Excessive Oxalic Acid Secreted by Sparassis latifolia Inhibits the Growth of Mycelia during Its Saprophytic Process

Abstract

Sparassis latifolia is an edible and medicinal mushroom in Asia commercially cultivated on substrates containing pine sawdust. Its slow mycelial growth rate greatly increases the cultivation cycle. In this study, we mainly studied the role of oxalic acid (OA) secreted by S. latifolia in its saprophytic process. Our results show that crystals observed on the mycelial surface contained calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) according to X-ray diffraction (XRD). Vegetative mycelia secreted large amounts of OA during extended culture periods. However, high concentrations of OA decreased the mycelial growth rate significantly. Moreover, the degradation of lignocellulose was significantly inhibited under high concentrations of OA. These changes could be attributed to the significantly decreased activities of lignocellulose-degrading enzymes. In conclusion, by establishing a link between OA secretion by the mycelium and the slow growth rate of its saprophytic process, this work provides fundamental information for shortening the cultivation cycle of S. latifolia.

Transcriptome Analysis Revealed the Mechanism of Inhibition of Saprophytic Growth of Sparassis latifolia by Excessive Oxalic Acid

“…The degradation ability of lignocellulose was directly related to wood degradation by the mycelium, and will directly affect the growth rate [ 29 ]. Furthermore, our previous study also demonstrated that the activities of the lignocellulose-degrading enzyme of S. latifolia significantly reduced under high oxalic acid concentration [ 5 ]. Therefore, the low degradation efficiency of S. latifolia to lignocellulose under high oxalic acid concentration will inevitably reduce its growth rate.…”

“…For the pure culture of S. latifolia , a mycelial disc (diameter: 5 mm) was cultured on the center of potato dextrose peptone agar (PDPA) at 24 °C [ 15 ]. To grow mycelia, 10 mycelial discs (5 mm) of S. latifolia were inoculated into pine sawdust medium (PSM, 76% pine sawdust, 18% bran, 2% corn flour, 1.5% sucrose, 1.5% gypsum, and 1% calcium superphosphate) in a bag (17 × 35 cm) and incubated at 24 °C [ 5 ]. To identify the expression profiles of target genes affected by excessive oxalic acid, the mycelia were collected from PSM (CK), PSM with 10 mM oxalic acid (OA), and 4 mM 3,3-difluorooxaloacetate (IOA, an inhibitor of oxalic acid synthesis) after 20 days of incubation at 24 °C, mixed thoroughly, frozen immediately in liquid nitrogen, and stored at −80 °C for RNA extraction.…”

“…Unlike most saprophytic edible fungi, S. latifolia possesses two different nutritional modes: saprophytic and parasitic [ 4 ]. In previous studies, we found that its mycelium secretes a large amount of oxalic acid during its growth, which can inhibit mycelial growth [ 5 ]. Nevertheless, the mechanism underlying the growth rate of S. latifolia mycelia to oxalic acid during the process of saprophytic growth has not been deeply elucidated.…”