Beneficial fungal root endophyte Piriformospora indica inhibits bitter gourd mosaic complex disease incited by combined infection of tomato leaf curl, papaya ringspot and cucumber mosaic viruses without compromising crop growth and yield by orchestrating ROS production and scavenging through retrograde signaling

Joy Michal Johnson^1*Deepa R. Chandran^1*Arockiasamy Mary Sharmila¹Saru Sara Sam¹Anuradha Theeyan²Sarada S.³Soni K B²Prathapan K D⁴

• ¹Department of Plant Pathology, College of Agriculture, Vellayani, Thiruvananthapuram - 695 522, Kerala, India, Kerala Agricultural University, Vellanikkara, India
• ²Department of Molecular Biology and Biotechnology, College of Agriculture, Vellayani, Thiruvananthapuram - 695 522, Kerala, India, Kerala Agricultural University, Vellanikkara, India
• ³Department of Vegetable Science, College of Agriculture, Vellayani, Thiruvananthapuram - 695 522, Kerala, India, Kerala Agricultural University, Vellanikkara, India
• ⁴Department of Entomology, College of Agriculture, Vellayani, Thiruvananthapuram - 695 522, Kerala, India, Kerala Agricultural University, Vellanikkara, India

Bitter gourd mosaic complex (BGMC) - caused by the combined infection of tomato leaf curl virus (ToLCV), papaya ringspot virus (PRSV), and cucumber mosaic virus (CMV) - poses a major threat to bitter gourd (Momordica charantia var. charantia L.) cultivation resulting in severe yield losses. In this study, Piriformospora indica, a beneficial fungal root endophyte, was evaluated for its potential in managing BGMC. BGMC produced a wide range of symptoms. Presence of ToLCV, PRSV, and CMV in BGMC was confirmed serologically and molecularly. P. indica-colonization in bitter gourd plants significantly reduced BGMC incidence from 100% to less than 20%; and the disease severity from 84.17% to 9.02%, when the viruses were inoculated by wedge grafting at 15 days after the colonization. Further, P. indica drastically reduced the disease severity from > 75% to < 30% under field conditions. Surprisingly, the fungus could significantly inhibit all the three viruses responsible for BGMC as evidenced by the repression of their coat protein genes and the virus titres in DAS-ELISA and PCR/RT-PCR. P. indica-colonization also enhanced the growth and fruit yield. Biochemical analyses demonstrated that P. indica-colonization significantly suppressed ROS and H2O2 accumulation in the viruses-infected plants, while markedly enhancing the activities of antioxidant enzymes such as catalase, superoxide dismutase and peroxidase (> 5 fold). Moreover, the concomitant down-regulation of ROS and H2O2 marker genes (upto 12 fold less), and up-regulation of antioxidant genes (upto 10 fold) located in nucleus (WRKY40, MYB51, CML37, AGP5, CSD1, APX1), chloroplast (LOX2, PTOX, FSD1, FSD2) and mitochondria (HSPRO1, DIC2, PRX, MSD1, AOX2) of leaves clearly indicate the role of P. indica in orchestrating the regulation of ROS production and its scavenging through systemic retrograde signaling molecule(s) produced by the root endophyte. This premier study highlights the inherent ability of P. indica to diminish the complex symptoms of BGMC by simultaneous inhibition of ToLCV, PRSV and CMV. ROS and H2O2 producing and scavenging genes, located in the nucleus, chloroplast and mitochondria, are modulated by the retrograde signaling molecule(s), produced during the multipartite interaction involving the endophyte, plant and the viruses.