Retrotransposons-based genetic regulation underlies the cellular response to two genetically diverse viral infections in tomato

Plant retrotransposons are activated in response to various external stresses. However, little is known regarding the molecular response of retrotransposons to the virus infections in tomato. Hence, the expression profile of tomato homologs (Retrolyc1 and Kielia) of Tnt1 and Tto1 retrotransposons, respectively were evaluated in response to two diverse species of viral infections viz., Groundnut bud necrosis virus (GBNV) and Tomato leaf curl New Delhi virus (ToLCNDV). GBNV infection caused up-regulation of both the retroposons viz., Retrolyc1 and Kielia however, expression of Retrolyc1 was significantly higher than the Kielia. Nonetheless, ToLCNDV infection caused an identical upregulation of both the retroposons throughout the infection cycle. Further, the production of reactive oxygen species (ROS; H₂O₂) and the extent of cell death were high under GBNV infection than in ToLCNDV infected leaves. Enzymatic anti-oxidants (CAT, POD, SOD, APX and GR) showed upregulation in GBNV infection than in ToLCNDV infection. Stress-responsive transcripts (encoding metacaspase-8, COX-5b, PHO2 and NBS-LRR) of tomato involved in oxidative burst and programmed cell death (PCD), exhibited induction during the early stage of GBNV infection subsequently subsiding concomitant with the appearance of necrosis symptoms. Similarly, activation of Retrolyc1 and Kielia could be co-related with an enhanced expression levels of genes involved in hormonal signaling such as Auxin Responsive Factors 3/8, NAC1 transcription factor and MAP kinase (MAPK3) during ToLCNDV infection. Hence, activation of retrotransposons modulates the expression of PCD-responsive genes during GBNV infection whereas genes involved in hormonal signaling were affected in response to ToLCNDV infection manifested as characteristic necrosis and leaf curl disease symptoms, respectively.