AUTHOR=Moon Hyeran , Jeong A-Ram , Kwon Oh-Kyu , Park Chang-Jin 

TITLE=Oryza-Specific Orphan Protein Triggers Enhanced Resistance to Xanthomonas oryzae pv. oryzae in Rice

JOURNAL=Frontiers in Plant Science

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.859375

DOI=10.3389/fpls.2022.859375

ISSN=1664-462X

ABSTRACT=<p>All genomes carry lineage-specific orphan genes lacking homology in their closely related species. Identification and functional study of the orphan genes is fundamentally important for understanding lineage-specific adaptations including acquirement of resistance to pathogens. However, most orphan genes are of unknown function due to the difficulties in studying them using helpful comparative genomics. Here, we present a defense-related <italic>Oryza</italic>-specific orphan gene, <italic>Xio1</italic>, specifically induced by the bacterial pathogen <italic>Xanthomonas oryzae</italic> pv. <italic>oryzae</italic> (<italic>Xoo</italic>) in an immune receptor XA21-dependent manner. Salicylic acid (SA) and ethephon (ET) also induced its expression, but methyl jasmonic acid (MeJA) reduced its basal expression. C-terminal green fluorescent protein (GFP) tagged Xio1 (Xio1-GFP) was visualized in the nucleus and the cytosol after polyethylene glycol (PEG)-mediated transformation in rice protoplasts and Agrobacterium-mediated infiltration in tobacco leaves. Transgenic rice plants overexpressing Xio1-GFP showed significantly enhanced resistance to <italic>Xoo</italic> with reduced lesion lengths and bacterial growth, in company with constitutive expression of defense-related genes. However, all of the transgenic plants displayed severe growth retardation and premature death. Reactive oxygen species (ROS) was significantly produced in rice protoplasts constitutively expressing Xio1-GFP. Overexpression of Xio1-GFP in non-<italic>Oryza</italic> plant species, <italic>Arabidopsis thaliana</italic>, failed to induce growth retardation and enhanced resistance to <italic>Pseudomonas syringae</italic> pv. <italic>tomato</italic> (<italic>Pst</italic>) DC3000. Our results suggest that the defense-related orphan gene <italic>Xio1</italic> plays an important role in distinctive mechanisms evolved within the <italic>Oryza</italic> and provides a new source of <italic>Oryza</italic>-specific genes for crop-breeding programs.</p>