Induced expression of a temperature-sensitive leucine-rich repeat receptor-like protein kinase gene by hypersensitive cell death and wounding in tobacco plant carrying the N resistance gene

Phenylalanine ammonia-lyase and phenolic compounds are related to hybrid lethality in the cross Nicotiana suaveolens×N. tabacum

Hybrid lethality observed in hybrid seedlings between Nicotiana suaveolens and N. tabacum is characterized by browning, initially of the hypocotyls and eventually of entire seedlings. We investigated the mechanism underlying this browning of tissues. A phenylalanine ammonia-lyase (PAL) gene codes an enzyme involved in a pathway producing phenolic compounds related to the browning of plant tissues. The expression of PAL rapidly increased with the induction of hybrid lethality. Phenolic compounds were observed to be accumulated in whole parts of hybrid seedlings. Treatment of hybrid seedlings with L-2-aminooxy-3-phenylpropionic acid (AOPP), an inhibitor for PAL, suppressed browning and decreased the phenolic content of hybrid seedlings. Although programmed cell death (PCD) was involved in hybrid lethality, AOPP treatment also suppressed cell death and enhanced the growth of hybrid seedlings. These results indicated that PAL is involved in hybrid lethality, and phenolic compounds could be the cause of hybrid lethality-associated tissue browning.

Host specific preference for low temperature in the multiplication of a tombusvirus, gentian virus A

Gentian virus A (GeVA), a novel tombusvirus isolated from Japanese gentian, has shown only a limited ability to infect Japanese gentians under experimental conditions. In this study, temperature was found to affect the efficient multiplication of GeVA in Japanese gentians. GeVA efficiently multiplied in inoculated leaves of gentians at 18 °C but not at 23 °C. This low-temperature (18 °C)-preferred GeVA multiplication was specifically observed in Japanese gentians and Arabidopsis thaliana but not in other experimental plants, including Nicotiana benthamiana. In A. thaliana, visible defense responses, including pathogenesis-related protein 1 expression, were not detected at 23 °C. Furthermore, several A. thaliana mutants, including those defective in RNA silencing, with altered plant immunities did not allow GeVA to multiply to detectable levels at 23 °C. Taken together, these data suggest that unique interaction between GeVA and gentians/A. thaliana, which is independent of RNA silencing, may underlie the low-temperature-preferred multiplication of GeVA.

The receptor-like kinase SlSOBIR1 is differentially modulated by virus infection but its overexpression in tobacco has no significant impact on virus accumulation

KEY MESSAGE

The role of the tomato receptor-like kinase SlSOBIR1 in antiviral defense was investigated. SlSOBIR1 was transcriptionally modulated by unrelated viruses but its ectopic expression had no effect on virus accumulation. Leucine-rich repeat receptor-like kinases (LRR-RLK) constitute a diverse group of proteins allowing the cell to recognize and respond to the extracellular environment. In the present study we focused on a gene encoding a tomato LRR-RLK (named SlSOBIR1) involved in the host defense against fungal pathogens. Curiously, SlSOBIR1 has been previously reported to be down-regulated by Pepper yellow mosaic virus (PepYMV) infection. Here, we show that SlSOBIR1 is responsive to wounding and differentially modulated by unrelated virus infection, i.e., down-regulated by PepYMV and up-regulated by Tomato chlorotic spot virus (TCSV). Despite these divergent expression profiles, SlSOBIR1 overexpression in transgenic tobacco plants had no evident effect on TCSV and PepYMV accumulation. On the other hand, overexpression of SlSOBIR1 significantly increased the expression of selected defense genes (PR-1a and PR-6) and exacerbated superoxide production in wounded leaves. Our data indicate that the observed modulation of SlSOBIR1 expression is probably triggered by secondary effects of the virus infection process and suggest that SlSOBIR1 is not directly involved in antiviral signaling response.

Wounding in the plant tissue: the defense of a dangerous passage

Plants are continuously exposed to agents such as herbivores and environmental mechanical stresses that cause wounding and open the way to the invasion by microbial pathogens. Wounding provides nutrients to pathogens and facilitates their entry into the tissue and subsequent infection. Plants have evolved constitutive and induced defense mechanisms to properly respond to wounding and prevent infection. The constitutive defenses are represented by physical barriers, i.e., the presence of cuticle or lignin, or by metabolites that act as toxins or deterrents for herbivores. Plants are also able to sense the injured tissue as an altered self and induce responses similar to those activated by pathogen infection. Endogenous molecules released from wounded tissue may act as Damage-Associated Molecular Patterns (DAMPs) that activate the plant innate immunity. Wound-induced responses are both rapid, such as the oxidative burst and the expression of defense-related genes, and late, such as the callose deposition, the accumulation of proteinase inhibitors and of hydrolytic enzymes (i.e., chitinases and gluganases). Typical examples of DAMPs involved in the response to wounding are the peptide systemin, and the oligogalacturonides, which are oligosaccharides released from the pectic component of the cell wall. Responses to wounding take place both at the site of damage (local response) and systemically (systemic response) and are mediated by hormones such as jasmonic acid, ethylene, salicylic acid, and abscisic acid.

Suppression of tobacco mosaic virus-induced hypersensitive-type necrotization in tobacco at high temperature is associated with downregulation of NADPH oxidase and superoxide and stimulation of dehydroascorbate reductase

Tissue necroses and resistance during the hypersensitive response (HR) of tobacco to tobacco mosaic virus (TMV) are overcome at temperatures above 28 degrees C and the virus multiplies to high levels in the originally resistant N-gene expressing plants. We have demonstrated that chemical compounds that generate reactive oxygen species (ROS) or directly applied hydrogen peroxide (H(2)O(2)) are able to induce HR-type necroses in TMV-inoculated Xanthi-nc tobacco even at high temperatures (e.g. 30 degrees C). The amount of superoxide (O(2)(*-)) decreased, while H(2)O(2) slightly increased in TMV- and mock-inoculated leaves at 30 degrees C, as compared with 20 degrees C. Activity of NADPH oxidase and mRNA levels of genes that encode NADPH oxidase and an alternative oxidase, respectively, were significantly lower, while activity of dehydroascorbate reductase was significantly higher at 30 degrees C, as compared with 20 degrees C. It was possible to reverse or suppress the chemically induced HR-type necrotization at 30 degrees C by the application of antioxidants, such as superoxide dismutase and catalase, demonstrating that the development of HR-type necroses indeed depends on a certain level of superoxide and other ROS. Importantly, high TMV levels at 30 degrees C were similar in infected plants, whether the HR-type necrotization developed or not. Suppression of virus multiplication in resistant, HR-producing tobacco at lower temperatures seems to be independent of the appearance of necroses but is associated with temperatures below 28 degrees C.

A novel wound-responsive cis-element, VWRE, of the vascular system-specific expression of a tobacco peroxidase gene, tpoxN1

The wound-induced expression of tpoxN1, encoding a tobacco peroxidase, is unique because of its vascular system-specific expression and insensitivity to known wound-signal compounds such as jasmonic acid, ethylene, and plant hormones [Sasaki et al. (2002) Plant Cell Physiol 43:108-117]. To study the mechanism of expression, the 2-kbp tpoxN1 promoter region and successive 5'-deletion of the promoter were introduced as GUS fusion genes into tobacco plants. Analysis of GUS activity in transgenic plants indicated that a vascular system-specific and wound-responsive cis-element (VWRE) is present at the -239/-200 region of the promoter. Gel mobility shift assays suggested that a nuclear factor(s) prepared from wounded tobacco stems binds a 14-bp sequence (-229/-215) in the -239/-200 region in a sequence-specific manner. A mutation in this 14-bp region of the -239 promoter fragment resulted in a considerable decrease in wound-responsive GUS activity in transgenic plants. An 11-bp sequence, which completely overlaps with the 14-bp sequence, was found in the 5' distal region (-420/-410) and is thought to contribute to the wound-induced expression together with the 14-bp. The -114-bp core promoter of the tpoxN1 gene was indispensable for wound-induced expression, indicating that the 14-bp region is a novel wound-responsive cis-element VWRE, which may work cooperatively with other factors in the promoter.

Involvement of wound-induced receptor-like protein kinase in wound signal transduction in tobacco plants

The wound-induced receptor-like protein kinase (WRK) gene, isolated as one of the genes whose transcripts accumulated during the early period of N gene-dependent synchronized cell death in tobacco mosaic virus-infected tobacco plants, encodes a leucine-rich repeat receptor-like protein kinase, and its transcript was transiently increased 15 min after wounding. In the present study, analysis of a green fluorescent protein fusion protein indicated that WRK is localized in the plasma membrane. In transgenic tobacco plant lines with elevated or suppressed levels of WRK transcript, the wound-induced accumulation of both basic PR-1 and PR-6 transcripts was slightly enhanced or significantly suppressed respectively. The decrease in wound-induced basic PR gene expression in WRK suppressed lines was restored by jasmonic acid (JA) treatment. Furthermore, the levels of wound-induced enzymatic activation of both salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK) and wound-induced accumulation of JA were reduced in the WRK suppressed lines in comparison with a control line. These results suggest that WRK functions upstream of SIPK and WIPK and regulates wound signal transduction in tobacco plants.