Genome-Wide Identification and Expression Analyses of the Thaumatin-Like Protein Gene Family in <i>Tetragonia tetragonoides</i> (Pall.) Kuntze Reveal Their Functions in Abiotic Stress Responses

Thaumatin-like proteins (TLPs), including osmotins, are multifunctional proteins related to plant biotic and abiotic stress responses. <i>TLP</i>s are often present as large multigene families. <i>Tetragonia tetragonoides</i> (Pall.) Kuntze (Aizoaceae, 2<i>n</i> = 2x = 32), a vegetable used in both food and medicine, is a halophyte that is widely distributed in the coastal areas of the tropics and subtropics. Saline–alkaline soils and drought are two major abiotic stress factors significantly affecting the distribution of tropical coastal plants. The expression of stress resistance genes would help to alleviate the cellular damage caused by abiotic stress factors such as high temperature, salinity–alkalinity, and drought. This study aimed to better understand the functions of <i>TLP</i>s in the natural ecological adaptability of <i>T. tetragonoides</i> to harsh habitats. In the present study, we used bioinformatics approaches to identify 37 <i>TtTLP</i> genes as gene family members in the <i>T. tetragonoides</i> genome, with the purpose of understanding their roles in different developmental processes and the adaptation to harsh growth conditions in tropical coral regions. All of the <i>TtTLP</i>s were irregularly distributed across 32 chromosomes, and these gene family members were examined for conserved motifs of their coding proteins and gene structure. Expression analysis based on RNA sequencing and subsequent qRT-PCR showed that the transcripts of some <i>TtTLP</i>s were decreased or accumulated with tissue specificity, and under environmental stress challenges, multiple <i>TtTLP</i>s exhibited changeable expression patterns at short (2 h), long (48 h), or both stages. The expression pattern changes in TtTLPs provided a more comprehensive overview of this gene family being involved in multiple abiotic stress responses. Furthermore, several <i>TtTLP</i> genes were cloned and functionally identified using the yeast expression system. These findings not only increase our understanding of the role that <i>TLP</i>s play in mediating halophyte adaptation to extreme environments but also improve our knowledge of plant TLP evolution. This study also provides a basis and reference for future research on the roles of plant <i>TLP</i>s in stress tolerance and ecological environment suitability.