Distinct thresholds condition sense posttranscriptional gene silencing initiation and amplification

It has long been known that Pro35S-driven sense transgenes have a high propensity to undergo posttranscriptional gene silencing (S-PTGS). However, what exactly conditions S-PTGS initiation and amplification to make it systemic remains unknown. Through genetic screens, we show that antagonistic chromatin-related mutations enhancing and reducing transgene expression result in enhanced and reduced S-PTGS amplification capacities, respectively, without affecting the initiation rate. Analysis of a large set of independent transgenic plants confirms a direct relationship between transgene expression and its capacity to amplify S-PTGS. Combining an inducible or a tissue-specifically expressed GUS transgene with a Pro35S:GUS transgene locus prone to amplify S-PTGS but unable to spontaneously initiate it induces systemic S-PTGS, indicating that transient and/or local passing of a discrete threshold is sufficient to initiate S-PTGS. Together, these results call for the existence of distinct thresholds related to transiently produced aberrant RNA and permanently produced target mRNA levels, which condition S-PTGS initiation and amplification, respectively. We show that this model also applies to endogenous genes for which RNA Quality Control (RQC) acts as a first layer of protection against S-PTGS, and DCL2's obscuration by DCL4 as a second layer, allowing RQC to dysfunction locally without translating into the drama of systemic S-PTGS.