leaf pathotype acquisition and assessment of poplar resistance to stem canker diseases in the laboratory

Rapid In Vivo Leaf Inoculation for Screening Poplar Stem Canker Resistance

Poplar stem canker diseases, mainly caused by two necrotrophic pathogens, Cytospora chrysosperma (Pers.) Fr. and Botryosphaeria dothidea (Moug. ex Fr.) Ces. &#38; de Not., severely threaten the development and survival of poplar plantations in the Northeast, North, and Northwest (Three-north) of China. Hybrid breeding is the most direct and efficient method to control and manage tree diseases; however, compared with the progress in breeding for high-yield, fast-growing, or other poplar hybrid clones, research on resistance breeding for poplar canker disease is scarce. Only limited studies of disease-resistance hybrid breeding have been reported1,2,3, and no canker-resistance hybrid clone has been cultivated in poplar afforestation.
The crucial step of regular hybrid breeding is the clone selection based on the phenotype acquisition of hybrid progeny. However, acquiring the acquisition of pathological phenotypes (pathotypes) is time-consuming, laborious, tiring, expensive, and experts-depending&#160;process. For woody plants, it is more challenging due to the time-, labor-, and economy-consuming nature caused by their long lifecycle, slow growth, and massive body. For example, in poplar, the canker resistance screening of hybrid progeny was conducted 5-7 years after hybridization through conventional in vitro stem inoculation methods1,2,3. Moreover, limited by this low-efficiency and high-consuming disease-resistant screening method, researchers re-selected the canker-resistant clones from a small subgroup (for example, the selected high-yield or fast-growing poplar clones), not from all hybrid progeny. Therefore, using the regular stem inoculation methods, it is not sure to identify the authentic resistant clones and can not reveal the disease-resistant diversity of the breeding progeny, thus limiting the exploration of disease-resistance-related genes or gene modules. Compared to the rapid development of poplar fast-growing/high-yield breeding, those breeding programs can obtain hybrids through phenotypic selection or even genomic selection in the first two years of breeding4, the poplar canker-resistance breeding developed slowly. The disease-resistant screening (or detection) of hybrid progeny, or the pathogen inoculation method, has become the crucial speed-limiting step in poplar canker disease-resistance breeding.
In this protocol, we introduce a novel inoculation method for poplar stem canker pathogens, in vivo leaf inoculation method. Using this method, we can quickly and efficiently test the canker disease resistance of dozens of poplar species (cultivars or clones) within 5-7 days. The validation assay illustrated that the in vivo leaf inoculation is consistent with the traditional in vitro stem inoculation on the resistance detection of stem canker disease, suggesting that the leaf inoculation method is suitable for large-scale resistance screening of poplar canker diseases, such as the whole genotype selection of resistance progeny in the breeding of stem canker disease. This method solves the pathological problem of selecting resistant offspring in the hybrid breeding of poplar canker diseases.

Author Spotlight: High-Throughput In Vivo Leaf Inoculation for Accelerating Disease Resistance Screening in Poplar Hybrid Breeding

In Vivo Leaf Inoculation: An Alternative Method to Assess the Disease Resistance of Hybrid Clones in Poplar Breeding of Stem Canker Disease

The scope of this research protocol is to examine the methodology of pathogen inoculation and disease-resistant assessment methods using the poplar hybrid breeding. We are proposing the in vivo leaf inoculation method as an alternative to the traditional method with the aim of advancing the breeding of poplar stem canker disease. Using the leaf inoculation method, researchers can screen the disease resistance of large-scale hybrid clones, in the second years of hybrid lesion, realizing earlier on the sliding selections in poplar hybrid grading of stem canker disease.The most advantageous aspects of the leaf inoculation method are as high efficiency and high throughput. First, the leaf method can sharply shorten the period of disease resistance selection from five to seven years after hybridization to two years after hybridization. Second, the method can assess the disease resistance of large-scale hybrid clones within several weeks.The leaf inoculation method will advance the hybrid breeding of poplar Botryosphaeria dothidea and Cytospora chrysosperma canker in China, and the poplar hybrid breeding of Septoria canker in North America. Moreover, the method can be used to mine disease resistance genes and improve the development of poplar molecular breeding. In addition to poplar hybrid breeding of stem canker disease, the leaf method can be used to screen the disease resistance of natural or mutant poplar populations, and the susceptibility of poplar stem canker pathogens.Moreover, the method can provide pathotypes for the mining of disease resistance genes, and the genetic analysis of disease resistance.

Leaf Pathotype Acquisition and Assessment of Poplar Resistance to Stem Canker Diseases in the Laboratory

To begin, take canker inoculated poplar leaves kept in plastic bags. Carefully remove the plastic protective films from the leaves and gently remove the mycelium innoculants from the leaf surfaces. Observe and identify the leaf pathotypes of each poplar clone by examining the shape and color of necrotic spots.Then place leaves in a ruler on a black background. Photograph the lower surface of the leaves with a camera, or scan them using a scanner to obtain images with a resolution of over 300 dots per inch. Open the images of the diseased leaves in the software ImageJ, 1.54G.Set the scale in ImageJ according to the ruler visible in the leaf images. Then using the Wand tool in ImageJ, identify the lesions on the leaves and measure the area of each necrotic spot. After measuring all necrotic spots, record the area values and export these values as a spreadsheet for further analysis.If the lesions around the mycelium covered peer sites are significantly larger than those around the PDA covered sites, classify them as disease sites. If the mycelium covered peer sites show changes like lesion color, hyphae-like structures pycnidia and conidia, define these as disease sites. Calculate the average area of effective PDA lesion spots for each poplar hybrid clone.Divide the mycelium inoculated spots into two categories, onset and non-onset. Then calculate the tested poplars clone disease incidence rate. Calculate the average area of the diseased spots on leaves.Based on the average areas of diseased spots across all tested poplar clones, set a five level disease grading standard. Calculate the disease index of each poplar clone using the five level disease grading standard. Next, using the Shapiro-Wilk test, verify the normal distribution of the numbers of poplar clones across different resistance levels.Based on the disease index, classify all tested poplar clones into five or seven resistance groups. Very high resistance, high resistance, resistance, no resistance, and no susceptibility, susceptibility, high susceptibility, and very high susceptibility. Inoculation of stem canker pathogen induced necrotic lesions, and even pycnidia-like structures on poplar leaves.Moreover, the leaf positions and light conditions of the leaves impact the disease severity of the leaves.

leaf-pathotype-acquisition-assessment-poplar-resistance-to-stem

Research

JoVE Journal

Biology

Stem canker diseases caused by the pathogen Cytospora chrysosperma (Pers.) Fr.) and Botryosphaeria dothidea (Moug. ex Fr.) Ces. &#38; de Not. are the two major forest diseases in the poplar plantations in China, sometimes which can destroy all the poplar seedlings or severely damage mature poplar forests. Hybrid breeding is the most direct and efficient method of controlling and managing tree diseases. However, assessing disease resistance or selecting disease-resistance clones based on In vitro stem inoculation is inefficient, time-consuming, and expensive, limiting the development of hybrid breeding of poplar stem canker disease. In this study, we proposed an alternative method to assess disease resistance to stem canker pathogens through in vivo leaf inoculation. The test materials used in this method can be on 1-year-old poplar saplings or the annual branches of perennial poplars in the greenhouse or the field. The critical step of this alternative method is the selection of inoculating leaves: the 5-7th newly matured leaves might be the most suitable. The second critical step of the leaf inoculation method is to make wounds on plant leaves through needle pierces, providing sufficient lesions to measure disease severity. For the adequate number of leaves produced in the early stage of poplar breeding, this in vivo leaf inoculation contributes to the rapid, accurate, and large-scale screening of the disease-resistance poplar clones to stem canker pathogens. Moreover, this leaf inoculation method will also serve as an efficient method for screening pathotypes of stem canker disease pathogen C. chrysosperma, B.dothidea, or other poplar stem canker pathogens.

We provide a step-by-step protocol for assessing poplar resistance to stem canker pathogens using an in vivo leaf inoculation method. This method is especially suitable for large-scale assessment of Cytospora chrysosperma and Botryosphaeria dothidea canker disease resistance in poplar breeding progeny in China.

Stem canker diseases caused by the pathogen Cytospora chrysosperma (Pers.) Fr.) and Botryosphaeria dothidea (Moug. ex Fr.) Ces. &#38; de Not. are the two ...

Preparing Reagents and Poplar Material for In Vivo Leaf Inoculation

Begin by adding potato extraction, dextrose, and auger in water to prepare potato dextrose auger or PDA culture medium for fungal strains. Heat the solution at 100 degrees Celsius for 10 minutes to completely dissolve the components. Poor PDA medium into 25 milliliter tubes.Sterilize all tubes at 121.1 degrees Celsius for 30 minutes. Then cool the sterilized tubes on a clean bench to 50 to 60 degrees Celsius. Pour the medium into 9.0 centimeter diameter culture plates and cool them at room temperature.Next, cut the mycelium of the fungal pathogen cultured in PDA medium at 28 degrees Celsius for seven days into approximately 0.5 centimeter square cubes. Inoculate the fungal cubes at the center of the PDA plates, ensuring the mycelium sides face the medium. Then culture, the inoculated fungal pathogens in a thermostatic incubator for seven to 10 days.After incubation, cut the PDA medium containing the fungal mycelium into square cubes with sides 1.0 to 1.2 centimeters long. Select the newly matured fully extended leaves from the top of the main branches of 1-year-old poplar clones, ensure the leaves are free from pests, diseases, and mechanical damage. For one year old branches of Perennial poplar hybrid clones, select the top fifth to seventh leaves of the selected branches as the inoculated materials.Spray the selected poplar clones or branches with clean water. After drying, wipe the selected leaves with 75%alcohol, either one hour or one day before the inoculation manipulations.

In Vivo Inoculation of Stem Canker Pathogens in Poplar Leaves to Test the Canker Disease Resistance

To begin, prepare fungal culture for canker pathogen and poplar material for leaf inoculation. For small leaves, inoculate two previously prepared square fungal mycelium cubes and PDA cubes onto the upper surface of the top fifth to seventh leaves of poplar clones. Ensure that the mycelium faces the leaves and the inoculation sites are located at the center of the half leaves.Inoculate four square fungal mycelium cubes and four PDA cubes onto the upper surface of the top fifth to seventh leaves of large poplar leaves. Then, wrap the inoculated leaves with transparent adhesive tape. Gently press the tape to make it adhere to the leaves, preventing the movement and water loss of the mycelium cubes during the experiment.Next, using a five milliliter syringe needle, pierce the leaves and the mycelium cubes at five sites until the needle penetrates the cubes from the upper to the lower surface of the poplar leaves. Place one piercing site at the center and the other four one to two millimeters near the four vertices of the cubes. Inspect the inoculated leaves within three days for any location shifts or water loss of the mycelium inoculants.Observe the onset of necrotic lesions around the piercing wound sites on the lower surface of the leaves. At the end of the experiment, pick off all inoculated leaves and place them in plastic sample bags.