In Defense of Plants: Salicylic Acid in a Host-Parasite-Pathogen System

Cuscuta pentagona is an obligate parasitic plant that uses a specialized organ, known as a haustorium, to penetrate host xylem and phloem to absorb water and nutrients from its hosts. As holoparasites, they lack chloroplast and therefore cannot photosynthesize. Bidirectional movement at the haustoria raises the possibility that the parasite can transmit defense signals from one host to another, though in what form is unclear. The biotrophic feeding mode, lack of chloroplast, and bidirectional movement across the haustoria lead me to hypothesize that (1) C. pentagona as a shoot parasite would elicit a systemic acquired resistance response in a host that would be observable in Cuscuta, (2) C. pentagona would be unable to synthesize salicylic acid without a host because it lacks chloroplast, and (3) that Cuscuta transmit salicylic acid molecules that are not utilized in the parasite between hosts because excess salicylic acid would not be beneficial to Cuscuta. I predicted that (1) salicylic acid would be higher in tomatoes infected with C. pentagona and a salicylic response to subsequent attack by the root pathogen Phytophthora nicotianae would be elevated, (2) C. pentagona would be deficient in salicylic acid without a host and would have increased salicylic acid content when challenged while attached to a host, and (3) C. pentagona would move the salicylic acid from a primary host to a secondary host and prime the secondary plant’s salicylic acid defense response. Cuscuta pentagona was attached to tomato (Solanum lycopersicum) plants and challenges - in the form of infection by a second parasite Phytophthora nicotianae (Experiments 1 & 3), needle puncture, and exposure to Methyl salicylic acid (Experiment 2) - were introduced to elicit a defense response in either the tomatoes or Cuscuta. Treatment by P. nicotianae decreased salicylic acid in tomato roots but Cuscuta did not elicit a response or alter the effect of P. nicotianae. Nonetheless, an increased amount of salicylic was observed in the Cuscuta when attached to a host infected by P. nicotianae. Significant levels of salicylic acid in unattached Cuscuta seedlings showed that they are able to independently produce salicylic acid, rejecting hypothesis (2). Treatment of the Cuscuta with needle and methyl salicylic acid did not produce significant changes in salicylic acid concentration. I found no evidence of direct transfer of salicylic acid by Cuscuta bridging, but there was diminished salicylic acid in the roots of secondary plants in pairs infected with P. nicotianae, rejecting hypothesis (3). There appears to be interaction between Cuscuta and its hosts via salicylic acid, but further experimentation is needed to fully elucidate the mechanisms of this interaction