Journal of Applied Horticulture (www.horticultureresearch.net) Journal of Applied Horticulture, 19(3): 191-195, 2017 MicroRNA mediated regulation of gene expression in response to soil-borne fungus Fusarium oxysporum f.sp. cubense (Foc1) infection in two contrasting banana genotypes K. Pavitra 1, 2 , A. Rekha 3 and K.V. Ravishankar 1 * 1 Division of Biotechnology, ICAR-Indian Institute of Horticultural Research, Hesaraghatta Lake Post, Bengaluru- 560 089, India. 2 Department of Biotechnology, Centre for Post Graduate Studies, Jain University, Jayanagar, Bengaluru- 560 011, India. 3 Division of Fruit Crops, ICAR-Indian Institute of Horticultural Research, Hesaraghatta Lake Post, Bengaluru-560 089, India. *E-mail: kvravi@iihr.res.in Abstract Fusarium wilt caused by the soil-borne fungus Fusarium oxysporum f.sp. cubense (Foc1) is one of the important diseases affecting banana production. MicroRNAs, the short non-coding RNAs containing 22 to 24 nucleotides, function in post-transcriptional regulation of target gene expression. MicroRNAs (miRNAs) as gene expression regulators relate to several abiotic stress responses that have already been reported. However, the evidence for the interaction of miRNAs-mRNA in plant response to biotic stresses is very limited. Hence, this study mainly focuses on microRNAs and their target genes in fusarium wilt infection in banana. Here, we have examined the miRNA-mRNA expression patterns between two contrasting banana genotypes in response to fungal infection using quantitative Real-time PCR (qPCR). A total of 6 miRNAs and 9 targets were examined for their expression at two-time points after infection (3 and 10 days post inoculation (dpi)) in both uninfected control and infected root samples. Based on expression analysis, we observed early and continuous down regulation of miRNAs and up-regulation of the nine targets in tolerant genotype “Calcutta-4”. This negative relation was not observed in the susceptible genotype “Kadali”. The mode of expression level of miRNAs and their putative target genes will help in understanding the roles of miRNAs imparting tolerance to fusarium wilt in banana (Musa spp.). Key words: Musa, microRNA, transcription factors, Fusarium wilt, Fusarium oxysporum f.sp. cubense. Introduction Banana is world’s most popular fruit crop produced in majority of tropical and subtropical countries. Fusarium wilt disease caused by Fusarium oxysporum f.sp. cubense (Foc) (Snyder and Hansen, 1954) is a major limiting factor and highly destructive disease, resulting in a signifcant reduction in yield and quality. Foc colonizes the vascular system of the host, resulting in wilting and death of the whole plant (Wardlaw, 1961; Stover, 1962). As a part of a defense mechanism, plants have evolved a complex network of cellular, physiological and molecular responses. Biotic stresses bring about the expression of several genes in plants at both the transcriptional and post-transcriptional levels. Over the years, several studies have reported that microRNAs (miRNAs) are involved in both abiotic and biotic stress responses (Sunkar et al., 2004). Former studies have reported that miRNAs have wide range of roles in biological and metabolic processes of plants like regulation of plant development, signal transduction and response to abiotic stresses and pathogen invasions (Chuck et al., 2009). A few studies have reported that the roles of miRNAs play crucial role in plant-microbe interaction and defense responses. The aim of this study was to understand the concept of miRNA mediated gene expression patterns in two contrasting genotypes during fungal infection. The miRNA-mRNA expression during disease progression in tolerant and susceptible genotypes for Foc was analysed by quantitative Real-time PCR (qPCR) approach. Here, we tried to examine two aspects: frst - the role of few selected miRNAs in fusarium wilt infection through their level of expression in two contrasting genotypes differing in their response to fusarium wilt disease; secondly, we also examined the expression for putative target genes of the selected miRNAs. Materials and methods Plant material, fungal inoculation and RNA extraction: Fusarium oxysporum f.sp. cubense (Foc1) was isolated from banana corm with fusarium symptoms. Two diploid Musa genotypes Calcutta-4 (tolerant) and Kadali (susceptible) were used in this study (Ravishankar et al., 2011). The plants were inoculated at four to six leaf stage under greenhouse conditions. For inoculations, wounds were made in the newly emerged roots of the plantlets (2 months old plantlets grown by macro- propagation and placed in pots containing sterilized cocopeat) and 50 mL conidial suspension (1x10 4 spores/mL) was poured onto the injured roots and covered with sterile cocopeat. Here, three plantlets for each treatment were inoculated with Foc1 isolate, while the remaining three uninfected control plantlets were used to serve as controls. All inoculated and un-infected control plantlets were maintained in the greenhouse throughout the experiment. Root samples were harvested before inoculation and at 0, 3 and 10 days post inoculation (dpi). These samples were frozen immediately using liquid nitrogen and kept at -80°C until further use (Swarupa et al., 2013). Total RNA was extracted from the root tissue of infected and uninfected samples (three biological replicates) using modifed Journal Appl