High-resolution metabolite imaging: luteolinidin accumulates at the host-cyanobiont interface during cold-acclimation inAzollasymbioses

Abstract

AbstractAquatic ferns of the genusAzolla(Azolla) form highly productive symbioses with filamentous cyanobacteria fixing N2in their leaf cavities,Nostoc azollae. Stressed symbioses characteristically turn red due to 3-deoxyanthocyanin (DA) accumulation, rare in angiosperms and of unknown function. To reveal DA functions upon cold acclimation and recovery, we integrated laser-desorption-ionization mass-spectrometry-imaging (LDI-MSI), a newA. filiculoidesgenome-assembly and annotation, and dual RNA-sequencing into phenotypic analyses of the symbioses.Azolla sp.Anzali recovered even when cold-induced DA-accumulation was inhibited by abscisic acid. Cyanobacterial filaments generally disappeared upon cold acclimation, andN. azollaetranscript profiles were unlike those of resting stages formed in cold-resistant sporocarps, yet filaments re-appeared in leaf cavities of newly formed green fronds upon cold-recovery.The high transcript accumulation upon cold acclimation ofAfDFR1encoding a flavanone 4-reductase activein vitrosuggested that the enzyme of the first step in the DA-pathway may regulate accumulation of DAs in different tissues. However, LDI-MSI highlighted the necessity to describe metabolite accumulation beyond class assignments as individual DA and caffeoylquinic acid metabolites accumulated differentially. For example, luteolinidin accumulated in epithelial cells, including those lining the leaf cavity, supporting a role for the former in the symbiotic interaction during cold acclimation.Summary statementDuring cold acclimation inAzollasymbioses, individual compounds from the same phenolic class accumulated in different host tissues: luteolinidin associated with biotic interactions at the symbiosis interface whilst apigenidin with photooxidative stress mitigation in the mesophyll.