Mechanisms of Aluminum Toxicity Impacting Root Growth in Shatian Pomelo

Aluminum (Al) toxicity in acidic soils poses significant challenges to crop growth and development. However, the response mechanism of Shatian pomelo (Citrus maxima 'Shatian Yu') roots to Al toxicity remains poorly understood. This study employed root phenotype analysis, physiological response index measurement, root transcriptome analysis, and quantitative PCR (qPCR) validation to investigate the effects of Al toxicity on Shatian pomelo roots. The findings revealed that Al toxicity inhibited root growth and development, resulting in reduced root biomass, total root length, total root surface area, root volume, average root diameter, and root tip count. Antioxidant enzyme activities (peroxidase, superoxide dismutase, ascorbate peroxidase, and catalase activity) and soluble protein content increased with rising Al toxicity, whereas malondialdehyde content initially increased and then declined. Additionally, Al toxicity stress increased Al (1439.25%) content and decreased boron (B, 50.64%), magnesium (Mg, 42.04%), calcium (Ca, 46.02%), manganese (Mn, 86.75%), and iron (Fe, 69.92%) levels in the roots. RNA sequencing (RNA-seq) analysis identified 3855 differentially expressed genes (DEGs) between 0 mmol/L Al (control) and 4 mmol/L Al (Al toxicity) concentrations, with 1457 genes up-regulated and 2398 down-regulated, indicating a complex molecular regulatory response. The qPCR results further validated these findings. This study elucidates the response mechanisms of Shatian pomelo roots to Al toxicity stress, providing insights into the regulatory pathways involved. The findings offer valuable reference points for breeding Al-resistant Shatian pomelo varieties. The results of this study provide important genetic tools and technical support for the screening and breeding of highly resistant varieties of Shatian pomelo. On the one hand, by detecting the key indexes (such as antioxidant enzyme activity and nutrient absorption capacity) of Shatian pomelo, varieties with excellent anti-Al toxicity characteristics can be selected. On the other hand, the Al-resistant genes identified in this study, such as TFM1 and ALERTFA0, can be used to develop molecular markers, assisted marker breeding, or transgenic breeding to accelerate the breeding process of Al-resistant strains.