Genome-Wide Identification and Biotic Stress Responses of TLP Gene Family in Citrus sinensis

Thaumatin-like proteins (TLPs), a subfamily of pathogenesis-related (PR) proteins, play a vital role in plant defense against pathogens. In this study, 23 <i>CsTLP</i> genes were identified in the <i>Citrus sinensis</i> genome. These genes encode proteins ranging from 203 to 512 amino acids, with molecular weights between 21.88 and 53.75 kDa, classifying them as small molecular weight proteins. The <i>CsTLP</i> genes are unevenly distributed across eight chromosomes, with chromosome 3 containing the highest number (6 genes). Subcellular localization predictions indicate that most <i>CsTLPs</i> are located in the extracellular space. Phylogenetic analysis with <i>Arabidopsis thaliana TLPs</i> classified the <i>CsTLPs</i> into 10 clades, with clade 5 being the largest. Three segmentally duplicated gene pairs were identified, suggesting a mechanism for the expansion of this gene family. Expression profiling revealed tissue-specific patterns, with the highest expression levels observed in roots and leaves. Under biotic stress, qRT-PCR analysis of 12 selected <i>CsTLPs</i> demonstrated pathogen-specific responses: <i>CsTLP9</i> and <i>CsTLP22</i> were strongly upregulated during Huanglongbing (HLB, bacterial) infection, by 21.70-fold and 9.47-fold, respectively. Multiple genes, including <i>CsTLP5</i>/<i>13</i>/<i>18</i>/<i>21</i>/<i>23</i>, exhibited over 10-fold upregulation following Citrus Anthracnose (CA, fungal) infection; however, most genes showed only weak responses to Citrus tristeza virus (CTV, viral). These findings underscore the regulatory significance of <i>CsTLPs</i> in pathogen responses and provide an important theoretical foundation for enhancing molecular disease-resistance breeding in <i>Citrus sinensis</i>.