Evidence for Early Evolution of Sulfated Peptide Signaling in Plant Development

This study demonstrates that the sulfation of PSY peptides by TPST is essential for cell expansion and development in the non-vascular plant *Physcomitrium patens*, revealing that this sulfated peptide signaling mechanism is evolutionarily conserved across land plants.

The Gist

Imagine a plant as a bustling city made of bricks (cells). In animals, these bricks can move around, migrate, and rearrange themselves to build a house or a heart. But in plants, the bricks are glued together in a rigid, semi-hard shell called a cell wall. They can't move. So, how does a plant grow from a tiny seed into a towering tree or a delicate flower?

The answer lies in two things: pushing (expanding the bricks) and multiplying (making more bricks). But for the city to grow in the right shape, the bricks need to talk to each other. They need a communication system to say, "Hey, expand here!" or "Stop dividing, we're done!"

This paper discovers that plants use a very specific type of "text message" to coordinate this growth, and this system is ancient—dating back to the very first plants that crawled out of the water onto land.

The "Sulfated" Text Message

Think of the plant's communication system as a postal service. The messages are small peptides (tiny protein snippets). But for these messages to work, they need a special stamp: a sulfate group attached to a specific letter (Tyrosine) in the message.

• The Stamp Maker: There is a specific enzyme (a biological machine) called TPST that acts as the post office clerk. Its only job is to slap that sulfate stamp onto the message.
• The Message: The most important message in this study is called PSY. Without the sulfate stamp, the PSY message is just a piece of paper with no meaning. The recipient (the cell) ignores it.

The Experiment: Breaking the Post Office

The scientists decided to see what happens if they break the post office. They used a tiny moss called Physcomitrium patens (a model organism that's like the "fruit fly" of the plant world).

They used gene-editing tools (CRISPR) to delete the gene for the TPST enzyme. They created a moss that couldn't stamp its messages.

The Result? The moss city collapsed.

• The "Stunted" City: The mutant moss was tiny, about half the size of normal moss.
• The "Round" City: Instead of growing long, branching filaments (like tree roots or vines), the mutant moss stayed round and clumpy. It couldn't send out its "branches" because it couldn't communicate where to grow.
• The "Frozen" City: When the moss tried to grow its leafy structures (gametophores), it got stuck. It started building them but couldn't finish. The cells stopped dividing and stopped expanding. It was like a construction crew that laid the foundation and then just walked away.
• The "Early Retirement": The mutant moss also got old and brown (senesced) much faster than normal moss.

The "Magic" Rescue

Here is the coolest part. The scientists took the broken moss and gave it a bottle of pre-stamped messages (synthetic PSY peptides) from two sources:

1. From the moss itself (PpPSY).
2. From a flowering plant (Arabidopsis, a cousin of mustard and cabbage).

The Magic Happened:
When they added these pre-stamped messages to the broken moss, the moss suddenly woke up!

• It started growing long branches again.
• It built its leafy structures perfectly.
• It stopped aging early.

This proved that the problem wasn't the moss's ability to receive the message, but its inability to stamp it. Once the stamp was provided externally, the system worked.

The Evolutionary "Aha!" Moment

This is where the story gets truly exciting.

The scientists found that the "stamp maker" enzyme (TPST) in the moss has a specific part (a Histidine amino acid at position 124) that is essential for its job. When they mutated just this one part, the enzyme stopped working, and the moss looked exactly like the broken one.

The Big Discovery:
This specific "Histidine" part is the same one found in animal enzymes that do the exact same job.

• For a long time, scientists thought plants and animals evolved this system independently (convergent evolution), like two inventors coming up with the lightbulb separately.
• But this paper suggests they didn't. The fact that the "screwdriver" (the catalytic residue) is identical in both moss and animals suggests that this communication system is ancient. It likely evolved in a common ancestor of all land plants and animals, or at least very early in the history of life.

The "Universal Remote"

To prove this system is truly universal, they tried a reverse experiment. They took the moss's message (PpPSY) and gave it to Arabidopsis (a flowering plant) and Rice.

The Result: The moss message worked on the flowering plants! It made their roots grow longer.

The Takeaway

Think of the plant world as a giant, ancient network. This paper shows that:

1. Plants need "stamped" text messages to grow and expand their cells.
2. The "stamp maker" is a critical machine that, if broken, stops the plant from growing properly.
3. The language is universal. A message written by a moss 400+ million years ago can still be read and understood by a modern rice plant or a mustard weed.

It's like finding out that the "Hello" used by your great-great-great-grandparents is still the exact same word used by your neighbors today. The way plants talk to build their bodies is a fundamental, ancient code that has survived billions of years of evolution.

Technical Summary

1. Problem Statement

Plant development relies on coordinated cell proliferation and expansion because plant cells are immobile due to their rigid cell walls. While sulfotyrosyl peptide signaling (involving peptides like PSY, PSK, RGF, and CIF) is known to regulate growth in seed plants (angiosperms), the evolutionary origins of this mechanism and its necessity in non-vascular plants remain unclear. Specifically, it is unknown whether:

• Tyrosine sulfation is essential for development in early-diverging land plants like mosses.
• The enzymatic machinery (Tyrosyl Protein Sulfotransferase, TPST) and its catalytic mechanisms are conserved between plants and animals.
• Sulfated peptide signaling pathways are evolutionarily conserved across the plant kingdom (i.e., can a peptide from a moss function in a flowering plant and vice versa?).

2. Methodology

The researchers utilized the moss Physcomitrium patens (formerly Physcomitrella patens) as a model organism due to its haploid genome, ease of genome editing, and distinct growth phases (tip-growing protonemata and diffuse-growing gametophores).

• Genetic Manipulation:
  • CRISPR/Cas9 Knockouts: Generated a null mutant ($\Delta$tpst) by targeting the amino-terminal coding region of the TPST gene (Pp3c16_24250) to create frameshift mutations.
  • Site-Directed Mutagenesis: Used Homology-Directed Repair (HDR) to create specific alanine substitutions at candidate catalytic residues: Arg-81 (5'-PSB motif), Arg-144 (3'-PB motif), and His-124 (putative catalytic base).
• Phenotypic Analysis:
  • Morphometrics: Quantified plant area, circularity (as a proxy for caulonemal filament formation), and gametophore expansion using brightfield and chlorophyll autofluorescence imaging.
  • Live-Cell Imaging: Employed confocal microscopy on microfluidic chambers to track bud development, cell division rates, and cellular expansion in real-time. Lines expressing mEGFP-tubulin and 3XmRuby2-TON1 (plasma membrane marker) were used.
  • Rescue Experiments: Applied synthetic sulfated peptides (PpPSY1, AtPSY1, AtPSK1) to $\Delta$tpst mutants to test for functional rescue.
  • Cross-Species Assays: Tested the effect of synthetic P. patens PSY1 (PpPSY1) on root elongation in Arabidopsis thaliana and rice (Oryza sativa).
• Molecular Analysis: RT-PCR confirmed transcript variants; Sanger sequencing verified mutations.

3. Key Contributions

• Establishment of TPST Necessity in Moss: Demonstrated that TPST-mediated tyrosine sulfation is not just a feature of seed plants but is essential for the development of non-vascular plants.
• Identification of a Conserved Catalytic Residue: Identified Histidine 124 (His-124) in P. patens TPST as the essential catalytic base, challenging the previous hypothesis that plant and animal TPSTs evolved via convergent evolution.
• Demonstration of Evolutionary Conservation: Proved that the PSY signaling pathway is functionally conserved across ~450 million years of evolution, as Arabidopsis PSY1 can rescue moss mutants, and moss PSY1 can promote root growth in Arabidopsis and rice.

4. Key Results

A. Phenotypic Defects in $\Delta$tpst Moss

• Growth Deficits: $\Delta$tpst plants were 48% smaller than wild type (WT) and exhibited a "round" morphology with 47% fewer caulonemal filaments (indicating a failure in the transition from chloronema to caulonema).
• Gametophore Failure: While $\Delta$tpst formed buds at normal rates, they failed to expand into gametophores. Instead, growth aborted after bud formation.
• Cellular Defects:
  • Division: Time-lapse imaging revealed $\Delta$tpst buds underwent significantly fewer cell divisions (3 divisions in 12 hours vs. 12 in WT) and displayed irregular first divisions.
  • Expansion: $\Delta$tpst cells showed negligible expansion (1.08-fold area increase) compared to WT (1.92-fold) during phyllid formation.
• Senescence: $\Delta$tpst plants exhibited early senescence (browning) compared to WT.

B. Enzymatic Mechanism and Catalytic Residues

• His-124 is Essential: The tpst-H124A mutant phenocopied the null $\Delta$tpst mutant (small, round, no gametophores, early senescence), confirming His-124 is the critical catalytic base.
• Partial Function in Other Mutants: tpst-R81A and tpst-R144A mutants (affecting PAPS binding motifs) showed intermediate phenotypes: they were small but could form expanded gametophores and had lower circularity than the null. This suggests residual enzymatic activity, likely due to weak PAPS binding.
• Evolutionary Implication: The conservation of the His-124 catalytic mechanism between P. patens and animal sulfotransferases suggests plant and animal TPSTs likely share a common evolutionary origin rather than evolving via convergent evolution.

C. Cross-Species Peptide Rescue and Conservation

• Rescue of Moss Mutants: Exogenous application of sulfated Arabidopsis PSY1 (AtPSY1) fully rescued the $\Delta$tpst phenotype, restoring protonemal growth, gametophore expansion, and delaying senescence. P. patens PSY1 (PpPSY1) also rescued the phenotype but required a higher concentration, suggesting lower efficacy.
• Specificity: AtPSK1 (a different sulfated peptide) failed to rescue $\Delta$tpst, likely because the P. patens PSK precursor lacks the conserved Asp-Tyr motif required for sulfation.
• Reciprocal Function: Synthetic PpPSY1 promoted root elongation in Arabidopsis (Col-0 and tpst-1 mutants) and rice (Kitaake), confirming that the PSY receptor and signaling pathway are conserved across land plants.

5. Significance

• Evolutionary Insight: This study provides strong evidence that sulfated peptide signaling is an ancient mechanism essential for plant development, predating the divergence of bryophytes and vascular plants.
• Mechanistic Clarity: By identifying His-124 as the conserved catalytic base, the paper refutes the "convergent evolution" hypothesis for plant and animal TPSTs, suggesting instead a shared ancestral origin.
• Developmental Biology: It elucidates that sulfated peptides are required for both tip growth (protonemata) and diffuse growth (gametophores), regulating both cell division rates and cell expansion.
• Biotechnological Potential: The cross-species functionality of PSY peptides suggests potential applications in using conserved peptide hormones to enhance growth in diverse crop species.

In conclusion, the paper establishes that TPST-mediated sulfation of PSY peptides is a fundamental, evolutionarily conserved requirement for plant growth and development, functioning through a conserved catalytic mechanism and signaling pathway.