NaKTI2, a Kunitz trypsin inhibitor transcriptionally regulated by NaWRKY3 and NaWRKY6, is required for herbivore resistance in Nicotiana attenuata

Overexpression of RpKTI2 from Robinia pseudoacacia Affects the Photosynthetic Physiology and Endogenous Hormones of Tobacco

Kunitz trypsin inhibitor genes play important roles in stress resistance. In this study, we investigated RpKTI2 cloned from Robinia pseudoacacia and its effect on tobacco. RpKTI2 was introduced into the tobacco cultivar NC89 using Agrobacterium-mediated transformation. Six RpKTI2-overexpressing lines were obtained. Transgenic and wild-type tobacco plants were then compared for photosynthetic characteristics and endogenous hormone levels. Transgenic tobacco showed minor changes in chlorophyll content, fluorescence, and photosynthetic functions. However, the maximum photochemical efficiency (Fv/Fm) increased significantly while intercellular CO2 concentration (Ci) decreased significantly. Stomatal size and hormone content (indole-3-acetic acid, zeatin riboside, gibberellin, and indole-3-propionic acid) were reduced, while brassinosteroid content increased. Random forest regression revealed that RpKTI2 overexpression had the biggest impact on carotenoid content, initial fluorescence, Ci, stomatal area, and indole-3-acetic acid. Overall, RpKTI2 overexpression minimally affected chlorophyll synthesis and photosynthetic system characteristics but influenced stomatal development and likely enhanced the antioxidant capacity of tobacco. These findings provide a basis for future in-depth research on RpKTI2.

Heterologous expression of the Glycine soja Kunitz-type protease inhibitor GsKTI improves resistance to drought stress and Helicoverpa armigera in transgenic Arabidopsis lines

Kunitz-like protease inhibitors (KTIs) have been identified to play critical roles in insect defense, but evidence for their involvement in drought stress is sparse. The aim of this study was to identify and functionally characterize a Kunitz-like protease inhibitor, GsKTI, from the wild soybean (Glycine soja) variety ED059. Expression patterns suggest that drought stress and insect herbivory may induce GsKTI transcript levels. Transgenic Arabidopsis lines overexpressing GsKTI have been shown to exhibit enhanced drought tolerance by regulating the ABA signaling pathway and increasing xylem cell number. Transgenic Arabidopsis leaves overexpressing GsKTI interfered with insect digestion and thus had a negative effect on the growth of Helicoverpa armigera. It is concluded that GsKTI increases resistance to drought stress and insect attack in transgenic Arabidopsis lines.

NaMLP, a new identified Kunitz trypsin inhibitor regulated synergistically by JA and ethylene, confers Spodoptera litura resistance in Nicotiana attenuata

We identified a miraculin-like protein (NaMLP) who is a new Kunitz trypsin inhibitor regulated synergistically by JA and ethylene signals and confers Spodoptera litura resistance in wild tobacco Nicotiana attenuata. The findings revealed a new source of trypsin inhibitor activities after herbivory, and provide new insights into the complexity of the regulation of trypsin inhibitor-based defense after insect herbivore attack. Upon insect herbivore attack, wild tobacco Nicotiana attenuata accumulates trypsin protease inhibitor (TPI) activities as a defense response from different protease inhibitor (PI) coding genes, including WRKY3-regulated NaKTI2, and JA-dependent NaPI. However, whether any other TPI gene exists in N. attenuata is still unclear. A miraculin-like protein gene (NaMLP) was highly up-regulated in N. attenuata after Alternaria alternata infection. However, silencing or overexpression of NaMLP had no effect on the lesion diameter developed on N. attenuata leaves after A. alternata inoculation. Meanwhile, the transcripts of NaMLP could be induced by wounding and amplified by Spodoptera litura oral secretions (OS). S. litura larvae gained significantly more biomass on NaMLP-silenced plants but less on NaMLP overexpressed plants. Although NaMLP showed low sequence similarity to NaKTI2, it had conserved reaction sites of Kunitz trypsin inhibitors, and exhibited TPI activities when its coding gene was overexpressed transiently or stably in N. attenuata. This was consistent with the worst performance of S. litura larvae on NaMLP overexpressed lines. Furthermore, NaMLP-silenced plants had reduced TPI activities and better S. litura performance. Finally, OS-elicited NaMLP was dramatically reduced in JA-deficient AOC silencing and ethylene-reduced ACO-silencing plants, and the expression of NaMLP could be significantly induced by methyl jasmonate or ethephon alone, but dramatically amplified by co-treatment of both methyl jasmonate and ethephon. Thus, our results demonstrate that in addition to JA-regulated NaPI, and WRKY3/6-dependent NaKTI2, N. attenuata plants also up-regulates TPI activities via NaMLP, which confers S. litura resistance through JA and ethylene signaling pathways in a synergistic way.

Water Deficiency and Induced Defense Against a Generalist Insect Herbivore in Desert and Mediterranean Populations of Eruca sativa

In natural and agricultural ecosystems, plants are often simultaneously or sequentially exposed to combinations of stressors. Here we tested whether limited water availability (LWA) affects plant response to insect herbivory using two populations of Eruca sativa from desert and Mediterranean habitats that differ in their induced defenses. Considering that such differences evolved as responses to biotic and possibly abiotic stress factors, the two populations offered an opportunity to study ecological aspects in plant response to combined stresses. Analysis of chemical defense mechanisms showed that LWA significantly induced total glucosinolate concentrations in the Mediterranean plants, but their concentrations were reduced in the desert plants. However, LWA, with and without subsequent jasmonate elicitation, significantly induced the expression of proteinase inhibitor in the desert plants. Results of a no-choice feeding experiment showed that LWA significantly increased desert plant resistance to Spodoptera littoralis larvae, whereas it did not affect the relatively strong basal resistance of the Mediterranean plants. LWA and subsequent jasmonate elicitation increased resistance against the generalist insect in Mediterranean plants, possibly due to both increased proteinase inhibitor expression and glucosinolate accumulation. The effect of LWA on the expression of genes involved in phytohormone signaling, abscisic acid (ABA-1) and jasmonic acid (AOC1), and the jasmonate responsive PDF1.2, suggested the involvement of abscisic acid in the regulation of defense mechanisms in the two populations. Our results indicate that specific genotypic responses should be considered when estimating general patterns in plant response to herbivory under water deficiency conditions.