Keywords

CDPK, CFK, Calcium signaling, Calmodulin, Phylogeny, Plant evolution, Protein phosphorylation, Symbiosis, Land colonization

Reference

DOI: 10.1038/s41598-017-03367-8

Abstract

The Calmodulin-Fused Kinase (CFK) gene family is identified as the major converter of calcium signals (CS) into protein phosphorylation responses (PPRs) in plants, functioning as a universal second messenger system.
By constructing a deep phylogenetic tree, the study reveals that CDPKs, CRKs, CCaMKs, PPCKs, and PEPRKs form a monophyletic group (CFK), likely originating from de novo fusion of a kinase gene and a calmodulin gene.
CFKs are structurally and functionally diversified into four subfamilies, evolving distinct calcium sensitivities, regulatory motifs, and membrane-targeting sequences.
The CFK system has been central to all six major plant adaptation events, including land colonization, plant-microbe symbiosis, and stress signaling, highlighting CFKs as pivotal calcium signal decoders and integrators in plant evolution.

Notes

1. Experimental Approach and Phylogenetic Analysis

  • Comprehensive tree built using all known Ca2+-activated kinases (CDPK-SnRK, CAMK, MAPK as outgroup).
  • Revealed a well-supported monophyly of CDPK, CRK, CCaMK, PPCK, and PEPRK families — now proposed as CFK family.
  • Absence of similar genes in prokaryotes suggests eukaryotic-specific evolution via fusion of kinase and calmodulin genes.
  • Domain structure, intron patterns, and sequence alignments were analyzed to confirm deep homology.

First time showing all plant calcium signal converters (except CBL-CIPK) share a common CFK ancestor!

2. Key Phylogenetic and Functional Findings

A. Origin and Expansion of CFKs

  • Absent in Glaucophytes, minimally present in red algae (1 gene in Porphyridium cruentum), ubiquitous in Chlorophyta, Charophyta, and land plants.
  • Emerged ~1500 million years ago in red algae.

B. Structural Diversification of CFKs

  • Subfamily B6 lost C-terminal CaM-LD & AID, becoming calcium-independent (PPCK & PEPRK).
  • Membrane targeting: N-terminal myristoylation and palmitoylation in marine Chlorophyta and PEST motifs/acetylation in freshwater species.
  • Some CFKs contain C2 domains for membrane localization.

Functional diversification linked to different calcium signaling needs and environmental adaptation.

C. Role in Water-to-Land Transition

  • In Chlamydomonas reinhardtii, subfamily B3/B4 CFKs regulate flagella biogenesis and nutrient uptake.
  • Streptophyte-specific EF-hand-less B5 CFKs decode low-frequency signals for symbiosis (arbuscular mycorrhizal fungi, rhizobia).
  • C3 group CFKs acquire membrane-targeting motifs and specialize in high-frequency stress signaling (40 s/cycle) for drought, salt, pathogen response.

D. Role in Seed Plant Evolution

  • Gymnosperm-originated C3 cluster: myristoylated, palmitoylated, but lacking EF hands — involved in seed maturation (pattern conserved from Ginkgo to Arabidopsis).

CFKs central to terrestrial life adaptations: symbiosis, stress signaling, seed development.

3. Functional and Evolutionary Implications

  • CFKs dominate Ca2+-signal-to-protein-phosphorylation pathways in plants, surpassing SnRK3 in many contexts.
  • Their evolution mirrors major plant adaptations:
    • Freshwater to land: New symbiosis and environmental resilience mechanisms.
    • Plant-microbe co-evolution: CFKs key to symbiotic signaling.
    • Seed evolution: Specialized CFKs regulating maturation.
  • Structural modularity (loss/gain of EF hands, lipidation motifs) enables precise tuning of calcium sensitivity and localization — highly adaptable signaling nodes.


Figure1: Some pic from the paper_1 (Click to enlarge)


Figure2: Some pic from the paper_2 (Click to enlarge)


Figure3: Some pic from the paper_3 (Click to enlarge)


Figure4: Some pic from the paper_4 (Click to enlarge)

4. RD’s Takeaways and Reflections

  • Absolutely love the framing of CFKs as “universal calcium-to-phosphorylation converters” in plants!
  • The evolutionary fusion of CaM + kinase to create a new, self-contained signaling module is conceptually beautiful.
  • Raises many questions:
    • Could artificial fusions (e.g., synthetic CFKs) be engineered to respond to novel calcium signals?
    • What regulates the calcium independence of B6 members — loss of EF hands vs. compensatory domains?
    • How do plants leverage EF-hand-less CFKs for symbiosis signaling that animal systems don’t have?
  • Love the evolutionary storytelling: from flagella to land colonization, to seeds — calcium signaling rewired over time via CFKs.
  • The focus on lipid modifications (myristoylation/palmitoylation) resonates with emerging literature on CDPK membrane targeting — need to explore their interplay.

If CDPKs are “plant CaMKs”, then CFKs are the “evolutionary super-family” of calcium-based signaling devices in plants — deeply ancient, deeply functional.

Take-Home Messages

  • CFK family (CDPK, CRK, CCaMK, PPCK, PEPRK) is the central plant system converting calcium signals into phosphorylation responses.
  • Emerging via de novo fusion of kinase and calmodulin genes, CFKs are plant-specific signaling innovations.
  • Diversified in calcium sensitivity, structural motifs, and regulatory domains, CFKs handle diverse signaling functions, from symbiosis to abiotic stress to development.
  • Core to major plant evolutionary events: land colonization, stress adaptation, seed development.
  • CFKs = the molecular engine converting environmental calcium signals into coordinated protein phosphorylation — shaping plant evolution itself.

RD’s Ongoing Series: “Signal Decoding Proteins”

Final Reflection

“The Scientific Reports paper redefines our understanding of plant calcium signaling — not just isolated CDPKs but an entire CFK family as evolutionary calcium-phosphorylation integrators. This is a landmark in decoding how plants ’think’ in calcium.”