Why do mixotrophic plants stay green? A comparison between green and achlorophyllous orchid individuals in situ

Abstract

Abstract. Some forest plants adapt to shade by mixotrophy, i.e., they obtain carbon both from photosynthesis and from their root mycorrhizal fungi. Fully achlorophyllous species using exclusively fungal carbon (the so-called mycoheterotrophic plants) have repeatedly evolved from such mixotrophic ancestors. However, adaptations for this evolutionary transition, and the reasons why it has happened a limited number of times, remain unknown. We investigated this using achlorophyllous variants (i.e., albinos) spontaneously occurring in Cephalanthera damasonium, a mixotrophic orchid. In two populations, we compared albinos with co-occurring green individuals in situ. We investigated vegetative traits, namely, shoot phenology, dormancy, CO 2 and H 2 O leaf exchange, mycorrhizal colonization, degree of mycoheterotrophy (using 13 C abundance as a proxy), and susceptibility to pathogens and herbivores. We monitored seed production (in natural or experimental crosses) and seed germination. Albinos displayed (1) more frequent shoot drying at fruiting, possibly due to stomatal dysfunctions, (2) lower basal metabolism, (3) increased sensitivity to pathogens and herbivores, (4) higher dormancy and maladapted sprouting, and, probably due to the previous differences, (5) fewer seeds, with lower germination capacity. Over the growing season, green shoots shifted from using fungal carbon to an increasingly efficient photosynthesis at time of fruiting, when fungal colonization reached its minimum. Conversely, the lack of photosynthesis in fruiting albinos may contribute to carbon limitation, and to the abovementioned trends. With a 10 3 3 fitness reduction, albinos failed a successful transition to mycoheterotrophy because some traits inherited from their green ancestors are maladaptive. Conversely, mycoheterotrophy requires at least degeneration of leaves and stomata, optimization of the temporal pattern of fungal colonization and shoot sprouting, and new defenses against pathogens and herbivores. Transition to mycoheterotrophy likely requires progressive, joint evolution of these traits, while a sudden loss of photosynthesis leads to unfit plants. We provide explanations for the evolutionary stability of mixotrophic nutrition and for the rarity of emergence of carbon sinks in mycorrhizal networks. More broadly, this may explain what prevents the emergence of fully heterotrophic taxa in the numerous other mixotrophic plant or algal lineages recently described.

Plant family identity distinguishes patterns of carbon and nitrogen stable isotope abundance and nitrogen concentration in mycoheterotrophic plants associated with ectomycorrhizal fungi

“…However, the patterns observed here of 13 C depletion in evergreen sclerophyllous PMH Ericaceae relative to deciduous PMH Orchidaceae do not fit with the general tendency of sclerophyllous plants towards lower stomatal conductance and therefore greater 13 C enrichment (Larcher, 2003). Furthermore, for non-photosynthetic FMH albino individuals of the orchid Cephalanthera damasonium, a significantly higher stomatal conductance and simultaneously higher 13 C enrichment than in PMH individuals has been found (Julou et al, 2005;Roy et al, 2013). Consequently, systematic differences in stomatal conductance are also unlikely to be responsible for the differences in 13 C enrichment found for FMH and PMH plants of the Orchidaceae and Ericaceae.…”

“…We identified 21 publications suitable for our study published between 2003 and 2015 and added one further so far unpublished data set (Table 1). We explicitly excluded from our data set investigations for which the sampling design did not allow calculation of enrichment factors (Trudell et al, 2003) or for which C and N isotope abundance was affected by experimental manipulations [shading and trenching (Hynson et al, 2012); fungicide application (Bellino et al, 2014); defoliation (Gonneau et al, 2014)], by investigation of chlorophyll concentration gradients (Stöckel et al, 2011) or by investigation of different developmental stages (Roy et al, 2013;Gonneau et al, 2014). We did include data from mutant achlorophyllous (albino) orchids that are fully mycoheterotrophic.…”

Mycorrhizal symbioses and the evolution of trophic modes in plants

“…Recent studies have shown that, in partially mycoheterotrophic orchids, carbon derived from mycorrhizal fungi mostly supports young spring shoots and below‐ground organs, whereas carbon originating from photosynthesis contributes most to sexual reproduction (Gonneau et al, ; Lallemand et al, ; Suetsugu, Ohta, & Tayasu, ). These results may explain why albino plants fail to produce similar levels of seeds than green plants, but show the same survival rates (Lallemand et al, ; Roy et al, ). Using fungicides, Bellino et al () were able to eliminate the mycorrhizal fungi associating with the partially mycoheterotrophic orchid Limodorum abortivum without impairing fruit production, supporting the idea that carbon derived from fungi contributes little to sexual reproduction.…”

“…Detailed physiological measurements and long‐term observations in mixed populations of the woodland orchid Cephalanthera damasonium have shown that albinos are in general less fit than their green counterparts. Albinos displayed more frequent shoot drying at fruiting, possibly due to stomatal dysfunctions, had a lower basal metabolism, showed increased sensitivity to pathogens and herbivores, had higher dormancy and showed signs of maladapted sprouting, and produced, probably due to the previous differences, fewer seeds, which in turn had a lower germination capacity (Roy et al, ). When all fitness costs were added, albinos were estimated to have a 10 3 × fitness reduction, compared to green plants, suggesting that a successful transition to full mycoheterotrophy is unlikely to occur from albino plants.…”

Two widespread green Neottia species (Orchidaceae) show mycorrhizal preference for Sebacinales in various habitats and ontogenetic stages

“…Full mycoheterotrophy evolved repeatedly and different adaptations were likely involved in this process (Roy et al . ). Notably, in the orchid family only ca .…”