Host defense responses of mesquite ( <i>Prosopis laevigata</i> , Fabaceae) trees against the mistletoe ( <i>Psittacanthus calyculatus</i> , Loranthaceae) during the early stage of infection

Mesquite (Prosopis laevigata, Fabaceae) is a legume widely distributed in Mexico, infested by the mistletoe Psittacanthus calyculatus, Loranthaceae) a parasitic plant that absorbs water and nutrients from the mesquite. For parasitism to become established, the mistletoe must evade host defenses. To date, these defenses have only been studied in hosts with advanced parasitic infestations, but the initial defense of mesquite against these infestations remains unknown. The objective of this work was to investigate the early stress responses of mesquite trees after the first physical contact during infection by P. calyculatus. To do so, we first artificially inoculated mistletoe seeds and placed them on mesquite branches to initiate the parasitism. Mistletoe inoculations induced the early production of the phytohormones salicylic acid and jasmonic acid. These changes were accompanied by higher cell wall invertase activity, which precedes the increase in sucrose, glucose, and fructose. H₂O₂ formation and superoxide dismutase and peroxidase activity were also induced, while catalase activity decreased. Analyses revealed the presence of phenolic compounds as a defensive response against the mistletoe and, finally, elevated phenylalanine ammonia lyase activity after mistletoe inoculation. These data suggest that host trees recognize the presence of parasitic plants and trigger immune signaling responses well before haustorium formation. This is the first step toward understanding the interaction between the host tree and the mistletoe at beginning of parasitism.