The biosynthesis and impacts of cytokinins on growth of the oyster mushroom, <i>Pleurotus ostreatus</i>

While a lot is known about cytokinins (CKs) and their actions at the molecular and cellular levels in plants, much less is known about the function of CKs in other kingdoms such as fungi. CKs have been detected in a wide range of fungal species where they play roles ranging from enhancing the virulence of phytopathogens to fortifying plant growth when secreted from fungal symbionts. However, the role of CKs where they concern fungal physiology, apart from plant associations, remains largely uncharacterized. Profiling by UHPLC-HRMS/MS (ultrahigh-performance liquid chromatography-high-resolution tandem mass spectrometry) revealed that <i>Pleurotus ostreatus</i> (oyster mushroom) produces CKs in vitro in both liquid and solid cultures. During fungal growth, CK profiling patterns were consistent with previous suggestions that tRNA degradation products might play a role in the physiological development of fungi. It confirms that those products are CKs that act as fungal growth regulators. Moreover, <i>P. ostreatus</i> was shown to respond to exogenous applications of aromatic and isoprenoid CKs, and their effects were dependent on the dose and CK type in a biphasic manner consistent with hormone action. <i>N</i>⁶-benzyladenine (BAP), kinetin (KIN), <i>N</i>⁶-isopentenyladenine (iP), and <i>trans</i>-zeatin (tZ) bioassays all revealed hormesis-type responses. Accordingly, at low doses, mycelium colony diameter, biomass accumulation, and changes in morphology were stimulated, whereas at high doses only inhibitory effects were observed. Thus, CKs may act as "mycohormones" and consequently have potential for applications in fungal agriculture and medicinal compound production.