Reaction of cabbage lines reveals resistance to infection by Cucumber mosaic virus

ABSTRACT

Cucumber mosaic virus (CMV) is one of the most economically damaging viral pathogens affecting vegetable crops around the world. In the present research, 50 cabbage (Brassica oleracea L.) lines chosen from the Black Sea Agricultural Research Institute (BSARI, Turkey) were evaluated for their reaction to CMV. The level of resistance to CMV was determined based on biological assays, symptom expression, disease severity rating, and viral titer, as determined by ELISA. Eight cabbage lines were identified as moderately susceptible and 3 were susceptible to the CMV-BA isolate. These plants exhibited various symptoms and accumulated high levels of virus titer. However, the results showed that 17 lines had high resistance, 12 were found to be resistant and 10 were found to be moderately resistant to the CMV. The lines that showed high levels of resistance to the virus in this study could be used as sources of CMV resistance in cabbage breeding programs.

Key words
Brassica oleracea; breeding; CMV; reaction

Cabbage (Brassica oleracea L.) is an important vegetable crop that is widely cultivated around the world. According to the Food and Agriculture Organization of the United Nations (FAO), records of 2011, Turkey annual cabbage production was 710.000 tons and was ranked 11^th in the world. Cabbage is mainly used as a green vegetable, widely grown in the Black Sea Region of Turkey (Balkaya and Karaagac 2006). Samsun province has 222.161 tons of cabbage production in a year, which accounts for 32% of Turkey production (TurkStat 2012).

Species of the genus Brassica may be infected by various viruses such as Cucumber mosaic virus (CMV), Turnip mosaic virus (TuMV), Cauliflower mosaic virus (CaMV), Beet western yellows virus (BWYV), and Turnip yellow mosaic virus (TYMV) (Raybould et al. 1999; Chen et al. 2000; Moreno et al. 2004). CMV is among the most economically damaging pathogens in Brassica crops (Moreno et al. 2004).

CMV is the type species of the genus Cucumovirus in the family Bromoviridae (Palukaitis and Garcia-Arenal 2003). CMV particles are isometric and composed of a coat protein shell which encapsidates the single-stranded, plus-sense RNA genome (Roossink 2001). It has the broadest host range among the known plant viruses, infecting more than 1,200 species of plants from monocotyledons to dicotyledons (Zitter and Murphy 2009). The CMV spreads through the sap of infected plants by leaf contact, through seeds of 19 plant species and by dodder. The large population of aphid vectors is one reason for the widespread nature of CMV (Zitikaitė and Urbanavičienė 2010; Saruhan et al. 2015). Although resistance is generally pathogen-specific, the use of disease-resistant crop varieties is regarded as an economical and durable method for controlling plant diseases, especially those caused by viruses (Ashfaq et al. 2014). The development of disease-resistant cultivars can provide a simple and cheap approach to reduce the economic losses caused by plant viruses. In this method, resistance is detected by inoculating accessions from a germplasm collection with a virus and screening the reactions of each accession (Balci 2005).

The objective of this study was to assess the reactions to CMV of cabbage lines from the Black Sea Agricultural Research Institute (BSARI). Fifty cabbage lines were screened under greenhouse conditions by sap inoculation method. The grade of reaction to CMV in cabbage tissues was evaluated using a combination of biological and serological assays.

A selected set of 50 advanced lines of B. oleracea var. capitata subvar. alba (Turkish origin) utilized in the breeding project white head cabbage was obtained from BSARI, Turkey, and used for screening purpose (Table 1). The CMV isolate obtained from field-collected samples in Samsun province of Turkey was used in this study. The samples were examined for the most widespread cabbage-infecting viruses of that growing area to detect any mixtures with other viruses. Only samples infected with CMV were studied. The CMV-BA, which is a severe isolate, was propagated on the laboratory host Yolo Wonder, a CMV-susceptible cultivar, in an insect-free controlled environment, and symptomatic leaves were harvested for use as inoculum sources. The samples used as inoculum source were free from tested viruses, such as TuMV, CaMV, TYMV, and BWYV. The presence of only CMV was confirmed by the ELISA and polymerase chain reaction (PCR) methods (Oliveira et al. 2012).

Table 1
Cabbage lines used in the current study and summarized results of reactions of the lines against CMV.

		Lines		Infection index (1 - 5)		Average of ELISA reading, 1 h after substrate addition		Observed symptoms		Types of reaction
1		W-1		1.0		0.122		---		Highly resistant
2		W-31		1.3		0.122		---		Highly resistant
3		W-33		1.2		0.111		---		Highly resistant
4		W-34		1.0		0.112		---		Highly resistant
5		W-35		1.3		0.110		---		Highly resistant
6		W-39		1.0		0.120		---		Highly resistant
7		W-41		1.0		0.120		---		Highly resistant
8		W-42		1.0		0.119		---		Highly resistant
9		W-43		1.0		0.112		---		Highly resistant
10		P-62/1		1.0		0.123		---		Highly resistant
11		P-83		1.0		0.109		---		Highly resistant
12		P-88		1.0		0.124		---		Highly resistant
13		P-93		1.0		0.114		---		Highly resistant
14		BY-27/2		1.0		0.084		---		Highly resistant
15		180		1.0		0.099		---		Highly resistant
16		SEP		1.0		0.080		---		Highly resistant
17		522		1.0		0.107		---		Highly resistant
18		W-24		1.3		0.113		Mosaic		Resistant
19		W-29		1.3		0.099		Mosaic		Resistant
20		W-32		1.2		0.126		Mosaic		Resistant
21		W-36		1.0		0.134		Mosaic		Resistant
22		W-37		1.2		0.132		Mosaic		Resistant
23		W-44		1.2		0.129		Mosaic		Resistant
24		P-63		1.0		0.130		Mosaic		Resistant
25		BY-31		1.0		0.115		Mosaic		Resistant
26		102-1		1.2		0.113		Mosaic		Resistant
27		541		1.2		0.137		Mosaic		Resistant
28		4		1.2		0.086		Mosaic		Resistant
29		145-SA		1.2		0.089		Leaf curling		Resistant
30		W-7		1.3		0.167		Mosaic		Moderately resistant
31		W-8		1.6		0.123		Mosaic		Moderately resistant
32		W-13		1.3		0.120		Mosaic		Moderately resistant
33		W-46		1.3		0.141		Mosaic		Moderately resistant
34		P-27		1.0		0.155		Chlorotic local lesions		Moderately resistant
35		YBB-36/2		1.3		0.117		Mosaic, Chlorotic local lesions		Moderately resistant
36		44-F3		1.2		0.140		Mosaic		Moderately resistant
37		508-T		1.3		0.129		Mosaic		Moderately resistant
38		530-2		1.0		0.133		Mosaic		Moderately resistant
39		145-4		1.2		0.122		Mosaic		Moderately resistant
40		W-38		1.3		0.154		Mosaic		Moderately susceptible
41		W-45		1.3		0.157		Mosaic		Moderately susceptible
42		P-43/1		1.3		0.143		Mosaic		Moderately susceptible
43		P-87		1.7		0.139		Mosaic		Moderately susceptible
44		P-92		1.6		0.193		Mosaic		Moderately susceptible
45		HBF-4/2		1.6		0.124		Necrotic local lesions		Moderately susceptible
46		23-1		1.7		0.139		Necrotic local lesions		Moderately susceptible
47		140 M		1.4		0.092		Yellowing, LC		Moderately susceptible
48		W-5		1.7		0.174		Severe mosaic		Susceptible
49		W-40		3.0		0.219		Severe mosaic		Susceptible
50		P-19/2		2.0		0.204		Chlorotic local lesions		Susceptible

Fresh symptomatic leaves of plants were harvested for use as inoculum sources, and these leaf tissues were used as inoculums and homogenized (1/5 w/v) in 0.05 M phosphate buffer, pH 7.2 (Ashfaq et al. 2014), containing 1% Na₂SO₃. The seedlings of each cabbage line, at the 2- to 4-leaf stage, were lightly dusted with carborundum (600 mesh), and the extracts were rubbed onto carborundum-dusted leaves of 50 plants of each cabbage line and grown for 6 weeks.

Forty-five days post-inoculation, the cabbage lines were visually assessed for symptom severity according to a 5-point rating scale proposed by Shin et al. (2013) and Ntui et al. (2014) with some modifications. A modified version of the scale was adopted for the study, and the lines were categorized in a 5-degree scale as highly resistant (HR), resistant (R), moderately resistant (MR), moderately susceptible (MS), and susceptible (S) on the basis of biological assay, symptoms expression, disease rating scale, and serological tests (Shin et al. 2013). All plants in each line were scored; the ratings totaled and were divided by the number of plants to give a disease index for the line.

CMV virions in plant tissue were analyzed by DAS-ELISA using polystyrene 96-well plates and a Bioreba (AG, Switzerland) phytodiagnostic ELISA kit according to the manufacturers’ guidelines. The plates were incubated at room temperature after pipetting the substrate buffer, and the absorbance values were read at 30, 60, and 120 min following the addition of substrate at 405 nm using a microplate reader (Tecan Spectra). It was also confirmed visually after incubation for 2 h at room temperature. All samples were tested in 2 replicate wells, and the absorbance value greater than 3 times that of non-inoculated negative control plants held in a growth chamber was rated as positive (Ashfaq et al. 2014).

The data were analyzed using the statistical software package SPSS v. 21.0 (SPSS, release V.21.0 for Windows; SPSS, Chicago, Illinois, USA). The results on the reaction of the 50 white head cabbage lines to the isolate of CMV under controlled conditions are given in Table 1. All lines were rated with disease severity indexes of 1 – 5 at the time of final observation. The appearance of the symptoms varied with the lines. The average intensity of symptoms expressed in the 5-degree scale is presented in Table 1.

Twenty-nine of the 50 B. olerecea lines tested showed symptoms of CMV including mosaic, severe mosaic, yellowing, leaf curling, chlorotic local lesions, and necrotic local lesions. Twenty-one lines did not exhibit symptoms and were found virus-free after testing with DAS-ELISA against CMV.

On the basis of host reactions and ELISA results, the lines were grouped as highly resistant, resistant, moderately resistant, moderately susceptible, and susceptible. Out of 50 lines screened for CMV resistance, 39 showed different levels of resistance. About 34% of total lines investigated showed high levels of resistance to CMV. Of the lines screened for CMV reactions, 12 (24%) were found to be resistant, 10 (20%) were found to be moderately resistant whereas 8 (16%) were moderately susceptible and 3 (6%) were susceptible (Table 1).

The reaction of these cabbage lines to CMV isolate has been summarized in Table 1. Based on both disease rating scale and ELISA, the results indicated that the lines W-1, W-31, W-33, W-34, W-35, W-39, W-41, W-42, W-43, P-62/1, P-83, P-88, P-93, BY-27/2, 180, SEP, and 522 were highly resistant; W-24, W-29, W-32, W-36, W-37, W-44, P-63, BY-31, 102-1, 541, 4, and 145-SA were resistant; W-7, W-8, W-13, W-46, P-27, YBB-36/2, 44-F3, 508-T, 530-2, and 145-4 were moderately resistant. Eight lines, i.e. W-38, W-45, P-43/1, P-87, P-92, HBF-4/2, 23/1, and 140 M were moderately susceptible whereas W-5, W-40, and P-19/2 were found to be susceptible against CMV.

There were significant differences (p < 0.05) among the lines with respect to the degree of susceptibility to CMV using the Tukey’s Studentized Range Test. Multiple linear regression analyses showed that there was a positive correlation between infection index and the degree of reaction against CMV.

Species of the family Brassicaceae (Cruciferae) may be infected by several viruses (Moreno et al. 2004). In a previous study in Turkey, CMV was newly reported by Erkan et al. (2013) on cabbage plants in Izmir province, Turkey. In the present investigation, 50 lines of B. oleracea were evaluated in the greenhouses of the BSARI, located in Samsun province. It has got the 1^st rank of Turkey’s cabbage production capacity for the reaction of resistance to CMV in 2013 – 2014. Forty-five days after inoculation, symptoms of infection were observed in approximately 58% of the tested lines. Similar observations were also reported by other researchers in cabbage plants.

In B. oleracea subsp. capitata f. alba ‘Amager’, TuMV presence was detected in 50% of plants inoculated with CAR39 isolate and in 42.5% of plants inoculated with CAR37A isolate. For the ‘Langedijker’ cultivar, the percentage of infected plants was 35 and 50, respectively, for CAR37A and CAR39 isolates (Gładysz and Hanus-Fajerska 2009).

The majority of tested lines were resistant to CMV. Seventeen of the 50 white cabbage lines tested showed highly resistance to CMV. These cabbage lines remained symptomless throughout the period of the study. Twelve and 10 lines were regarded as resistant and moderately resistant, respectively, based on both disease rating scale and ELISA tests. Similarly, 40 Chilli pepper genotypes were evaluated by mechanical inoculation, and resistance to CMV Chilli isolate was examined by visual observations and DAS-ELISA. Nine genotypes, C-2, CV-2, CV-5, BSS-269, PGRI, M-2001, CM-2001, M-97, and CP-328, were remained free of infection and catalogued as highly resistant. Among these genotypes, 5 were categorized as resistant, 7 as moderately resistant, 8 as moderately susceptible, and 11 as susceptible (Ashfaq et al. 2014). These results are in agreement with Rashid et al. (2007), who did not observe any infection by ELISA in the lines C-1, C-2, C-5, C-7, C-9, and C-11, which did show negative reaction to CMV.

In the present study, 8 cabbage lines tested were detected as moderately susceptible and 3 (W-5, W-40, and P-19/2) as susceptible against CMV. Similar results were reported by Pink and Walkey (1990) with ‘Polinius F1’ cultivar of white cabbage, and ‘Amager’ was also recognized as susceptible to TuMV isolates. The majority of tested lines were resistant to CMV, and the majority of highly resistant and resistant lines showed no reaction under mechanical inoculation. The present findings suggest that the lines showing resistance to CMV local isolate should be maintained for further studies for breeding purpose (Ashfaq et al. 2014).

In conclusion, the reactions of the cabbage lines evaluated in this study to CMV isolate ranged between highly resistant and susceptible. The tested lines showed different levels of susceptibility to CMV isolate. However, the majority of the lines were screened by mechanical inoculation in the present study, and certain amount of resistance was evident. Hence, they could be used for resistance breeding programs.

ACKNOWLEDGEMENTS

The authors would like to thank Ondokuz Mayis University and the Scientific and Technological Research Council of Turkey (TUBITAK) (112O578) for their financial support. We thank S. Sari for kindly providing the isolate used in the study.

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Publication Dates

• Publication in this collection
  12 Jan 2017
• Date of issue
  Jan-Mar 2017

History

• Received
  05 May 2016
• Accepted
  07 June 2016