Abstract
Recent studies with Y satellite RNA (Y-sat) of cucumber mosaic virus have demonstrated that Y-sat modifies the disease symptoms in specific host plants through the silencing of the magnesium protoporphyrin chelatase I subunit (CHLI), which is directed by the Y-sat derived siRNA. Along with the development of peculiar yellow phenotypes, a drastic decrease in CHLI-transcripts and a higher accumulation of Y-sat derived siRNA were observed. To investigate the molecular mechanisms underlying the Y-sat-induced chlorosis, especially whether or not the reduced expression of CHLI causes the chlorosis simply through the reduced production of chlorophyll or it triggers some other mechanisms leading to the chlorosis, we have established a new experimental system with an inducible silencing mechanism. This system involves the expression of artificial microRNAs targeting of Nicotiana tabacum CHLI gene under the control of chemically inducible promoter. The CHLI mRNA levels and total chlorophyll content decreased significantly in 2 days, enabling us to analyze early events in induced chlorosis and temporary changes therein. This study revealed that the silencing of CHLI did not only result in the decreased chlorophyll content but also lead to the downregulation of chloroplast and photosynthesis-related genes expression and the upregulation of defense-related genes. Based on these results, we propose that the reduced expression of CHLI could activate unidentified signaling pathways that lead plants to chlorosis.