Spider mite tetranins elicit different defense responses in different host habitats

Spider mites (Tetranychus urticae) are a major threat to economically important crops. Here, we investigated the potential of tetranins, in particular Tet3 and Tet4, as T. urticae protein-type elicitors that stimulate plant defense. Truncated Tet3 and Tet4 proteins showed efficacy in activating the defense gene pathogenesis-related 1 (PR1) and inducing phytohormone production in leaves of Phaseolus vulgaris. In particular, Tet3 caused a drastically higher Ca2+ influx in leaves, but a lower reactive oxygen species (ROS) generation compared to other tetranins, whereas Tet4 caused a low Ca2+ influx and a high ROS generation in the host plants. Such specific and non-specific elicitor activities were examined by knockdown of Tet3 and Tet4 expressions in mites, confirming their respective activities and in particular showing that they function additively or synergistically to induce defense responses. Of great interest is the fact that Tet3 and Tet4 expression levels were higher in mites on their preferred host, P. vulgaris, compared to the levels in mites on the less-preferred host, Cucumis sativus, whereas Tet1 and Tet2 were constitutively expressed regardless of their host. Furthermore, mites that had been hosted on C. sativus induced lower levels of PR1 expression, Ca2+ influx and ROS generation, i.e., Tet3- and Tet4-responsive defense responses, in both P. vulgaris and C. sativus leaves compared to the levels induced by mites that had been hosted on P. vulgaris. Taken together, these findings show that selected tetranins respond to variable host cues that may optimize herbivore fitness by altering the anti-mite response of the host plant.