The mixotrophic orchids paradox: versatile energy sources but constrained metabolism
The so-called mixotrophic orchids combine photosynthesis with carbon recovery from mycorrhizal fungi. Compared to autotrophy and mycoheterotrophy, mixotrophy allows greater adjustment of the carbon source in response to the environment. Omics studies have opened a way to understand the underlying physiological mechanisms of such nutritional strategies. We investigated the leaves and roots of nine orchid species collected from multiple sites, using a multi-omics analysis. By comparing gene and metabolite repertoires, we detected an extensive photosynthetic-related gene loss in achlorophyllous species, but no metabolite specific to the nutrition mode. Our transcriptome analysis highlighted specifically regulated biological pathways depending on the organ, and emphasized the similarities between the two chlorophyllous nutrition modes. Because of inconsistent gene expression regulation between species within a nutrition mode, we hypothesized the existence of many strategies for autotrophy and mixotrophy. Our analyses revealed subtle differences in expression between autotrophs and mixotrophs, particularly in photophosphorylation, the Calvin cycle and solute transport genes. Finally, we demonstrated the existence of distinct metabolite correlation profiles between nutrition modes. In mixotrophs, we observed a higher proportion of correlations, indicating greater dependency between metabolites and showing a paradox: their plasticity is not in agreement with their constrained metabolism.