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The hormone ethylene influences plant growth, development, and some defense responses. The fungal elicitor Ethylene-Inducing Xylanase (EIX) elicits ethylene biosynthesis in tomato (Lycopersicon esculentum) and tobacco (Nicotiana tabacum) leaves by induction of 1-aminocyclopropane-1-caboxylic acid synthase (Acs) gene expression. A minimal promoter element in the LeAcs2 gene required for EIX responsiveness was defined by deletion analysis in transgenic tomato plants. The sequence between -715 and -675 of the tomato Acs2 gene was found to be essential for induction by EIX. A Cys protease (LeCp) was isolated that specifically binds to this cis element in vitro. Ectopic expression of LeCp in tomato leaves induced the expression of Acs2. Moreover, chromatin immunoprecipitation showed that LeCp binds in vivo to the Acs promoter. We propose a mechanism for the dual function of the LeCp protein. The protease acts enzymatically in the cytoplasm. Then, upon signaling, a small ubiquitin-related modifier protein binds to it, enabling entrance into the nucleus, where it acts as a transcription factor. Thus, LeCp can be considered a dual-function protein, having enzymatic activity and, upon elicitor signaling, exhibiting transcriptional factor activity that induces LeAcs2 expression. [ABSTRACT FROM AUTHOR]

An ethylene-inducing xylanase (EIX) is a potent elicitor of plant defense responses in specific cultivars of tobacco (Nicotiana tabacum) and tomato (Lycopersicon esculentum). The LeEix locus in tomatoes was characterized by map-based cloning, which led to the identification of a novel gone cluster from which two members (LeEix1 and LeEix2) were isolated. Similar to the tomato Ve resistance genes in tomato plants, the deduced amino acid sequences encoded by LeEix1 and LeEix2 contain a Leu zipper, an extracellular Leu-rich repeat domain with glycosylation signals, a transmembrane domain, and a C-terminal domain with a mammalian endocytosis signal. Silencing expression of the LeEix genes prevented the binding of EIX to cells of an EIX-responsive plant and thus inhibited the hypersensitive response. Overexpression of either LeEixl or LeEix2 genes in EIX-nonresponsive tobacco plants enabled the binding of EIX, although only LeEix2 could transmit the signal that induced the hypersensitive response. Overexpressing LeEix2 in mammalian COS-7 cells enables binding of EIX, indicating physical interaction between the EIX elicitor and LeEix2 gone product. Structural analysis of the LeEix proteins suggests that they belong to a class of cell-surface glycoproteins with a signal for receptor-mediated endocytosis. Mutating the endocytosis signal in LeEix2 (Tyr 993 to Ala) abolished its ability to induce the hypersensitive response, suggesting that endocytosis plays a key role in the signal transduction pathway. [ABSTRACT FROM AUTHOR]