An Easy and Flexible Inoculation Method for Accurately Assessing Powdery Mildew-Infection Phenotypes of Arabidopsis and Other Plants

Summary

We present a protocol for constructing a simple spore-distribution system consisting of an inoculation box with a ~50 µm mesh and a transparent plastic chamber. This can be used to evenly inoculate plants with powdery mildew spores, thereby enabling accurate and reproducible assessment of disease phenotypes of plants under study.

Abstract

Reducing crop losses due to fungal diseases requires improved understanding of the mechanisms governing plant immunity and fungal pathogenesis, which in turn requires accurate determination of disease phenotypes of plants upon infection with a particular fungal pathogen. However, accurate disease phenotyping with unculturable biotrophic fungal pathogens such as powdery mildew is not easy to achieve and can be a rate-limiting step of a research project. Here, we have developed a safe, efficient, and easy-to-operate disease phenotyping system using the Arabidopsis-powdery mildew interaction as an example. This system mainly consists of three components: (i) a wooden inoculation box fitted with a removable lid mounted with a stainless steel or nylon mesh of ~50 µm pores for inoculating a flat of plants with fungal spores, (ii) a transparent plastic chamber with a small front opening for minimizing spore escape while conducting inoculation inside, and (iii) a spore-dislodging and distribution method for even and effective inoculation. The protocols described here include the steps and parameters for making the inoculation box and the plastic chamber at a low cost, and a video demonstration of how to use the system to enable even inoculation with powdery mildew spores, thereby improving accuracy and reproducibility of disease phenotyping.

Introduction

Powdery mildew is one of the most common and important diseases of numerous food crops and ornamental plants¹. Studies of powdery mildew diseases have been very popular, as evidenced by over 10,500 publications as the search result with "powdery mildew" as key word at the Web of Science (as of November 2020). Indeed, powdery mildew (represented by Blumeria graminis) is considered to be one of the top 10 fungal pathogens by the journal of Molecular Plant Pathology². Quantification of disease susceptibility is a necessary step in characterization of plant genes contributing to disease resistance or suscep....

Protocol

1. Making a standard inoculation box with a removable top lid mounted with a mesh

1. Purchase a roll of 50 µm nylon membrane mesh or 48 µm stainless steel mesh (recommended) from stores. Make sure to order enough for cutting into multiple pieces of 14 in x 26 in for replacement of worn mesh.
2. Purchase one 1/4 in x 2 ft x 4 ft medium density fiberboard or plywood, and cut two 24-1/2 in x 10 in pieces and two 12 in x 10 in pieces for making an inoculation box.Use 8 corner clamps for supporting t.......

Discussion

Our meshed-box-based inoculation method has several advantages over other inoculation methods. First, it can achieve even distribution of spores if operated properly, as demonstrated in Figure 5. Second, the use of ~50 µm mesh, plus spore-dislodging by gentle shaking of infected leaves can reduce plant infection by thrips or other plant-infecting insects that are present in source plants. Third, the use of different-sized inoculation boxes for inoculating plants or detached leaves insid.......

Materials

Name	Company	Catalog Number	Comments
48 µm stainless steel grid mesh screen; Size: 24" X 48"	Amazon	NA	For making the lid of an inoculation box
#6-32 x ¾" machine screws, flat washers and nuts	Home Depot	NA	For making an inoculation chamber
#6-32 zinc plated nylon lock nut (4-Pack)	Home Depot	NA	For making an inoculation chamber
#6-32x3/8” Phillips flat head machine screws, flat washers and nuts	Home Depot	NA	For securing  magnet door catch plates
#8-32x1/2" machine screws, flat washers and nuts	Home Depot	NA	For securing corner braces and door hinge
0.250 thick clear extruded acrylic film-masked sheet;  Size: 17 ½" X 20"	Professional Plastics	SACR.250CEF	For making an inoculation chamber
0.250 thick clear extruded acrylic film-masked sheet; Size: 18" X 20"	Professional Plastics	SACR.250CEF	For making an inoculation chamber
0.250 thick clear extruded acrylic film-masked sheet; Size: 18" X 30"	Professional Plastics	SACR.250CEF	For making an inoculation chamber
0.250 thick clear extruded acrylic film-masked sheet; Size: 20" X 29 ½ "	Professional Plastics	SACR.250CEF	For making an inoculation chamber
1-5/8" cabinet door magnetic catch white	Home Depot	Model #P110-W	For making an inoculation chamber
2" steel zinc-plated corner brace (8-Pack)	Home Depot	Model #13611	For making an inoculation box & chamber
3" Corner Clamp	Harbor Freight Tools	SKU 63653, 1852, 60589	For making inoculation chamber
3/4"  steel zinc plated corner brace (4-Pack)	Home Depot	Model #13542	For making an inoculation box & chamber
4-7/8" zinc-plated light duty door pull handles	Home Depot	Model #15184	For making an inoculation box
Fine fan-blender brushes	Michaels Store	M10472846	For inoculation
Kelleher 3/4" x 3/4" x 36" wood square dowel	Home Depot	NA	For making the lid of an inoculation box
Medium density fiberboard (1/4" x 2' x 4');	Home Depot	Model# 1508104	For making an inoculation box
Round glass coverslips with a 500 µm grid	ibidi USA Inc.	10816	For determining  spore density