Diel dynamics of multi-omics in elkhorn fern provide new insights into weak CAM photosynthesis

Crassulacean acid metabolism(CAM)has high water-use efficiency(WUE)and is widely recognized to have evolved from C3 photosynthesis.Different plant lineages have convergently evolved CAM,but the molecular mechanism that underlies C3-to-CAM evolution remains to be clarified.Platycerium bifurcatum(elkhorn fern)provides an opportunity to study the molecular changes underlying the transition from C3 to CAM photosynthesis because both modes of photosynthesis occur in this species,with sporotrophophyll leaves(SLs)and cover leaves(CLs)performing C3 and weak CAM photosynthesis,respectively.Here,we report that the physiological and biochemical attributes of CAM in weak CAM-performing CLs differed from those in strong CAM species.We investigated the diel dynamics of the metabolome,proteome,and transcriptome in these dimorphic leaves within the same genetic background and under identical environmental conditions.We found that multi-omic diel dynamics in P.bifurcatum exhibit both tissue and diel effects.Our analysis revealed temporal rewiring of biochemistry relevant to the energy-producing pathway(TCA cycle),CAM pathway,and stomatal movement in CLs compared with SLs.We also confirmed that PHOSPHOENOLPYRUVATE CARBOXYLASE KINASE(PPCK)exhibits convergence in gene expression among highly divergent CAM lineages.Gene regulatory network analysis identified candidate transcription factors regulating the CAM pathway and stomatal movement.Taken together,our results provide new insights into weak CAM photosynthesis and new avenues for CAM bioengineering.

A Putative Chloroplast-Localized Ca2＋/H＋ Antiporter CCHA1 Is Involved in Calcium and pH Homeostasis and Required for PSII Function in Arabidopsis

Calcium is important for chloroplast, not only in its photosynthetic but also nonphotosynthetic functions. Mul- tiple Ca2＋/H＋ transporters and channels have been described and studied in the plasma membrane and organ- elle membranes of plant cells; however, the molecular identity and physiological roles of chloroplast Ca2＋/H＋ antiporters have remained unknown. Here we report the identification and characterization of a member of the UPFO016 family, CCHA1 （a chloroplast-localized potential Ca2＋/H＋ antiporter）, in Arabidopsis thaliana. We observed that the ccha I mutant plants developed pale green leaves and showed severely stunted growth along with impaired photosystem II （PSII） function. CCHA1 localizes to the chloroplasts, and the levels of the PSII core subunits and the oxygen-evolving complex were significantly decreased in the ccha I mutants compared with the wild type. In high Ca2＋ concentrations, Arabidopsis CCHA1 partially rescued the growth defect of yeast gdtl3 null mutant, which is defective in a Ca2＋/H＋ antiporter. The cchal mutant plants also showed significant sensitivity to high concentrations of CaCI2 and MnCI2, as well as variation in pH. Taken these results together, we propose that CCHA 1 might encode a putative chloroplast-localized Ca2＋/H＋ antiporter with critical functions in the regulation of PSII and in chloroplast Ca2＋ and pH homeostasis in Arabidopsis.