Visualizing Stromule Frequency with Fluorescence Microscopy

Summary

Protocols to investigate the dynamics of chloroplast stromules, the stroma-filled tubules that extend from the surface of chloroplasts, are described.

Abstract

Stromules, or "stroma-filled tubules", are narrow, tubular extensions from the surface of the chloroplast that are universally observed in plant cells but whose functions remain mysterious. Alongside growing attention on the role of chloroplasts in coordinating plant responses to stress, interest in stromules and their relationship to chloroplast signaling dynamics has increased in recent years, aided by advances in fluorescence microscopy and protein fluorophores that allow for rapid, accurate visualization of stromule dynamics. Here, we provide detailed protocols to assay stromule frequency in the epidermal chloroplasts of Nicotiana benthamiana, an excellent model system for investigating chloroplast stromule biology. We also provide methods for visualizing chloroplast stromules in vitro by extracting chloroplasts from leaves. Finally, we outline sampling strategies and statistical approaches to analyze differences in stromule frequencies in response to stimuli, such as environmental stress, chemical treatments, or gene silencing. Researchers can use these protocols as a starting point to develop new methods for innovative experiments to explore how and why chloroplasts make stromules.

Introduction

Chloroplasts are dynamic organelles in plant cells responsible for photosynthesis and a host of other metabolic processes. Signaling pathways from the chloroplast also exert significant influence on plant physiology and development, coordinating plant responses to environmental stress, pathogens, and even leaf shape^1-6. Recently, biologists have gained interest in a poorly understood aspect of chloroplast structure: stromules, very thin stroma-filled tubules that extend from the surface of the chloroplast⁷.

The biological functions of stromules remain unknown, although stromule frequency is known to vary in response to....

Protocol

NOTE: For this protocol, we have focused on assaying stromule frequency in the epidermis of N. benthamiana leaves. Several stable transgenic lines have been generated that can be used for this purpose, including 35S_PRO:FNRtp:EGFP¹³ and NRIP1:Cerulean⁶. Both of these lines show robust expression of fluorophores in the chloroplast stroma of leaves grown under a wide range of conditions. Alternatively, chloroplast-targeted fluorophores may be transiently expressed in N. benthamiana using Agrobacterium transformations¹³. This is less ideal than the transgenic lines, since Agrobact....

Results

This protocol was used to visualize stromule frequency at day and at night in the cotyledons of young N. benthamiana seedlings. Slices from a z-stack were merged into a single image (Figure 1A). For visual purposes, that image was then desaturated and inverted so that stroma appears black (Figure 1B). The chloroplasts were labeled either as having no stromules (green asterisk) or having at least one stromule (magenta asterisk). Of the 87.......

Discussion

When investigating stromules, three important factors must be considered throughout: (i) manipulation of the plant tissue must be kept to an absolute minimum, (ii) the experimental system must be kept consistent, and (iii) sampling strategies must be carefully planned to ensure robust, reproducible data are analyzed.

Stromules are remarkably dynamic: they can extend and retract rapidly before an observer's eyes under the microscope. Moreover, stromule frequency varies significantly in resp.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Hepes	Sigma-Aldrich	H3375
NaOH	Fischer-Scientific	S320-1
Sorbitol	Sigma-Aldrich	S1876
EDTA	Fischer-Biotech	BP121
MnCl2	Sigma-Aldrich	221279
MgCl2	Sigma-Aldrich	M0250
KCl	Sigma-Aldrich	P3911
NaCl	Sigma-Aldrich	S9625
Laser Scanning Confocal Microscope	Carl Zeiss Inc	Model: LSM710
Carboxyfluorescein diacetate (CFDA)	Sigma-Aldrich	21879
Dimethyl sulfoxide (DMSO)	EMD	MX1458-6
Waring blender	Waring	Model: 31BL92
Fiji	fiji.sc		Open-source software for analyzing biological images