Narboh D, a Respiratory Burst Oxidase Homolog in Nicotiana attenuata, is Required for Late Defense Responses After Herbivore Attack

The superoxide (O-2(-))-generating NADPH oxidases are crucial for the defense of plants against attack from pathogens; however, it remains unknown whether they also mediate responses against chewing insect herbivores. The transcripts of the respiratory burst NADPH oxidase homolog Narboh D in Nicotiana attenuate are rapidly and transiently elicited by wounding, and are amplified when Manduca sexta oral secretions (OS) are added to the wounds. The fatty-acid-amino-acid-conjugates (FACs), demonstrably the major elicitors in M. sexta OS, are responsible for the increase in Narboh D transcripts. Silencing Narboh D significantly reduced reactive oxygen species (ROS) levels after OS elicitation, but neither OS-elicited jasmonic acid (JA) or JA-isoleucine (JA-Ile) bursts, pivotal hormones that regulates plant resistance to herbivores, nor early transcripts of herbivore defense-related genes (NaJAR4 and NaPAL1), were influenced. However, late OS-elicited increases in trypsin proteinase inhibitors (TPIs), as well as the transcript levels of defense genes such as polyphenol oxidase, TPI and Thionin were significantly reduced. In addition, Narboh D-silenced plants were more vulnerable to insect herbivores, especially the larvae of the generalist Spodoptera littoralis. We thus conclude that Narboh D-based defenses play an important role in late herbivore-elicited responses.