Arsenic Stress Resistance in the Endophytic Fungus Cladosporium cladosporioides: Physiological and Transcriptomic Insights into Heavy Metal Detoxification

This study aims to evaluate the tolerance of an endophytic fungus isolated from the fibrous roots of <i>Gentiana yunnanensis</i> Franch. to arsenic (As) and elucidate the underlying physiological and molecular mechanisms. The filamentous fungus is identified as <i>Cladosporium cladosporioides</i> based on morphological characteristics and phylogenetic tree analysis, belonging to the family Moniliaceae and Phyla Hyphomycetes. The tolerance of <i>C. cladosporioides</i> to As(V) was assessed by measuring its biomass under varying concentrations of As(V). The fungus exhibited remarkable As(V) tolerance, with an EC₅₀ value of 2051.94 mg/L, and accumulated high concentrations of As in its mycelium. Subcellular distribution analysis revealed that As was predominantly localized in the cell wall fraction, with levels 4.06 times higher than those in the non-cell wall fraction. Notably, the concentrations of total organic As and As(III) in the mycelium were 852.75 μg/g and 24.94 μg/g, respectively, with conversion ratios of 76.64% and 2.24%. The organic As levels significantly surpassed both As(V) and As(III) concentrations in all cellular fractions (cell wall and non-cell wall components), demonstrating particularly efficient As transformation in <i>C. cladosporioides</i>. Under As(V) stress, the membrane antioxidant system, including superoxide dismutase (SOD), metallothionein (MT), glutathione (GSH), and melanin, was activated and significantly enhanced to mitigate oxidative damage. Transcriptomic analysis identified 4771 differentially expressed genes (DEGs; 2527 upregulated), including highly expressed As-responsive genes (<i>CcArsH_1</i>, <i>CcARR_1</i>, <i>CcARR_</i>3, <i>CcGST_1</i>, and <i>CcGST</i>_<i>3</i>). Strong correlations emerged between As speciation (total/organic/As(V)/As(III)), antioxidant levels, and DEG expression patterns. Taken together, these findings demonstrate that <i>C. cladosporioides</i> employs a multi-faceted As detoxification strategy involving subcellular distribution and reductive transformation (As(V) to As(III)/organic As), antioxidant system enhancement, transcriptomic adaptations, and integrated defense strategy. This work highlights <i>C. cladosporioides</i> potential for As bioremediation and elucidates As accumulation mechanisms in <i>G. yunnanensis</i>.