Evaluation of Photosynthetic Efficiency in Photorespiratory Mutants by Chlorophyll Fluorescence Analysis

Summary

We describe an approach to measure changes in photosynthetic efficiency in plants after treatment with low CO₂ using chlorophyll fluorescence.

Abstract

Photosynthesis and photorespiration represent the largest carbon fluxes in plant primary metabolism and are necessary for plant survival. Many of the enzymes and genes important for photosynthesis and photorespiration have been well studied for decades, but some aspects of these biochemical pathways and their crosstalk with several subcellular processes are not yet fully understood. Much of the work that has identified the genes and proteins important in plant metabolism has been conducted under highly controlled environments that may not best represent how photosynthesis and photorespiration function under natural and farming environments. Considering that abiotic stress results in impaired photosynthetic efficiency, the development of a high-throughput screen that can monitor both abiotic stress and its impact on photosynthesis is necessary.

Therefore, we have developed a relatively fast method to screen for abiotic stress-induced changes to photosynthetic efficiency that can identify uncharacterized genes with roles in photorespiration using chlorophyll fluorescence analysis and low CO₂ screening. This paper describes a method to study changes in photosynthetic efficiency in transferred DNA (T-DNA) knockout mutants in Arabidopsis thaliana. The same method can be used for screening ethyl methanesulfonate (EMS)-induced mutants or suppressor screening. Utilizing this method can identify gene candidates for further study in plant primary metabolism and abiotic stress responses. Data from this method can provide insight into gene function that may not be recognized until exposure to increased stress environments.

Introduction

Abiotic stress conditions commonly seen in farmer's fields can negatively impact crop yields by reducing photosynthetic efficiency. Detrimental environmental conditions such as heat waves, climate change, drought, and soil salinity can cause abiotic stresses that alter CO₂ availability and reduce a plant's response to high light stress. The two largest terrestrial carbon fluxes are photosynthesis and photorespiration, which are essential for plant growth and crop yields. Many of the important proteins and enzymes involved in these processes have been characterized under laboratory conditions and identified at the genetic level¹

Protocol

1. Seed preparation and sterilization

NOTE: Seed preparation consists of seed imbibing and seed sterilization. It is important to note that all these steps are to be carried out in a laminar flow hood to maintain sterile conditions. All necessary materials, reagents, and growth media must be autoclaved (see the Table of Materials).

1. Seed imbibation and stratification
   NOTE: The seed lines used are plgg1-1 (salk_053463), abcb26 (salk_085232), and wild type (WT, Col-0).
   1. Dispense the seeds into 1.5 mL microcentrifuge tubes. Imbibe the seeds in sterile water in a l....

Results

The results show plate images of raw and fluorescence images from ambient and low CO₂ screening of WT and test mutants. Each plantlet is labeled by area number, with corresponding fluorescence readings given as QY. The data are exported as a text file and can be opened in a spreadsheet for analysis (see Supplemental Table S1). Mutant lines plgg1-1 and abcb26 were selected to demonstrate the positive and negative identification of genes associated with photorespiratory stress........

Discussion

The experimental methods outlined in this paper come with some advantages and limitations. One advantage is that this method can screen many plant seedlings, although some precautions must be taken to prevent contamination of the plant media plate during the plating and growing process. Therefore, it is critical to seal the Arabidopsis plates with surgical tape. Another advantage of this experiment is that it has a shorter 12 h photorespiratory stress period compared to the previously published work⁸

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1.5 mL microcentrifuge tube	VWR	10810-070	container for seed sterilization
agarose	VWR	9012-36-6	chemical used to suspend seeds for ease of plating
Arabidopsis thaliana seeds (abcb26)	ABRC, ordered through TAIR www.arabidopsis.org	SALK_085232	arabidopsis seeds used as experimental group
Arabidopsis thaliana seeds (plgg1-1)	ABRC, ordered through TAIR www.arabidopsis.org	SALK_053469C	parental arabidopsis seeds
Arabidopsis thaliana seeds (WT)	ABRC, ordered through TAIR www.arabidopsis.org	Col-0	arabidopsis wild type seeds used as a control group
bleach	clorox	generic bleach	chemical used to sterilize seeds
Carbolime absorbent	Medline products	S232-104-001	CO2 absorbent
Closed FluorCam	Photon Systems Instruments	FC 800-C	Fluorescence imager
FluoroCam FC 800-C	Photon Systems Instruments	Closed FluorCam FC 800-C/1010-S	Fluorescence imager
FluoroCam7	Photon Systems Instruments	Closed FluorCam FC 800-C/1010-S	Fluorescence image analysis software
Gelzan (plant agar)	Phytotech labs	71010-52-1	chemical used to solidify MS media as plates
glass flask 1 L	Fisherbrand	FB5011000	container for making and autoclaving MS media
growth chamber	caron	7317-50-2	growth chamber used to grow plants
Murashige & Skoog Basal Medium with Vitamins & 1.0 g/L MES (MS)	Phytotech labs	M5531	growth media for arabidopsis seedlings
potassium Hydroxide (KOH)	Phytotech labs	1310-58-3	make as 1 M solution for ph adjustment
spider lights	Mean Well Enterprises	XLG-100-H-AB	lights used in the light assay
Square Petri Dish with Grid, sterile	Simport Scientific	D21016	used to hold MS media for arabidopsis seedlings
surgical tape	3M	1530-1	tape used to seal plates
tween 20	biorad	9005-64-5	surfactant used to assist seed sterilization