Phloem evolved gradually and asynchronously to xylem in early vascular plants

By re-examining exceptionally preserved 407-million-year-old Rhynie chert fossils, this study reveals that early vascular plants possessed large, primitive food-conducting cells rather than true phloem, suggesting that phloem features evolved gradually and asynchronously to the earlier emergence of xylem.

The Gist

Imagine the history of plants as a grand construction project. For a long time, scientists believed that when plants first learned to grow tall and move water and food around their bodies, they built two essential "pipes" at the exact same time: Xylem (the water pipes) and Phloem (the sugar/food pipes).

Think of it like building a house: you assume the plumbing (water) and the electrical wiring (food) were installed together in the same week.

However, a new study by Laura Cooper and Alexander Hetherington suggests that nature didn't build these systems simultaneously. Instead, they were built asynchronously—one after the other, over millions of years, and in a very different order than we thought.

Here is the story of their discovery, broken down into simple concepts:

1. The Mystery of the Missing "Food Pipe"

For decades, paleontologists have been digging up fossils from the Rhynie Chert, a 407-million-year-old "time capsule" of ancient plants preserved in rock. They found plenty of Xylem (water pipes). Because these pipes are made of tough, woody material (like lignin), they fossilize easily. It's like finding a cast-iron pipe in an old ruin; it survives the ages.

But they rarely found Phloem (food pipes). Phloem is made of soft, thin-walled cells, like delicate tissue paper. In the fossil record, this tissue usually rots away before it can turn to stone. Because of this, scientists only found definitive "food pipes" about 40 million years after they found the "water pipes."

This created a confusing gap: Did plants have water pipes for 40 million years before they figured out how to transport food? Or did the food pipes just disappear from the fossil record?

2. The "Food-Conducting Cells" (FCCs): The Prototype

The researchers took a fresh look at these ancient fossils using high-tech microscopes. They found a tissue that looked sort of like modern food pipes, but not quite. They called these Food-Conducting Cells (FCCs).

Think of FCCs as the prototype or the "beta version" of the modern food pipe. They had some features of the real thing, but they were missing the finishing touches.

The Three Big Differences:

• No "Fence" (The Pericycle): In modern plants, the food pipes are neatly wrapped in a protective layer called a pericycle, which acts like a fence separating the "food zone" from the "storage zone" (cortex). In the ancient Rhynie plants, there was no fence. The food cells just slowly blended into the storage cells, like a gradient fading from dark to light.
• Oversized Pipes: Modern food pipes are very narrow and long (like a thin straw). The ancient FCCs were massive—about six times wider than modern ones. It's like comparing a garden hose to a firehose.
• The "Pores" Discovery: The most exciting find was in a plant called Asteroxylon mackiei. The researchers found tiny holes in the walls of these giant cells. These are sieve pores—tiny doors that allow sugar to flow from cell to cell. This is the "smoking gun" that these cells were indeed moving food, even if they looked different from today's plants.

3. The New Evolutionary Story

Based on these findings, the authors propose a new timeline for how plants evolved:

1. Step 1: The Food First. Long before plants had tough water pipes, they had these giant, soft "Food-Conducting Cells" with sieve pores. They were the first to figure out how to move sugar around.
2. Step 2: The Water Arrives. Later, plants evolved the tough, woody Xylem to move water efficiently.
3. Step 3: The Upgrade. Once water pipes were established, the "Food-Conducting Cells" went through a major renovation. They shrank down to become narrow, efficient straws, and they got their protective "fence" (the pericycle) added. This turned them into the Phloem we see in plants today.

The Analogy: The Evolution of a Delivery Service

Imagine a delivery company in the ancient world:

• Phase 1: They start with giant, slow-moving trucks (the FCCs) that can carry food, but they are clumsy and don't have a specific route. They just drive through the neighborhood.
• Phase 2: They build a high-speed train system (Xylem) to move water. This is a huge engineering feat.
• Phase 3: Realizing the giant trucks are inefficient, they upgrade the food delivery. They switch to tiny, fast motorcycles (modern Phloem) and build a dedicated highway with barriers (the pericycle) to keep traffic flowing smoothly.

Why This Matters

This study changes our understanding of plant history. It tells us that the complex systems we see in trees and flowers today didn't appear all at once. Nature experimented with "beta versions" first.

The ancient plants in the Rhynie Chert weren't "failed" plants; they were the innovators. They figured out how to move food first, using a different design, and only later refined that design to match the water-transport system. It's a reminder that evolution is a gradual, messy, and asynchronous process, not a single moment of perfection.

Technical Summary

1. Problem Statement

The evolutionary origin of plant vasculature (xylem and phloem) is a central question in plant biology. While xylem (water-conducting tissue) and phloem (sugar-conducting tissue) are functionally linked and always co-occur in extant vascular plants, their evolutionary origins remain unclear.

• The Fossil Bias: The fossil record is heavily skewed toward xylem because lignified tracheids fossilize readily, whereas thin-walled phloem cells rarely preserve. Consequently, the first definitive fossil record of xylem predates that of phloem by approximately 40 million years.
• The Knowledge Gap: This bias obscures the origins of the vascular system. It is debated whether the "phloem-like" tissues found in the 407-million-year-old Rhynie chert (early Devonian) represent true phloem or a precursor tissue. Resolving the identity of these tissues is critical for understanding whether xylem and phloem evolved simultaneously or asynchronously.

2. Methodology

The authors conducted a multi-scale comparative anatomical study using exceptionally preserved fossils from the Rhynie chert and extant lycophytes.

• Specimens:
  • Fossils (Rhynie Chert): Four species were analyzed: the non-vascular polysporangiophyte Aglaophyton majus (possessing water-conducting cells but no true xylem), and three vascular plants: Rhynia gwynne-vaughanii, Trichopherophyton teuchansii (zosterophyll), and Asteroxylon mackiei (lycopsid).
  • Extant Controls: Four species of extant lycophytes (Lycopodium clavatum, Huperzia selago, Huperzia goebellii, Selaginella uncinata) were used for comparative anatomy.
• Imaging Techniques:
  • Light Microscopy: Used for histological analysis of thin sections to compare tissue organization (FCCs vs. cortex) and cell morphology.
  • Scanning Electron Microscopy (SEM): Used on HF-etched samples of A. mackiei to visualize subcellular wall structures without cell collapse.
  • Airyscan Confocal Laser Scanning Microscopy (CLSM): Used on thin sections of A. mackiei to resolve sub-micron features non-destructively, complementing SEM data.
• Quantitative Analysis:
  • Histological: Assessment of the presence/absence of a pericycle (a bounding layer between vasculature and cortex).
  • Cellular: Measurement of cell diameter and aspect ratio (length/width) for Food-Conducting Cells (FCCs) in fossils vs. phloem sieve elements in extant species.
  • Subcellular: Identification and measurement of pores in cell walls to distinguish between plasmodesmatal pits and sieve pores.

3. Key Contributions

• Redefinition of Rhynie Chert Tissue: The authors propose renaming the "phloem-like" tissue in Rhynie chert plants as Food-Conducting Cells (FCCs) to distinguish them from the true phloem of extant vascular plants, which requires specific histological and cellular criteria.
• Discovery of Earliest Sieve Pores: The study reports the earliest known record of sieve pores in the fossil record, identified within the FCCs of Asteroxylon mackiei.
• Evolutionary Decoupling: The paper provides evidence that the evolution of phloem features was asynchronous with the evolution of xylem, challenging the view that vascular tissues evolved as a single, simultaneous innovation.

4. Results

A. Histological Differences (The Absence of a Pericycle)

• Extant Plants: Phloem is delimited from the cortex by a distinct pericycle (a layer of 1–3 cells).
• Rhynie Chert Plants: No pericycle was observed. Instead, the FCCs grade gradually into the cortex.
  • In A. majus and R. gwynne-vaughanii, the transition is smooth: cells adjacent to the water-conducting strand are elongated and close-packed, becoming progressively shorter, rounder, and more spaced (cortical) toward the periphery.
  • Even in the more derived lycophytes (T. teuchansii and A. mackiei), a sharp boundary layer (pericycle) is absent, though the transition is sharper than in the non-vascular A. majus.

B. Cellular Differences (Cell Size and Shape)

• Diameter: FCCs in Rhynie chert plants are significantly larger than extant phloem sieve elements.
  • Extant Lycophytes: Average diameter ~8.9 µm (range 2.2–41.4 µm).
  • Rhynie Chert FCCs: Average diameter ~56.2 µm (range 9.2–231.3 µm).
  • The fossil cells are, on average, six times larger in diameter than modern counterparts ($p \le 0.001$).
• Aspect Ratio: Extant sieve elements are extremely elongated (AR $\le$ 0.02). Rhynie FCCs are elongated but significantly shorter and wider (AR $\approx$ 0.20) compared to modern sieve elements.

C. Subcellular Discovery (Sieve Pores in A. mackiei)

• Method: Using SEM and Airyscan CLSM, the authors identified regular circular perforations in the lateral and end walls of A. mackiei FCCs.
• Dimensions: These pores measure approximately 0.2 µm in diameter.
• Differentiation: This size is distinct from plasmodesmatal pits (approx. 0.07 µm in Huperzia) and matches the size range of sieve pores in extant lycophytes (0.15–0.86 µm).
• Significance: This is the earliest definitive record of sieve pores, predating previous fossil records by tens of millions of years.

5. Significance and Evolutionary Scenario

The findings support a model where phloem evolved gradually and asynchronously relative to xylem:

1. Pre-Xylem Origin of Sugar Transport: The presence of FCCs with sieve pores in early-diverging polysporangiophytes (like Eophytes and A. majus) suggests that sugar-conducting tissue with sieve pores evolved before the origin of true water-conducting xylem.
2. Stepwise Assembly of Phloem: The "true" phloem of extant plants is a derived state assembled from ancestral FCCs.
   • Early Stage: Ancestral FCCs were large, lacked a pericycle, and possessed sieve pores.
   • Later Stage: After the origin of xylem, specific adaptations for efficient long-distance transport evolved independently in the lycophyte and euphyllophyte lineages. These included:
     • Reduction in cell diameter (narrowing).
     • Increase in cell length.
     • Development of a distinct pericycle to separate vascular tissue from the cortex.
3. Convergent Evolution: The study suggests that sieve pores may have evolved convergently in bryophytes (gametophyte generation) and early polysporangiophytes (sporophyte generation), indicating that the enlargement of plasmodesmata into sieve pores is a readily accessible developmental adaptation.

Conclusion: The paper overturns the assumption that xylem and phloem originated simultaneously. Instead, it proposes that the vascular system arose through the independent and asynchronous assembly of traits, with sugar-conducting capabilities (FCCs with sieve pores) preceding the full histological and cellular specialization of modern phloem.