Identification and Analysis of Aluminum-Activated Malate Transporter Gene Family Reveals Functional Diversification in Orchidaceae and the Expression Patterns of Dendrobium catenatum Aluminum-Activated Malate Transporters

Aluminum-activated malate transporter (<i>ALMT</i>) genes play an important role in aluminum ion (Al³⁺) tolerance, fruit acidity, and stomatal movement. Although decades of research have been carried out in many plants, there is little knowledge about the roles of <i>ALMT</i> in Orchidaceae. In this study, 34 <i>ALMT</i> genes were identified in the genomes of four orchid species. Specifically, ten <i>ALMT</i> genes were found in <i>Dendrobium chrysotoxum</i> and <i>D. catenatum</i>, and seven were found in <i>Apostasia shenzhenica</i> and <i>Phalaenopsis equestris</i>. These <i>ALMT</i> genes were further categorized into four clades (clades 1-4) based on phylogenetic relationships. Sequence alignment and conserved motif analysis revealed that most orchid <i>ALMT</i> proteins contain conserved regions (TM1, GABA binding motif, and WEP motif). We also discovered a unique motif (19) belonging to clade 1, which can serve as a specifically identified characteristic. Comparison with the gene structure of <i>AtALMT</i> genes (<i>Arabidopsis thaliana</i>) showed that the gene structure of <i>ALMT</i> was conserved across species, but the introns were longer in orchids. The promoters of orchid <i>ALMT</i> genes contain many light-responsive and hormone-responsive elements, suggesting that their expression may be regulated by light and phytohormones. Chromosomal localization and collinear analysis of <i>D. chrysotoxum</i> indicated that tandem duplication (TD) is the main reason for the difference in the number of <i>ALMT</i> genes in these orchids. <i>D. catenatum</i> was chosen for the RT-qPCR experiment, and the results showed that the <i>DcaALMT</i> gene expression pattern varied in different tissues. The expression of <i>DcaALMT1-9</i> was significantly changed after ABA treatment. Combining the circadian CO₂ uptake rate, titratable total acid, and RT-qPCR data analysis, most <i>DcaALMT</i> genes were highly expressed at night and around dawn. The result revealed that <i>DcaALMT</i> genes might be involved in photosynthate accumulation. The above study provides more comprehensive information for the <i>ALMT</i> gene family in Orchidaceae and a basis for subsequent functional analysis.