During the course of evolution, organisms have developed genetic mechanisms in response to various environmental stresses including wounding from mechanical damage or herbivory-caused injury. A previous study of wounding response in the plant tobacco identified a unique wound-induced gene, aptly named KED due to its coding for a protein that has an unusually high content of amino acids lysine (K), glutamic acid (E) and aspartic acid (D). However, by far little is known about this intriguing gene. In this study, we investigated the evolutionary aspects of the KED-rich coding genes. We found that a consistent pattern of wound-induced KED gene expression is maintained across representative species of angiosperm and gymnosperm. KED genes can be identified in species from all groups of land plants (Embryophyta). All the KED proteins from vascular plants (Tracheophyta) including angiosperm, gymnosperm, fern and lycophyte share a conserved 19-amino acid domain near the C-terminus, whereas bryophytes (moss, liverwort and hornwort) possess KED-rich, multi-direct-repeat sequences that are distinct from the vascular plant KEDs. We detected KED-rich sequences in Charophyta species but not in Chlorophyta wherever genome sequences are available. Our studies suggest diverse and complex evolution pathways for land plant KED genes. Vascular plant KEDs exhibit high evolutionary conservation, implicating their shared function in response to wounding stress. The extraordinary enrichment of amino acids K, E and D in these groups of distinct and widely distributed proteins may reflect the structural and functional requirement for these three residues during some 600 million years of land plant evolution.
A promoter is an essential structural component of a gene that controls its transcription activity in different development stages and in response to various environmental stimuli. Knowledge of promoter functionality in heterologous systems is important in the study of gene regulation and biotechnological application. In order to explore the activity of the pepper capsaicin synthase gene (PUN1) promoter, gene constructs of pPUN1::GUS (for β-glucuronidase) and pPUN1::NtKED (for a tobacco wound-responsive protein) were introduced into tobacco and tomato, respectively, and their activities were examined. Higher levels of GUS staining intensity and transcription were detected in ovary, anther and pollen than other tissues or organs in tobacco plants. Likewise, transgenic tomato fruits had a higher level of pPUN1::NtKED gene expression than the leaf and flower. The PUN1-driven gene expression can be transiently induced by wounding, heat (40 °C) and the capsaicinoid biosynthetic pathway precursor phenylalanine. When compared to the reported pPUN1::GUS-expressing Arabidopsis, the PUN1 promoter exhibited a more similar pattern of activities among pepper, tobacco and tomato, all Solanaceae plants. Our results suggest the potential utility of this tissue-preferential and inducible promoter in other non-pungent Solanaceae plants for research of gene function and regulation as well as in the biotechnological applications.
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