Tomato (
Solanum lycopersicum L.) is one of the most popular solanaceous vegetable widely grown in India. It is one of the most important protective food because of its high nutritive value, fruit is good source of vitamin A, C and also contains fibers, organic acids and antioxidants. In India tomato is grown in an area of 812.00 m.ha with a production of 205.73 mt. Andra Pradesh, Madhya Pradesh, Karnataka and Gujarath are the major producing states
(Anonymous, 2018). Intensive cultivation of tomatoes in some areas has lead to a significant increase in farmer’s income, but a complex of pests and diseases threatens its production and productivity. In India major constraint for tomato production is regular outbreaks of tomato leaf curl virus disease
(Saikia and Muniyappa 1989;
Ramappa et al., 1998) and causes yield loss to an extent of 100 per cent during summer
(Mishra et al., 2014). Tomato leaf curl disease can be effectively managed by control of vectors
i.
e Bemisia tabaci, by the application of pesticides, use of physical barriers like nylon nets
(Muniyappa et al., 2002) and botanicals like neem leaf and garlic cloves extract as an alternative to pesticides
(Mirza et al., 2018). However use of plant protection chemical is not economical because of higher cost of chemical, residual effect and ultimately affect the human health. Hence development of resistant lines or varieties is more important through different breeding methods, by introgressing the resistant gene from different sources.
Symptomology
Tomato leaf curl virus infected plants produce disease symptoms like clearing of veins, reduction in leaf size, stunted growth, deformation of leaf lets, inward and outward curling and puckering of leaflets. The infected plants produced only few fruits in case of late infection and no fruits, at very early stage of infection. The diseased plants usually developed purple patches especially on older leaves
(Vasudeva and Samraj 1948;
Sastry and Singh, 1973,
Saklani and Mathai, 1977; Raychaudhary and Nariani, 1977;
Capoor, 1981;
Saikia and Muniyappa, 1989).
Vector and virus
Tomato leaf curl disease is caused by the
Begomovirus belongs to the family
Geminiviridae and transmitted by the vector white fly (
Bemisia tabaci Gen.). The wide range of geographical distribution with variety of host range makes it difficult to control
(Naresh et al., 1980).
ToLCV genome is composed of circular single stranded DNA molecule of 2800 nucleotide which is monopartite (Tomato leaf curl Karnataka and Bangalore virus) found in southern parts of India
(Abinder et al., 2009). Bipartite begomovirus (Tomato leaf curl Delhi, India and Gujarat virus) has DNA-A and DNA-B components and found most commonly in northern parts of India
(Pandey et al., 2010). Chowdareddy et al., (2005) reported that in South India, tomato leaf curl Bangalore virus4 (ToLCV- Ban4) is most predominant.
The TYLCV genome consists of six open reading frames (ORFs), these open reading frames encode proteins for encapsidation (CP), movement of virus particle (MP), replication initiator (Rep), transcriptional activator (TrAP), replication enhancer (REn) and a determinant for expression of symptom and virus spreading (C4). They are partially overlapped and organized in two transcriptional directions, separated by a conserved inverted repeat termed intergenic region (IR)
(Gronenborn 2007). All the six open reading frames are essential for the establishment of a successful infection, from efficient replication to long distance movement within the host plant
(Castillo et al., 2007).
The knowledge of virus-vector relationship is most important in developing appropriate screening test for ToLCV resistance.
Mansour and Al-musa (1992) reported that a single whitefly can able to transmit virus with the minimum acquisition and inoculation period of 30 and 60 minutes respectively and latent period was 20-24 hr, the virus was retained by the vector for 11 days.
Disease intensity
Incidence of tomato leaf curl disease occurs throughout the year but the per cent of incidence and intensity vary with the seasons depend upon the climatic condition like temperature, humidity and rainfall and it was highly influenced by the vector population.
Sharma et al., 2017 studied the correlation between whitefly population and various abiotic factors and reported that whitefly population has positive correlation with temperature (both maximum and minimum) and sunshine hours. Whereas negative correlation was observed with relative humidity (maximum and minimum) and rainfall. The positive correlation with the temperature can be attributed to the enhanced rate of development and reproduction of whitefly. The relatively higher temperatures can be conducive for rapid multiplication and activity of
B.
tabaci.
Negative correlation with rainfall attributed to the destruction of eggs, nymphs and pupae of whitefly during continuous rains. The time intervals over which rainfall was associated with reduction in
B.
tabaci populations can be interruptive in terms of length of the insect’s life cycle. Rains may suppress oviposition, increase the mortality of nymphs, adults and induce insect emigration. Cooler weather, high relative humidity and rainfall can therefore be detrimental to whitefly population and its spread. Hence a strategy should be planned to minimize the pest and disease attack either by manipulation in agronomic practices.
Khandre et al., (2019) reported that there is a strong positive correlation between vector population dynamics and per cent disease incidence, in
Rabi season per cent disease incidence was 25.94 per cent with a white fly population of 2.06/leaf, where as in summer 5.13/leaf with 31.60 per cent disease incidence. Karnataka 90-100 per cent of plant was infected with ToLCD during February to end of May and from July to November per cent of disease incidence is low due to fall in whiteflies population brought about by low temperature
(Saikia and Muniyappa, 1989).
Effect of environment factors on tomato leaf curl disease incidence has positive relation with co efficient of infection (%) and climatic factors. Percent of co efficient of infection was maximum in winter compared to rainy season grown crop, this may be due to vector population don’t survived and multiplied in summer due to high temperature where as winter season was more favorable for faster multiplication of white fly
(Singh et al., 2015).
Crop loss
Crop losses mainly depend on the stage of the plant at which disease infection occurs. In tomato plants early infection causes severe losses and the losses reduced with delay in infection. They were minimum when the plants were infected at the time of maturity. Maximum yield losses of about 98.43 per cent were recorded at 30 days after transplanting , while minimum losses were 5.44 per cent, when it was infected at 75 days after transplanting
(Khandre et al., 2019).
Genetics and sources of resistance
Due to lack of resistant source in genepool of cutivated species, wild species have been explotied to transfer the resistance into cultivated specises. Tomato leaf curl disease resistance is mainly governed by polygenes,
viz.,
Ty1,
Ty2,
Ty3,
Ty4 and
Ty5. These genes are most commonly present in wild species and having different mode of inheritance pattern.
The resistant
Ty1 gene originated from the
S.
chilense accession LA1969, having inheritance pattern of partial dominance and mapped on the chromosome number 6
(Zamir et al., 1994). Dominant resistant gene
Ty2 originated from the from
S.
habrochaites accession B6013 in the tomato resistant line H24 and mapped on the chromosome number 11
(Hanson et al., 2006).
Partial dominant
Ty-3 resistant gene has been identified in
S.
chilense accessions LA1932, LA1938 and LA2779 and mapped to the long arm of chromosome 6
(Verlaan et al., 2013). Since both the
Ty1 and
Ty3 genes are present on the same chromosome, these two genes can be easily introgressed in to single genotype. T
y4 gene with partial dominance present in the
S.
chilense accessions LA1932 was mapped on the long arm of chromosome 3
(Ji et al., 2009). Recessive
Ty5 gene has been identified in the tomato breeding line TY172, which was derived from four different accessions of
S.
peruvianum (PI126926, PI126930, PI390681 and LA0441) and mapped on chromosome 4
(Abinder et al., 2009).
New
Solanum pimpinellifolium accessions INRA, LA1478, PI407543 and PI407544 with different resistance levels were found
(Ji et al., 2007a) TYLCV resistance derived from
Solanum pimpinellifolium Hirsute-INRA was shown to be under the control of a single dominant gene. However, its map location is different from that of
Ty 1. The location of this new gene is between markers TG 153 and CT 83. Along with this gene, there was a QTL located on chromosome 6 accounting for up to 27.7% of the variation in symptom severity
(Anbinder et al., 2009).. Apart from these wild species
Solanum pimpinellifolium is one of the most suitable species in tomato breeding, since there is no hybridization barrier in crossing programme with the cultivable species and fruit size is recovered in few backcrosses
(Agnihotri et al., 2013).
Breeding for tomato leaf curl diseseae resistance
In order to develop a resistant varieties/hybrids, existing germplasm has to be screened through different screening methods like natural (Hot spot screening), Artificial method (Artificial screening by using whiteflies, Agro inoculation) of screening and use of some of the morphological, physiological and molecular markers to identify the resistant one. Resistant materials are used in the hybridization programme to transfer the disease resistance in to the required material/ cultivated species.
Screening technology
Different screening technologies are used for selecting a resistant genotype based on the symptoms exhibited by plant.
• Hot spot screening.
• Artificial screening by using whiteflies.
• Agroinoculation is a new method of screening.
Hot spot screening
Screening is done in field condition during particular season
i.
e. summer in high disease intensity area. Resistant genotype is selected based on the symptom exhibited by plant.
Artificial screening by using whiteflies
It consists of two methods namely cage and mass inoculation methods.
Cage inoculation
Seedlings were planted in small plastic pots and kept in the glasshouse. Each pot was exposed separately to 15-20 viruliferous whiteflies for 48 h in insect-proof cages covered with insect-proof cotton cloths, with the help of whitefly collectors and transferring tubes. The plants were examined at weekly intervals for expression of ToLCV symptoms and disease incidence for a maximum of 12 weeks. The inoculated plants were maintained in an insect-proof glasshouse at 28±2°C for symptom development.
(Singh et al., 2015).
Mass inoculation
Seedlings were transplanted into plastic pots filled with soil and compost, kept in a 1.8x1.2x1.5 m (length x breadth x height) insect-proof net chamber (made of silk net). Tomato seedlings from each genotype will be subjected to whiteflies collected from the culture maintained in the glasshouse. The plants need to be examined at weekly intervals for expression of ToLCV symptoms and disease incidence and should be continued up to 60 days after transplanting
(Singh et al., 2015).
Agro inoculation
In this method of screening, cloned virus particles were mobilized into vector
i.
e.
Agrobacterium tumefaciens later cultured on Luria Bertani media over night, culture were mixed and used for inoculation, plants of thirty days old seedlings were injected two times in a week by puncturing the plant using needle
(Hou et al., 1998).
Resistance from the wild species has to be successfully introgressed in to the cultivated species through different conventional and modern breeding method like hybridization, backcross, marker assisted selection and gene pyramiding.
Hybridization
Genetically diverse parents of different wild species has been used in Interspecific hybridization, to identify good combiner for tomato leaf curl disease resistance along with the yield components, based on combining ability and gene action.
Singh et al., 2014 reported that based on GCA estimates, EC-520061 (
S.
habrochaites), WIR-5032 (
S.
chilense) were good general combiner. Based on the estimates of specific combining ability the hybrids PBC x EC-520061 and PBC x EC-521080 were best specific combiner for coefficient of infection and fruit yield per plant.
Gene pyramiding
Tomato leaf curl disease resistance is governed by polygenes
Viz.,
Ty1,
Ty2,
Ty3,
Ty4 and
Ty5 combining all the genes in to a single genotype gives stable resistance plant. Level of diseseae resistance varies with the genes with respect to different isolates of viruses. For example
Ty1 resistance gene carrying line showed level of resistance against some brazalian isolates and bipartite tomato mottle virus (ToMoV) in Florida.
Ty2 gene carrying lines showed differential responses to TYLCV strains
(Ji et al., 2007b). In such situation, pyramiding of multiple
Ty disease resistance genes may improve the spectrum, durability and level of resistance, thereby providing resistance to diverse tomato-infecting begomo viruses.
Prasanna et al., (2014) reported that Ty3 gene carrying line and pyramided line (
Ty2+
Ty3) exhibited high level of resistance to both monopartite (Tomato leaf curl Bangalore virus) and bipartite begomovirus (Tomato leaf curl Palmpur and New Delhi virus).
Ty2 gene carrying line posses moderate level of resistance to Bangalore virus and sucessptible to both the bipartite viruses. Whereas
Ty1 gene carrying line showed lower level of resistance to Palmpur virus but posses resistance to Bangalore and New Delhi virus.
Barriers in breeding programme
Linkage drag
Desired resistant gene is tightly linked with the genes of undesirable trait in this condition it is difficult to introgress the gene of interest
(Malav et al., 2016).
Hybrid sterility
Inter specific hybridization causes embryo abortion.
Unilateral incompatibilit
y
Some wild species like hirsutum posses compatibility only as male parent with cultivated species but as female parent won’t set the fruits.
Overcoming of breeding barrier
Embryo rescue
Embryos are manually excised and placed immediately onto a culture medium that provides the proper nutrients to support survival and growth
Eg:
Solanum lycopersicum ×
S.
peruvianum (Kharkongar
et al., 2013).
Somatic hybridization
Development of hybrid plant through the fusion of somatic protoplast of two different plant species
(Wolters et al., 1994).
Bridge crosses
In this method highly cross incompatible species is crossed with compatible species the progeny derived from the cross is again crossed with incompatible species
Eg: Progeny derived from the cross
S.
peruvianum ×
S.
chilenseis crossed with the
S.
lycopersicum (Poysa 1990).
