The Interplay between Enucleated Sieve Elements and Companion Cells

Area of Science:

• Plant Biology
• Cell Biology
• Molecular Biology

Background:

• Vascular plants possess specialized sieve elements (SEs) and companion cells for efficient transport of photosynthetic products and signals.
• SE differentiation involves enucleation and loss of translational capacity, yet retains essential components like plasmalemma and gains specific phloem proteins (P-proteins).
• Transcription factors (MYB, NAC) and OCTOPUS proteins are crucial for SE differentiation, with maturing SEs relying on companion cells via plasmodesmata.

Purpose of the Study:

• To review the current knowledge on bound and soluble phloem sap proteins.
• To elucidate the role of companion cells in regulating sieve tube protein content.
• To explore the transport mechanisms of phloem proteins and factors influencing their retention within SEs.

Main Methods:

• Review of existing literature on SE differentiation, companion cell function, and phloem protein analysis.
• Analysis of the impact of plasmodesmata morphology and physiology on phloem sap heterogeneity.
• Consideration of a newly developed protocol for isolating pure sieve tube proteins.

Main Results:

• Phloem proteins are transported via bulk flow and selective movement, with companion cells playing a key role in their presence.
• Plasmodesmata between SEs and companion cells allow passage of 20-70 kDa compounds, influencing sieve tube content.
• Understanding plasmodesmata is critical for clarifying phloem sap heterogeneity and protein retention mechanisms.

Conclusions:

• Companion cells are indispensable for maintaining sieve tube protein composition and function.
• Further investigation into plasmodesmata and protein retention is needed to fully understand phloem sap regulation.
• This review consolidates current understanding of phloem proteins, highlighting areas for future research.