Involvement of two nitric oxide-associated genes, NOA1 and GSNOR, in Nicotiana attenuata's resistance to the specialist insect herbivore Manduca sexta
Nitric oxide-associated protein 1 (NOA1) is involved in various abiotic stress responses and is required for plant resistance to pathogen infections. The first chapter of this dissertation explores for the first time the role of NOA1 in plant-herbivore interactions. We created NOA1-silenced Nicotiana attenuata plants (irNaNOA1), which had highly decreased photosynthesis rates. We further examined plant defense traits against its specialist herbivore Manduca sexta by treating WT and irNaNOA1 plants with mechanical wounding and M. sexta oral secretions (OS). NOA1-silenced plants showed elevated levels of herbivory-induced jasmonic acid (JA), but decreased JA-isoleucine conjugate (JA-Ile) levels that did not result from compromised JAR (jasmonic acid resistant) activity in irNOA1 plants. Moreover, nitrogen-rich defensive compounds, nicotine and trypsin proteinase inhibitors (TPI), did not differ between WT and irNaNOA1 plants. In contrast, concentrations of most carbon-based defensive compounds were lower in these plants than in WT plants, although the levels of chlorogenic acid were not changed. Therefore, silencing NOA1 alters the allocation of carbon resources within the phenylpropanoid pathway. These data suggest the involvement of NOA1 in N. attenuata’s defense against M. sexta attack, and highlight its role in photosynthesis, and biosynthesis of jasmonates and secondary metabolites. The second chapter explores GSNOR-involvement in plant–herbivore interactions. S-nitrosoglutathione reductase (GSNOR) reduces the nitric oxide (NO) adduct S-nitrosoglutathione (GSNO) and has been found to be important in resistance to phytopathogens. Using a virus-induced gene silencing (VIGS) system, the activity of GSNOR in N. attenuata was knocked down and the function of GSNOR in defense against the insect herbivore M. sexta was examined. Silencing GSNOR decreased the herbivory-induced accumulation of plant defense regulating phytohormones JA and ethylene without compromising the activity of salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK). Decreased activity of trypsin proteinase inhibitors (TPIs) were detected in GSNOR-silenced plants after simulated M. sexta feeding. Hence, GSNOR-silenced plants have elevated susceptibility to M. sexta attack. GSNOR is also required for methyl jasmonate (MeJA)-induced accumulation of defense-related secondary metabolites (TPI, caffeoylputrescine, and diterpene glycosides) but is not needed for the transcriptional regulation of JAZ3 (jasmonate ZIM-domain 3) and threonine deaminase, indicating that GSNOR mediates certain but not all jasmonate-inducible responses. This work highlights the important role of GSNOR in plant resistance to herbivory and jasmonate signaling and suggests the potential involvement of NO in plant–herbivore interactions.