multiple disease resistance gene of tomato against fungal diseases. Hence gene pyramiding is so critical technique to attain durable resistance against biotic and abiotic stresses in crops , and supply the great demand of food to steadily increasing population and alleviate the erroneously changing climatic condition.
So there are options to select parental lines form those wild tomato sources and make cross with an integration of marker assisted selection throughout each generation. Hence these helps for the confirmation of inherited traits to the next generation and amend the breeding techniques.

Conclusion and Recommendation
Though the importance of tomato (S, lycopersicum) sympathetic in all of our dishes and day to day consumption, it have been reported that fungal diseases of tomato are causing critical yield loss estimates almost 100% in sever condition. So to overcome this problem the cost effective, efficient and environmental friendly approach is developing new resistance variety that could be durable for long period and for multiple diseases. In modern and basic plant science, S, lycopersicum is the second next to Arabidopsis thaliana as it have been excellent model plants because it has a relatively small genome and is suitable for genome manipulation. Using this as an opportunity and use different conventional and molecular techniques for gene pyramiding is indispensable. Though the success of gene pyramiding depends on a lot of factors such as distance between the closest markers and the target gene, number of target genes to be transferred, genetic base of the trait, number of individuals that can be analysed, genetic background in which the target gene has to be transferred, type of molecular marker used and available technical facilities, it stacks multiple genes leading to the simultaneous expression of more than one gene in a variety to develop durable resistance expression and improve the efficiency of plant breeding leading to the development of genetic stocks and precise development of broad spectrum resistance capabilities. There are 16 wild relatives of tomato which are reaching in its biotic and abiotic resistance like S. pimpinellifolium, S. pennellii, S. chilense,, S. hirsutum and others, which can be used as parental line to introgress their gene towards farmer preferred cultivar of tomato. On those wild relative sources of tomato around 22 genes are available for fungal resistance along 10 pair of chromosomes. So this is interesting to incorporate in the breeding scheme with the help of molecular techniques. Especially in the developing world there is deepen need of using molecular techniques relative to the conventional breeding system, hence and unless there would a great far away behind to supply food to the increasing population and its demand, and the effect is massive by biotic and abiotic factors.