HubHLH36 promotes oxalate degradation through HuAAE3 to enhance salt tolerance in pitaya (Hylocereus polyrhizus)

Pitaya ( Hylocereus polyrhizus ) is one of the subtropical and tropical fruits, has gained global cultivation interest due to its nutritional, medicinal, and economic value. To improve the adaptability of pitaya on islands or soil salinization, it is of great significance to explore the salt tolerance mechanisms of pitaya. While oxalate has been conventionally characterized as an antinutrient metabolite, emerging evidence underscores its dual role as a stress signaling molecule through homeostatic regulation. Here, we demonstrate that dynamic control of oxalate catabolism is essential for pitaya's salt adaptation. Transcriptomic profiling revealed coordinated upregulation of the oxalate-degrading gene HuAAE 3 under salt stress, accompanied by enhanced oxalate accumulation. Notably, the transcription factor HubHLH36 emerged as a central regulator responding to both ionic stress and exogenous oxalate. Molecular validation through yeast one-hybrid and dual-luciferase assays unveiled confirmed that HubHLH36 directly activates HuAAE3 transcription by binding to its promoter. Transient overexpression of HuAAE3 or HubHLH36 in pitaya plants significantly reduced oxalate levels, enhanced salt/oxalate tolerance, and alleviated reactive oxygen species (ROS) accumulation. Conversely, virus-induced gene silencing (VIGS) of these genes elevated oxalate content, compromised stress tolerance, and exacerbated ROS production. Heterologous expression in Arabidopsis further validated the conserved function of HubHLH36-HuAAE3 module in conferring salt/oxalate tolerance with ROS mitigation. Our findings establish a novel transcriptional regulatory module where HubHLH36 modulates oxalate homeostasis via HuAAE3 activation, thereby mitigating oxidative stress under salt stress. These mechanistic insights provide genetic targets and theoretical frameworks for developing salt-tolerant pitaya cultivars, offering promising solutions for sustainable agriculture in saline environments.