Expression Profiling of the Aluminum-Activated Malate Transporter (ALMT) Gene Family in Pumpkin in Response to Aluminum Stress and Exogenous Polyamines

Aluminum (Al) toxicity is a key constraint on plant growth in acidic soils. To counteract Al stress, plants secrete organic acids such as malate from their roots to chelate Al³⁺, a process facilitated by Al-activated malate transporters (ALMTs). In this study, we identified 15 <i>ALMT</i> genes in the pumpkin (Cucurbita moschata) genome, which were phylogenetically classified into four subclades. Expression analysis revealed that several ALMTs, including <i>CmaALMT5</i>, <i>CmaALMT6</i>, and <i>CmaALMT12</i>, were upregulated in response to increasing Al concentrations. Exogenous application of polyamines (spermine, spermidine, and putrescine) alleviated Al-induced root growth inhibition, correlating with enhanced malate secretion. Notably, each polyamine differentially regulated specific ALMT genes: spermidine elevated the expression of <i>CmaALMT1</i>, <i>CmaALMT6</i>, <i>CmaALMT13</i>, and <i>CmaALMT15</i>; spermine induced <i>CmaALMT1</i>, <i>CmaALMT2</i>, <i>CmaALMT3</i>, <i>CmaALMT11</i>, and <i>CmaALMT14</i>; while putrescine significantly upregulated <i>CmaALMT1</i>, <i>CmaALMT3</i>, and <i>CmaALMT4</i>. These results suggest that polyamines may enhance Al tolerance in pumpkin through gene-specific transcriptional regulation of the ALMT family and promotion of root malate secretion, though further evidence is required.