Cauliflower mosaic virus Tav protein induces leaf chlorosis in transgenic tobacco through a host response to virulence function of Tav

Abstract

To study the precise mechanisms underlying the chlorosis caused by plant viruses, we previously established a synchronous experimental system using transgenic plants expressing Cauliflower mosaic virus multifunctional protein, Tav (transactivator/viroplasmin), under the control of an artificially inducible promoter. Shortly after the induction of Tav expression, pathogenesis-related protein (PR) 1a gene expression is upregulated in the transgenic tobacco lines, which show visible chlorosis within a week. The present study showed that the expression of Tav also induces some salicylic acid (SA)-and ethylene-responsive PR genes. In contrast to transiently expressed Tav, which suppressed Agrobacterium-induced and SA-induced PR1a expression, the artificial induction of Tav from the transgene did not affect SAinduced PR1a expression, rather it alone induced PR1a expression. In a deletion analysis, chlorosis and PR1a induction function in transgenic tobacco were mapped to a region in Tav that had been shown to have a role in pathogenesis in a susceptible host, elicitation of the hypersensitive response in a resistant host, suppression of RNA silencing, and the suppression of Tomato bushy stunt virus P19-mediated cell death in tobacco. The results suggest that Tav-induced chlorosis results from a host response, which accompanies PR1a induction, to pathogenic function of Tav.

Inducible expression of magnesium protoporphyrin chelatase subunit I (CHLI)-amiRNA provides insights into cucumber mosaic virus Y satellite RNA-induced chlorosis symptoms

“…8a-c). Consistently with our previous reports [41,42], iTav-3 showed the upregulation of PR1a expression by 15-fold (Fig. 8a).…”

“…Taken together, these results strongly suggest that the silencing of CHLI induces chlorosis, not through the simple decrease of chlorophyll biosynthesis but the activation of some plant responses. We have found the downregulation of CPRGs expression and the induction of PR1a expression in transgenic plants expressing Hsp90C-hpRNA [5] as well as in iTav-tobacco plants [41,42]. The present results and these reports collectively suggest that a rapid downregulation of CPRGs and a rapid upregulation of defense-related genes are common pathways of chlorosis development.…”

“…Increased accumulation of ROS inside the chloroplast would give rise to the elevated level of SA [11], which activates PR1a gene. On the contrary, we reported that the PR1a would be induced in Dex-treated iTav-tobacco plants through an SA-independent manner [41]. The upregulation of PR1a gene expression was much higher in CHLIamiRNAs and Hsp90C-hpRNA transgenic plants than in iTav-tobacco.…”

“…Previously, we have reported the induction of PR1a gene in iTav-tobacco plants shortly after the induced expression of CaMV-Tav transgene [41,42]. Also, our recent study has demonstrated the drastic upregulation of PR1a gene during the chlorosis development in Hsp90C silenced transgenic tobacco [5].…”

“…To this end, we employed artificial microRNAs (amiRNAs) designed to target CHLI mRNA and chemically inducible promoter. In this system, chlorosis could be induced in transgenic tobacco plants as in the previously reported inducible chlorosis systems, in which CaMV Tav [41,42] and hairpin RNA of Hsp90C [5] are expressed in a chemical-inducible manner. The present system enabled us to analyze early events and temporary changes therein, revealing that CHLI silencing did not only resulted in the decreased chlorophyll content but also caused downregulation of CPRGs expression.…”