Cell-lytic activity of tobacco BY-2 induced by a fungal elicitor from alternaria alternata attributed to the expression of a class I beta-1,3-glucanase gene

Comprehensive analysis of glucan elicitor-regulated gene expression in tobacco BY-2 cells reveals a novel MYB transcription factor involved in the regulation of phenylpropanoid metabolism

We previously demonstrated that a beta-1,3-, 1,6-oligoglucan (AaGlucan) from the fungus Alternaria alternata 102 shows strong elicitor activity in tobacco BY-2 cells. We have used cDNA microarray analysis to monitor global changes in gene expression in tobacco cells treated with this A. alternata fraction or with laminarin. In total, we identified 265 genes that were induced 1 h after treatment with an AaGlucan-enriched fraction or laminarin. Among them, we characterized in detail a novel tobacco R2R3 MYB-type transcription factor homolog (NtMYBGR1) and two DC1 domain-containing genes (NtDC1A and NtDC1B). Microarray data, together with overexpression and metabolic analyses, indicated that NtMYBGR1, but not the NtDC1 proteins, primarily targets the phenylpropanoid synthesis-related genes PAL and 4CL. These results suggest that NtMYBGR1 specifically regulates defense responses in BY-2 cells by enhancing phenylpropanoid metabolism in response to AaGlucan and laminarin elicitors.

Characterization of NtChitIV, a class IV chitinase induced by beta-1,3-, 1,6-glucan elicitor from Alternaria alternata 102: Antagonistic effect of salicylic acid and methyl jasmonate on the induction of NtChitIV

Tobacco BY-2 class IV chitinases (TBC-1, TBC-3) were rapidly and transiently induced by the beta-1,3-, 1,6-glucan elicitor from Alternaria alternata 102 (AaGlucan). The full-length cDNA and 5'-flanking region of a gene encoding class IV chitinases were isolated on the basis of the amino acid sequence of TBC-1. Sequence analysis indicated that NtChitIV encoded TBC-1, TBC-3, or both. Since purified TBC-1 and TBC-3 from BY-2 cells lack a chitin binding domain in the N-terminal region, these enzymes suggested to be derived from NtChitIV by post-translational proteolytic processing. The transcripts of NtChitIV accumulated rapidly within 1h after treatment with AaGlucan. Accumulation was maximal 3h after treatment. Reporter gene assays were used to analyze the promoter regions involved in the transcriptional control of NtChitIV, and these assays revealed that the 1.89-kb NtChitIV promoter was activated by AaGlucan but not by salicylic acid (SA) or methyl jasmonate (MeJA). The AaGlucan-induced transcriptional activation via 1.89-kb NtChitIV promoter was attenuated by pretreatment with SA or MeJA. These results suggest that NtChitIV expression is particularly induced by AaGlucan and that the AaGlucan-dependent signaling pathway is different from the SA- and MeJA-dependent signaling pathways.

Novel beta-1,3-, 1,6-oligoglucan elicitor from Alternaria alternata 102 for defense responses in tobacco

A novel elicitor that induces chitinases in tobacco BY-2 cells was isolated from Alternaria alternata 102. Six other fungi, including A. alternata IFO 6587, could not induce, or weakly induce chitinase activity. The purified elicitor was soluble in 75% methanol and showed the chitinase-inducing activity when applied at concentrations of as low as 25 ng x mL(-1). Structural determination by methylation analysis, reducing-end analysis, MALDI-TOF/MS, and NMR spectroscopy indicated that the elicitor was a mixture of beta-1,3-, 1,6-oligoglucans mostly with a degree of polymerization of between 8 and 17. Periodate oxidation of the elicitor suggested that the 1,6-linked and nonreducing terminal residues are essential for the elicitor activity. Further analysis of the elicitor responses in BY-2 cells indicated that the activity of this beta-1,3-, 1,6-glucan elicitor was about 1000 times more potent than that of laminarin, which is a known elicitor of defense responses in tobacco. Analyzing the expression of defense-related genes indicated that a phenylalanine ammonia-lyase gene and a coumaroyl-CoA O-methyltransferase gene were transiently expressed by this beta-1,3-, 1,6-glucan elicitor. The elicitor induced a weak oxidative burst but did not induce cell death in the BY-2 cells. In the tissue of tobacco plants, this beta-1,3-, 1,6-glucan elicitor induced the expression of basic PR-3 genes, the phenylpropanoid pathway genes, and the sesquiterpenoid pathway genes. In comparison with laminarin and laminarin sulfate, which are reported to be potent elicitors of defense responses in tobacco, the expression pattern of genes induced by the purified beta-1,3-, 1,6-glucan elicitor was more similar to that induced by laminarin than to that induced by laminarin sulfate.