Do compounds involved in plant resistance to fungal attack facilitate germination in the fully mycoheterotrophic orchid Cyrtosia septentrionalis?

• Published in: Symbiosis (Philadelphia, Pa.) Vol. 85; no. 1; pp. 47 - 56
• Main Authors: Umata, Hidetaka, Gale, Stephan W., Yamada, Masayoshi
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.09.2021; Springer Nature B.V
• Subjects: Atmosphere; Biomedical and Life Sciences; Carbon dioxide; Developmental Biology; Ecology; Evolutionary Biology; Germfree; Gibberellic acid; Hydrogen peroxide; Kinetin; Life Sciences; Microbiology; Plant resistance; Plant Sciences; Potassium; Salicylic acid; Seed germination; Seeds; Thermoinhibition; Phytohormone; Mycodependency; Defence response; Mycoheterotrophic germination
• ISSN: 0334-5114; 1878-7665
• DOI: 10.1007/s13199-021-00788-y

Plants produce compounds to resist fungal attack. However, most orchids require fungal colonisation for germination, and fully mycoheterotrophic (MH) orchids are dependent upon fungi for their entire nutritional needs. To elucidate the manner in which MH orchids interact with and accommodate a mycorrhizal partner, we examined the effect of compounds involved in pathogenic resistance on seed germination in Cyrtosia septentrionalis , an MH orchid for which a temperature of 30 °C and an atmosphere comprising reduced O 2 (5%) and elevated CO 2 (8%) have been shown to be optimal for axenic germination. Seeds were either co-cultured with a Physisporinus mycobiont in sealed containers or monocultured on nutrient-free water agar (WA) supplemented with various compounds under micro-aerophilic conditions (6–12% O 2 , 5–8% CO 2 ) at 19, 23 or 30 °C. Co-cultured seeds germinated and underwent onward development at all temperatures. In contrast, monocultured seeds failed to germinate on WA alone except at 30 °C, but germination was initiated, as indicated by seed coat rupture, at 19 °C when K + , kinetin and NAA were added. Ca 2+ and H 2 O 2 increased germination initiation, whereas salicylic acid and gibberellic acid inhibited it. Asymbiotic initiation also occurred at 30 °C in sealed vessels with a more-or-less regular atmosphere when K + and kinetin were added. We hypothesise that germination in C. septentrionalis is triggered by the micro-aerophilic conditions established as a result of its mycobiont’s wood-rotting activity, with certain compounds secreted as part of the early defence response having a potentiating effect. However, subsequent mycobiont colonisation is essential for full germination and onward development to ensue.