# The Extended Synaptotagmins of Physcomitrium patens

## TL;DR

This paper studies ESYTs in Physcomitrium patens, a moss, revealing new classes and functions related to membrane contact sites and plant stress responses.

## Contribution

The paper identifies a new ESYT class in P. patens and explores its potential roles in plant evolution and stress adaptation.

## Key findings

- P. patens contains ESYTs across all known classes and an additional class not found in A. thaliana.
- Motif analysis in the SMP domain suggests functional and evolutionary insights into ESYTs.
- P. patens is proposed as a model for studying ESYT roles in tip growth and plasmodesmata transport.

## Abstract

Membrane contact sites (MCSs) between the endoplasmic reticulum and the plasma membrane enable the transport of lipids without membrane fusion. Extended Synaptotagmins (ESYTs) act at MCSs, functioning as tethers between two membrane compartments. In plants, ESYTs have been mainly investigated in A. thaliana and shown to maintain the integrity of the plasma membrane, especially during stress responses like cold acclimatization, mechanical trauma, and salt stress. ESYTs are present at the MCSs of plasmodesmata, where they regulate defense responses by modulating cell-to-cell transfer of pathogens. Here, the analysis of ESYTs was expanded to the bryophyte Physcomitrium patens, an extant representative of the earliest land plant lineages. P. patens was found to contain a large number of ESYTs, distributed over all previously established classes and an additional class not present in A. thaliana. Motif discovery identified regions in the Synaptotagmin-like mitochondrial (SMP) domain that may explain phylogenetic relationships as well as protein function. The adaptation mechanisms of P. patens necessary to conquer land and its simple tissue structure make it highly suitable as a model organism to study ESYT functions in tip growth, stress responses, and plasmodesmata-mediated transport, and open new directions of research regarding the function of MCSs in cellular processes and plant evolution.