New insights on signaling mechanism involved in host-pathogen interaction between sugarcane and the hemi-biotroph Colletotrichum falcatum

Abstract

Colletotrichum falcatum is an intriguing hemibiotroph, causing red rot in sugarcane, which caused significant loss to sugarcane production for more than a century in the country. To comprehensively understand the molecular interaction among different components of disease resistance mechanism in sugarcane against C. falcatum, detailed studies were conducted on transcript profiling, induction of 3-deoxyanthocyandin phytoalexins and proteomics. A set of sugarcane varieties varying in red rot resistance were challenged with the pathogen and tissue samples were harvested at different time intervals for the analyses. In subtractive libraries ESTs associated with Ca2+ signalling such as calmodulin-like protein, calnexin, putative calcium dependent protein kinase (CDPK) and ethylene (ET) induced calcium binding protein were upregulated in response to pathogen challenge in resistant varieties. Here, ROS and calcium signals emerge at early intervals in response to C. falcatum, thus putatively involved in signalling and defense response in the host tissues. To circumvent the damage caused by ROS, L-ascorbate peroxidase, class III peroxidase and superoxide dismutase were upregulated in the resistant cultivar. Further, transcriptional induction of cysteine and serine proteases is associated with H2O2 mediated-hypersensitive response (HR) cell death at the pathogen infection site. Besides that, MAPK7/MAPK4 and Avr9/Cf9 proteins were upregulated in response to C. falcatum at the early intervals in resistant interaction. We also found up-regulation of ESTs associated with secretory pathway include coatomer  subunit, putative coatomer protein complex subunit β 2, signal recognition particle 9 kDa protein and a putative polyphosphoinositide binding protein. ESTs associated with ubiquitin proteasome system (UPS) system namely putative small zinc finger-related protein, 26S proteasome ATPase subunit Rpt6 and putative cullin 1 were upregulated in this study. These molecules may have a direct role to target and recycle several proteins during C. falcatum pathogenesis in the sugarcane tissues. Interestingly, SAM synthetase 1 and a radical SAM domain-containing protein were upregulated in response to the pathogen possibly leading to phytoalexin biosynthesis. The magnitude of synthesis and phytoalexin accumulation proximal to the pathogen infected region is likely to determine the red rot resistant phenotype of sugarcane cultivars. , Incompatible interactions maintained a higher induction of phytoalexins than the compatible ones as observed in the HPLC assays, wherein, upregulation of chalcone synthase 8 and SAM synthetase 1 involved in phytoalexin biosynthesis pathway was also found.