Engineering mobile RNA in Carrizo to enhance plant defense responses to control citrus greening
S. Zhang (1), G. Perazzo (1), D. Gabriel (1) (1) Integrated Plant Genetics, U.S.A.
Plants modulate plant defense responses against pathogens with both pro- and anti-programmed cell death (PCD) proteins. By suppressing expression of anti-PCD proteins, PCD can occur more rapidly and with stronger defense responses. There are many PCD triggers, including Reactive Oxygen Species (ROS) which activate plant defense signaling pathways over long distances. Citrus greening is caused by Ca. Liberibacter asiaticus (Las). Las produces a peroxidase that has been suggested to dampen the ROS signaling pathway. A gene silencing strategy was used to suppress an anti-PCD gene in Carrizo rootstocks. The expression levels of three citrus defense response genes (NDR1, PR1 and EDS1) were significantly higher in silenced lines than in nontransgenic (NT) controls after inoculation with Las flagellin (flg22), used as a proxy for Las. Silenced Carrizo lines were also more tolerant to heat and cold stresses than NT controls. Challenge inoculations are currently underway using approach-grafted, Las infected citrus; all sampling was taken from leaves above the approach graft union. To date, three transgenic Carrizo lines failed to become infected over a 9 month challenge period as compared with two transgenic lines that became infected within two months. Following removal of the infected source tree, two NT scions that were consistently positive have now tested negative, possibly indicating movement of the silencing signal from transgenic rootstock to NT scion.