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Effect of Zeatin on the Infection Process and Expression of MAPK-4 During Pathogenesis of Alternaria Brassicae in Non-Host and Host Brassica Plants

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Effect of Zeatin on the Infection Process and Expression of MAPK-4 During Pathogenesis of Alternaria Brassicae in Non-Host and Host Brassica Plants Vol. 12(17), pp. 2164-2174, 24 April, 2013 DOI: 10.5897/AJB12.2725 ISSN 1684–5315 © 2013 Academic Journals African Journal of Biotechnology http://www.academicjournals.org/AJB Full Length Research Paper Effect of zeatin on the infection process and expression of MAPK-4 during pathogenesis of Alternaria brassicae in non-host and host Brassica plants K. Kumar Marmath, P. Giri, G. Taj*, D. Pandey and A. Kumar Molecular Biology and Genetic Engineering, College of Basic Science and Humanities, G.B. Pant University of Agriculture and Technology, Pantnagar 263145, Dist. US Nagar, Uttarakhand, India. Accepted 4 April, 2013 Recent studies have revealed an important role of hormones in plant immunity. Cytokinins are phytohormones that are involved in various regulatory processes including plant defense. Zeatin a cytokinin antagonizes the effect of Alternaria brassicae pathotoxin in cell culture of Brassica juncea. Phytohormones are also the inducers of MAP Kinase signaling pathways which are the important signaling modules in eukaryotic cells. In this paper, an attempt was made to study the exogenous application of zeatin on the disease score, infection behavior of A. brassicae, expression pattern of MAPK-4 in the non host Sinapsis alba, host B. juncea susceptible c.v Varuna, B. juncea moderately tolerant c.v. Divya and transgenic Brassica to confer its role in plant immunity. We observed that high concentration of zeatin led to increased defense responses by delaying the infection process as well as significantly reducing the disease score. Semi-quantitative RT-PCR reveals that zeatin also increases the expression of MAPK-4 at early hours of infection. Our result supports that zeatin up regulates plant immunity via an elevation of MAPK-4 and clearly reflects that it antagonizes the effect of A. brassicae. The crosstalk between zeatin and MAPK signaling pathway may help plants fine-tune defense responses against A. brassicae in the disease Alternaria blight. Key words: Alternaria blight, Brassica, disease score, MAPK, pathogenesis, infection process. INTRODUCTION Phytohormones have long been implicated in both biotic essential role in sustaining juvenility of plant tissues and and abiotic interactions. Among the plant hormones, have been investigated to understand the relationship ethylene (ET), salicylic acid (SA) and jasmonate (JA) are between senescence and susceptibility towards several known for differentially regulating defense responses plant pathogens (Pogany et al., 2004). CKs also promote against biotrophic and necrotrophic pathogens and are resistance against biotrophs by enhancing salicylic acid considered as the immunity hormones (Grobkinsky et al., response (Choi et al., 2010). These are perceived by 2011). The balance of hormonal crosstalk strongly membrane-bound histidine kinase proteins similar to the influences the outcome of plant-pathogen interactions. In two-component system of bacteria (Muller and Sheen, addition to these hormones, many researchers 2007; To and Kieber, 2008). uncovered the role of several other hormones in plant CKs are also produced by a range of various microbial defense like auxin, gibberellins, abscisic acid, pathogens, including Pseudomonas syringae (Akiyoshi et brassinosteroid and cytokinins (CKs) (Spoel and Dong, al., 1987; Morris et al., 1991), Alternaria brassicae (Tiwari 2008; Robert- Seilaniantz et al., 2007). CKs play an 1993; Pandey et al., 2001) and leaf-mining insects *Corresponding author. E-mail: [email protected]. Marmath et al. 2165 (Engelbrecht, 1968) causing the formation of green Arabidopsis MAPK4 has been involved in osmotic islands, galls, growth abnormalities and manipulation of stress response pathway (Droillard et al., 2004) as well primary carbon metabolism (Balibrea et al., 2004). For as implicated in plant defense regulation as mpk4 several pathogens, it was shown that this CK production knockout plants exhibited constitutive activation of is essential for infection (Crespi et al., 1994; Hwang et al., salicylic acid (SA) dependent defenses, but failed to 2010). Based on the induction of sink metabolism by CKs induce jasmonic acid (JA) defense marker genes in (Ehness and Roitsch, 1997; Walters et al., 2008), it has response to JA (Brodersen et al., 2006). Considering been suggested that the host physiology is altered in these observations, it was hypothesized that A. brassicae response to CKs to allow the pathogen maximum access and its toxin may affect these MAPKs. Phytohormones to nutrients in hemi biotrophic interactions (Walters et al., are also the inducers of pathogenesis related proteins 2008). In contrast, the role of plant-derived CKs in which also play important role in defense. Earlier, our defense responses against pathogens that are unable to laboratory has reported that overexpression of osmotin produce CKs is largely unknown. Compared to auxin and PR-5 in Brassica develop tolerance against A. brassicae abscisic acid signaling, our understanding of the roles of (Taj et al., 2004). Since it is well known that CKs cytokinin in disease, and its interactions with other antagonize the effects of abscisic acid (ABA) and delay hormones, is relatively limited. Exogenous application of senescence and induce the expression of defense- CKs has been reported to enhance plant resistance related genes, we investigated whether CK (zeatin) can during some viral and fungal infections (Dekker, 1963; be used to develop the tolerance against A. brassicae in Clarke et al., 1998). However, the effect of zeatin on the B. juncea c.v. Varuna, Divya, non host Sinapsis alba and tolerance of plants to disease caused by the necrotrophic transgenic B. juncea c.v. Divya harbouring osmotin gene. fungus A. brassicae (Berk.) Sacc. has not been We also investigated the effect of exogenous zeatin on previously investigated. A. brassicae cause Alternaria the infection process of A. brassicae and the expression blight (AB) disease in the economically important pattern of MAPK4. Brassica species, Brassica juncea. AB disease causes chlorotic and necrotic lesions mostly on aerial parts of the MATERIALS AND METHODS plant (Verma and Saharan, 1994) and is responsible for substantial reduction in seed yield and oil quality (Meena Selection of non host/host genotypes et al., 2010). Over the years, much effort has been made to understand the plant-pathogen/host specific toxin Seeds of a non-host (S. alba), susceptible host B. juncea cultivar interaction during pathogenesis of AB of Brassica. (Varuna), moderately tolerant host B. juncea cultivar (Divya) and Interestingly, A. brassicae toxin mediated signal tolerant transgenic B. juncea c.v. Divya harbouring osmotin gene transduction pathway was antagonized by a cytokinin (Taj et al., 2004) were sown in plastic inserts (7.5 × 5 cm; 2 seeds per insert) containing mixture of soil, sand and vermicomposte in (zeatin) in cell culture studies (Pandey et al., 2001). the ratio of 2:1:1. Plants were grown in the greenhouse (22°C Phytohormones are also known to induce mitogen day/18°C night; 16 h photoperiod), and watered at appropriate activated protein kinase (MAPK) cascade, a more amount and time. conserved signalling pathway involved in plant response against fungal attack (Meskiene and Hirt, 2000). MAPK pathways are minimally comprised of three tiers MAP Alternaria brassicae Kinase Kinase Kinase (MAPKKK), MAP Kinase Kinase (MAPKK) and MAP Kinase (MAPK). The Arabidopsis A. brassicae (Berk.) Sacc. was isolated from a diseased leaf of B. juncea cultivar ‘Varuna’ at Crop Research Centre (CRC), Pantnagar thaliana genome contains 20 MAPKs, 10 MAPKKs and Uttarakhand, India. Pure single spore culture of A. brassicae was 80 MAPKKKs which can engage in various MAPK developed with the help of stereo microscope (Nikon make) and modules to regulate responses of plant to different maintained on potato dextrose agar (PDA) slants at 4°C. environmental and developmental signals. Recently, some members of the MAPK cascade have been reported to be involved in mediating defense response Inoculum preparation against various plant pathogens (Pitzschke et al., 2009). The best characterized MAPKs 3, 4 and 6 were activated A. brassicae was subcultured from the 7-day old culture on V-8 agar medium (10% V-8 juice, 0.02% CaCO3 and 2% agar) and by a diversity of stimuli including abiotic stresses and incubated at 22°C. A conidial suspension was prepared by scraping pathogens. Flagellin derived peptide flg22 triggers a rapid the mycelia and spores from surface of the actively growing fungal and strong activation of MAPK3, MAPK4 and MAPK6 culture into autoclaved distilled water and filtered using four layered (Droillard et al., 2004). MAPK4 and MAPK6 are also cheese cloth to remove most of the mycelia. The filtered spore activated by harpin protein followed by the induction of suspension was centrifuged at 2000 x g for 5 min and resuspended pathogenesis- related (PR) genes (Desikan et al., 2001). in deionized water. This centrifugation was repeated one more time in order to ensure a clear spore suspension free of metabolites. A number of studies have been identified where MAPK4 After the final wash, supernatant was discarded and spores were is component of pathogen signalling cascade (Gao et al., resuspended in water containing 0.05% Tween-20 as an adhesive. 2008; Qiu et al., 2008; Pitzschke et al., 2009). The concentration of spore suspension was adjusted to 5 x 104 2166 Afr. J. Biotechnol. Table 1. The list of primers used for semiquantitative RT-PCR analysis. Name of
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