When One Leaf Is Eaten, Do the Other Leaves Feel It?

Abstract

Plants have developed a number of defense mechanisms against attacks by herbivores. In the case of Nicotiana attenuata (coyote tobacco), these mechanisms include the production of jasmonic acid (JA) and ethylene, two compounds with well-characterized roles in host defense. The oral secretions (OS) from larvae of the herbivore Manduca sexta (the tobacco hornworm) contain factors that are important in eliciting these defense responses; however, the signal transduction pathways involved are not well understood. Mitogen-activated protein kinase (MAPK) signaling is known to be involved in mediating the response of plants to various forms of stress, and two members of this family, salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK), are rapidly activated after leaf wounding. Wu et al . investigated the response of N. attenuata to puncture wounds of the leaves that were treated either with water, as a control, or with the OS of M. sexta . Real-time polymerase chain reaction and Western blotting analyses demonstrated that, whereas wounding resulted in the production of SIPK and WIPK mRNAs and proteins, as expected, the addition of OS resulted in much higher amounts of these kinases being produced. Other responses to OS included the production and activation of transcription factors and other MAPKs, as well as the synthesis of JA. Notably, these responses to OS were much reduced in plants that had been modified by virus-induced gene silencing to block production of either SIPK or WIPK. SIPK, but not WIPK, was necessary to mediate the production of ethylene. By selectively wounding and testing different parts of a given leaf, the authors found that OS elicited SIPK and WIPK activation in both wounded and unwounded locations. Other leaves on the same plant showed evidence of increased transcription of other genes (downstream of JA signaling) that are involved in host defense, but without the activation of MAPKs. These data suggest that attack by an herbivore results in the acute localized activation of SIPK, WIPK, and the MAPK cascade, resulting in the production of JA and ethylene, which then prime other leaves to be ready to respond in case one leaf is not enough of a meal for the larva. J. Wu, C. Hettenhausen, S. Meldau, I. T. Baldwin, Herbivory rapidly activates MAPK signaling in attacked and unattacked leaf regions but not between leaves of Nicotiana attenuata . Plant Cell 19 , 1096-1122 (2007). [Abstract] [Full Text]