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Since the beginning of agriculture, large crop losses caused by Plant pathogenic organisms including bacteria, fungi, viruses, and nematodes have probably contributed to human hunger and malnutrition. The control of plant diseases is thus of fundamental importance for global food security issue and is a major objective of plant-breeding and pathology programs and the agricultural chemical industry during the last 100 years. The development of agrochemicals has contributed significantly to decrease crop losses, but most effective chemicals are costly and harmful to environment as well as to health of both human and animals. plant breeders have been using classical breeding methods to achieve an effective control practices from both the environmental and economic perspective, however, single disease resistance trait and pathogens rapidly evolve mechanisms became the bottleneck for the cultivation of plant with durable resistance. Only through recent molecular studies has it become apparent that identified resistance genes encode components of the plant immune system that confer the capacity to recognize and respond to specific pathogens. This molecular study can be helped to engineering a plant with durable resistance.
. During the past decade an increasing number of plant disease resistance (R) genes from different species have been identified by map-based cloning or transposon tagging approach. However, the markers used for such maps generated polymorphic data based on restriction sites (restriction fragment length polymorphism [RFLP] and amplified fragment length polymorphism [AFLP]), random sequences (random amplified polymorphic DNA [RAPD]), and repetitive elements (simple sequence repeats). Such markers may not represent the segregating gene, and the likelihood of identifying a marker linked to the target gene is a function of the distribution of the marker type and location of the gene in the genome.( Suren K et al, 2008) and become unlikely to be useful for detecting new R genes.
The recent advance in biotechnology has opened a new era for greater creativity for the breeder and germplasm by offering an effective method for creating new varieties that selectively targets a specific interested gene or a few heterologous traits. This approach provided new opportunities for accessing the great diversity of disease resistance genes in crop plants through cloning resistance genes from diverse plant species, in combination with various advances, technology A significant effort by several laboratories in the past 5-10 years has resulted in the identification and cloning of numerous R proteins from model or crops species (Chen et al. 2007. Recently, the use of PCR with degenerate primers targeted to the short conserved regions in the NBS has proven to be an efficient method for identifying resistance gene analogues (RGAs) as has been shown in many crop plants such as potato (Leister soybean, lettuce ,Arabidopsis thaliana ,Brassica spp, apple and in several other plant species. A novel molecular technique called Nucleotide Binding Site (NBS) profiling can also be used by researcher for (Van der Linden et al. 2004 specifically targets resistance genes and their analogs. NBS profiling generates a reproducible polymorphic multi-locus banding pattern and has already been successfully used to identify and map RGAs in potato, apple and lettuce (Van der Linden et al. 2004; Calenge et al. 2005; Syed et al. 2006).
Strawberry along with other species within the Rosaceae family has suffered from a dearth of molecular genetics study as compared to the grasses and major horticultural crops like tomato, grape and citrus. Molecular-marker technologies have developed very rapidly in the last decade; however octoploid genetic structure of commercial strawberry makes it difficult to associate molecular markers with disease resistance genes. Relatively few reports of the molecular cloning of putative resistance genes have been appeared in strawberry. Recently mapping RGA in diploid and octopolid strawberry contains the conserved motifs characteristic of NBS-LRR R genes; 28 of them contained uninterrupted ORFs. All the strawberry RGAs detected were closely related to sequences of known R genes and RGAs from other species. Thus, some of them may encode resistance gene products of unknown specificity.(2004). The availability of these markers has greatly facilitated genetic analyses and the utilization of tagged resistance genes in resistant cultivar development, and it is even allowing the molecular cloning of resistance genes for transfer into desirable cultivars by genetic transformation and develop transgenic disease resistance plants,
Plant breeders have long recognized the importance of resistance genes for
preventing disease in crop plants. Many of these genes have now been found to
encode effectorâ€‘triggered immunity (ETI) receptors, and we know that pathogens
can evolve to overcome these genes through loss or alteration of the effectors
that are recognized. The careful deployment of resistance genes in crop plants,
particularly by using multiple effective receptors in combination and by
selecting target effectors that have crucial virulence functions, should allow
more durable resistance.
Many nucleotideâ€‘binding (NB)â€‘leucineâ€‘rich repeat (LRR) genes have now been
cloned, and this can facilitate their application in agriculture either through
conventional breeding approaches, in which the cloned sequences are used as
molecular markers, or through transgenic means. Widespread genome
sequencing of plant pathogens is now yielding long lists of effector proteins that
could be recognized by plant immune receptors, and these can now be screened
against wild relatives of crop plants to identify new sources of resistance. This
approach has been useful already in identifying new sources of resistance to the
potato blight pathogen Phytophthora infestans in wild potatoes120. Pathogenassociated
molecular pattern (PAMP)â€‘triggered immunity (PTI) receptors are
typically not variable within species and thus have not contributed widely to
traditional breeding efforts. However, the transfer of these receptors among
species has tremendous potential to deliver durable resistance, as the recognition
components are highly conserved among pathogens. Although pathogens that
are adapted to a particular host plant may be adept at suppressing the pattern
recognition receptors (PRRs) of that host, their effectors might not recognize
PRRs from other host plants. For instance, the Arabidopsis thaliana EFâ€‘Tu receptor
occurs only in the Brassicaceae family, and transfer of this gene into tomato
provided good resistance against various bacterial pathogens121.