Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 36.6 The Symplast Is Highly Dynamic
The symplast, the interconnected cytoplasm of plant cells linked by plasmodesmata, is far more dynamic than once believed. It not only transports materials but also coordinates growth, development, defense, and systemic communication, acting like an internal information superhighway.
Experimental Support for the Pressure-Flow Hypothesis
- Experiments using aphids showed:
- Aphids tap into sieve tubes and extract sap under positive pressure.
- Severed stylets continue to exude sap, indicating a pressure-driven system.
- Closer to sugar sources, sap has higher sucrose concentration—supporting pressure-flow from sources to sinks.
The Phloem as a Superhighway
- Phloem transports more than sugars:
- Moves proteins, RNAs, and signaling molecules.
- Coordinates development, defense, and flowering.
- Examples:
- Flowering signals from leaves reach meristems.
- Infection triggers defense gene activation in distant tissues.
- Phloem-based signaling integrates physiological functions across the plant.
Electrical Signaling in the Phloem
- Plants like Mimosa pudica and Venus flytrap transmit electrical signals through phloem.
- These signals can:
- Change gene expression, photosynthesis, hormones, and phloem unloading.
- Though slower than animal nerves, this nerve-like communication enables fast coordination across organs.
Plasmodesmata: Dynamic Cell Connections
- Plasmodesmata:
- Open and close based on pressure, pH, and calcium.
- Form and degrade dynamically—even post-cell division.
- Transport macromolecules like proteins and RNAs.
- Symplastic domains: groups of interconnected cells coordinating activity.
- Viruses use movement proteins to widen plasmodesmata and spread viral RNA.
In a Nutshell
The symplast is a living, adaptable network that integrates transport and communication across plant tissues. Phloem not only distributes sugars but also carries systemic signals and even electrical impulses. Plasmodesmata are regulated portals, not just passive holes, and play vital roles in development, defense, and viral infection. This dynamic system ensures the plant operates as a unified whole.