Induction of disease symptoms of a non-cognate helper begomovirus by CLCuMB βC1 through the salicylic acid-dependent pathway

Cotton leaf curl Multan betasatellite (CLCuMuB) encodes beta C1 protein that induces disease symptoms in different monopartite begomovirus/betasatellite complexes. Iranian isolate of Tomato yellow leaf curl virus (TYLCV[Ab]) is a monopartite begomovirus without any associated betasatellite. It causes either no or mild symptoms in Nicotiana glutinosa. At the present study, the possibility of symptom induction by CLCuMuB-encoded beta C1 was investigated by employing TYLCV[Ab]/N. glutinosa pathosystem. N. glutinosa plants were agroinoculated with TYLCV[Ab], TYLCV[Ab] + CLCuMuB and TYLCV[Ab] + CLCuMuB Delta C1 (deleted beta C1 ORF). The expression level of two defensive genes, NPR1 and CAT1 was determined at 10 and 40 days' post-inoculation (dpi) using quantitative real time-PCR. The transcription level of CAT1 remained unchanged, while NPR1 expression was upregulated upon agroinoculation by TYLCV[Ab] + CLCuMuB. As a positive regulator of SA-dependent gene expression, NPR1 transcript level in plants agroinoculated with TYLCV[Ab] + CLCuMuB was 2 and 1.9-fold higher than that in plants agroinoculated with TYLCV[Ab] or TYLCV[Ab] + CLCuMuB Delta C1 at 10 dpi, respectively. NPR1 transcript level was also found to be 1.8 and 1.7-fold higher at 40 dpi. Furthermore, disease symptoms were evaluated in the plants pre-treated with jasmonic acid (JA) and salicylic acid (SA) hormones. Symptoms severity was significantly decreased in plants infected with TYLCV[Ab] + CLCuMuB when plants were pre-treated by JA, while pre-treatment with SA led to an increase in disease symptoms. As a result, the increased transcript level of a positive regulator of SA-dependent gene (NPR1) and severe symptoms caused by the spraying of SA suggest that beta C1 induces disease symptoms in N. glutinosa through a SA-dependent pathway.