Utilization of exogenous saccharides by protocorms of two terrestrial orchids

Abstract

Orchid protocorms are completely mycoheterotrophic structures. Although saccharides are proposed as the main energy and carbon (C) sources provided by fungi, there is only limited knowledge on their effects. For the first time, utilization of a wide range of saccharides by in vitro axenic protocorms of two terrestrial orchids from two subfamilies, Ophrys iricolor subsp. lojaconoi and Oeceoclades, was tested. Protocorm size and, in the first of these also rhizoid length and soluble saccharide contents, were analysed. The endogenous saccharide spectra reflected the supplied saccharides and their metabolism. In both species, sucrose supported protocorm growth best. Surprisingly, fructose inhibited O. iricolor subsp. lojaconoi protocorm growth while O. decaryana ones grew well on it. Interestingly, mannitol abundant in mycorrhizal fungi was not utilized while sorbitol not found in fungi was usable. Galactose was toxic at pre-germination stage. Protocorm rhizoid length correlated with protocorm size but revealed several signalling effects of some saccharides. In conclusion, the orchid’s ability to utilize various saccharides reflects more likely species life strategy rather than phylogenetic relations or saccharide abundance in mycorrhizal fungi.

Integrative Study Supports the Role of Trehalose in Carbon Transfer From Fungi to Mycotrophic Orchid

“…Other soluble carbohydrates, however, differ in their effects. Galactose is toxic for D. majalis embryos, which corresponds well with the effects observed in other orchid species ( Quednow, 1930 ; Wynd, 1933 ; Ernst, 1967 ; Ernst et al, 1971 ; Ponert and Lipavská, 2017 ). Surprisingly, the seeds cultivated on medium with galactose contained large amount of starch.…”

“…Mannitol is probably not utilized by D. majalis . An inability to grow on mannitol has been observed in several other orchid species ( Ernst, 1967 ; Smith, 1973 ; Purves and Hadley, 1976 ; Van Waes, 1984 ; Rasmussen, 1995 ; Johnson and Kane, 2013 ; Ponert and Lipavská, 2017 ) but Galeola septentrionalis ( Nakamura, 1982 ) and Phalaenopsis hybr. ( Ernst, 1967 ) are able to utilize mannitol to some extent.…”

“…Similarly, Ophrys iricolor subsp. lojaconoi and Oeceoclades decaryana grew only poorly on maltose ( Ponert and Lipavská, 2017 ). Contrastingly, other orchid species tested (all from the subfamily Epidendroideae) were able to readily grow on maltose ( Ernst, 1967 ; Ernst et al, 1971 ; Rahman et al, 2005 ; Sopalun et al, 2010 ).…”

“…Asymbiotic cultivations were performed on medium SMS without inositol ( Ponert et al, 2013 ; Ponert and Lipavská, 2017 ) consisting exclusively of defined substances and thus enabling to control precisely soluble carbohydrate supply in experiments. D-carbohydrates were added prior autoclaving (100mM monosaccharides, 50mM disaccharides, 33mM raffinose).…”

“…Protocorms from each Petri dish were analyzed separately. Soluble carbohydrates were extracted in aqueous solution as described ( Ponert and Lipavská, 2017 ) and analyzed using an HPLC system with refractometric detection (column 250 × 8mm filled with IEX Pb form 8μm; Watrex, Czech Republic) following the protocol of Vojtíšková et al (2006) .…”