Plants routinely combat pathogenic microbes and survive. Different layers of plant defence have been uncovered by genetic, genomic and biochemical analyses. Plants possess many preformed barriers, but also activate species level (non-host) resistance, race-specific and race non-specific resistance, as well as basal defence. Plant resistance (R) proteins recognise pathogen avirulence (Avr) determinants and in turn trigger signal transduction cascades that lead to rapid defence mobilisation.

Constitutive defences include many preformed barriers such as waxy epidermal cuticles, cell walls and bark (specialized morphological structures). Inducible defences include production of repellents, toxic chemicals, pathogen-degrading enzymes, anti-nutritional effects and deliberate cell suicide (Ghosh et al., 2017).

The sensing of biotic or abiotic stress conditions induces signaling cascades that activate ion channels, kinase cascades, production of reactive oxygen species (ROS), accumulation of hormones such as salicylic acid, ethylene, jasmonic acid and abscisic acid. These signals ultimately induce expression of specific subsets of defense genes that lead to the assembly of the overall defense reaction (Hassani et al., 2018).

Emerging technologies such as comparative analysis, transcriptome analysis, reverse genetics etc., which have provided us with a vast number of potential ways to decipher plant pathogen communications. The conventional methods such as introduction and selection using backcross breeding or pedigree method, and the molecular methods such as marker assisted selection and engineering plant systems has become an outbreak incase of resitance breeding. The omics approaches could be combined with the potential to map different QTLs contributing to a given agronomical trait and to identify linked molecular markers. This will open the possibility to transfer simultaneously several QTLs and to pyramid QTLs for several agronomical traits in one improved cultivar (Onaga and Wydra  2017).

            Vandana and Bhai (2018) conducted experiment on differential expression of three important genes encoding pathogenesis related proteins (PR proteins) viz., β-1,3-glucanase (PR-2), osmotin (PR-5) and cytosolic ascorbate peroxidase (cAPX, PR-9) in Phytophthora susceptible (Sreekara) and resistant (04-P24) black pepper lines compared to uninoculated plants using quantitative reverse transcription PCR (qRT-PCR). Upon Phytophthora capsici inoculation, expression of these three genes were either up-regulated or downregulated. In the susceptible line, all three genes were expressed maximally on one day after inoculation (DAI) and thereafter the expression declined. In the resistant line, a steady increase in the expression pattern of genes was noticed during the course of infection. Highest expression levels of cAPX were noticed on 3 DAI and that of β-1, 3-glucanase and osmotin genes were maximum on 5 DAI. Soil inoculation of P. capsici affected the transcriptional activity of these genes in stem tissue also, indicating systemic defense response against the pathogen.

            Yuan et al., 2007 studied that the key regulator of salicylic acid (SA)-mediated resistance, NPR1, is functionally conserved in diverse plant species, including rice (Oryza sativa L.). The rice genome contains five NPR1 -like genes. Three rice homologous genes, OsNPR1 / NH1, OsNPR2 / NH2 and OsNPR3, were found to be induced by rice bacterial blight Xanthomonas oryzae pv. Oryzae and rice blast Magnaporthe grisea. Studies confirmed that OsNPR1 is the rice orthologue by complementing the Arabidopsis npr1 mutant. Over-expression of OsNPR1 conferred disease resistance to bacterial blight in transgenic plants. The OsNPR1-green fluorescent protein (GFP) fusion protein was localized in the cytoplasm and moved into the nucleus after redox change. Mutations in its conserved cysteine residues led to the constitutive localization of OsNPR1 (2CA)-GFP in the nucleus of transgenic rice. The study demonstrates that rice has evolved an SA-mediated systemic acquired resistance similar to that in Arabidopsis, and also provides a practical approach for the improvement of disease resistance in rice.

            Understandings of plant pathogen-communications throw light on the different compounds associated with plant defense. Using new technologies, it might be possible to achieve more durable and long term resistance through various genetic approaches. Therefore, it is the need of the hour to combat yield losses caused by diseases on a global scale. Also, an increased and stable yield is required to address decreasing land availability issues. Engineering disease resistance with new tools available needs to be made a priority. 

References: 

Ghosh, P, Anjanabha, B. and Bharat, C., 2017, Manipulating disease and pest resistance             pathways in plants for enhanced crop improvement. Biosci. Biotech. Res. Comm., 10(4): 631-644.

Hassani, M, A, Duran, P. and Hacquard, S., 2018, Microbial interactions within the plant             holobiont. Microbiome, 6(2): 1-17.

Onaga, G. and Wydra, K., 2017, Advances in plant tolerance to biotic stresses. Intech plant    genomics, 10(2): 57-67.

Vandana, V. V. and Bhai, R. S., 2018, Differential expression of PR genes in response to             Phytophthora capsici inoculation in resistant and susceptible black pepper (Piper nigrum            L.) lines. Eur. J. Pl, Pathol., 150(3): 713-724

Yuan, Y., Zhong, S., Li, Q., Zhu, Z., Lou, Y., Wang, L., Wang, J., Wang, M., Li, Q., Yang, D.         and He, Z., 2007, Functional analysis of rice NPR1 like genes reveals that OsNPR1/NH1             is the rice orthologue conferring disease resistance with enhanced herbivore         susceptibility. Pl, Biotech. J., 5(2): 313-324.