Effect of Temperature Regime and Growth Stage Interaction on Pattern of Virus Presence in TSWV-Resistant Accessions of Capsicum chinense

Effect of Temperature on Symptoms Expression and Viral RNA Accumulation in Groundnut Bud Necrosis Virus Infected Vigna unguiculata

Background

Groundnut bud necrosis virus (GBNV) (Tospovirus genus, Bunyaviridae family) infects the major crops of solanaceae, leguminosae and cucurbitaceae in India. Temperature is an important factor which influences the plant growth and development under diseased conditions.

Objective

In the present study, we evaluated the effect of four different temperatures on the symptoms expression and viral RNA accumulation in the GBNV inoculated cowpea plants.

Material and Methods

For the evaluation of viral symptoms and RNA accumulation, we used 2–3 leaf stage cowpea plants grown in the controlled conditions. GBNV was mechanically inoculated by sap method to the cowpea plants and inoculated plants were incubated at four different temperatures (30, 25, 20 and 15 °C).

Results

The first visible symptom of GBNV infection at the inoculated site was observed in the form of chlorotic spots which were converted into the necrotic spots as the infections succeeded. Some yellow mosaic symptoms were also observed at the systemic site during viral infection cycle. Plants incubated at higher (30 and 25 °C) temperatures showed a severe necrosis and a higher viral RNA accumulation at the inoculated site and facilitated the viral spread at the systemic site. However, viral RNA accumulation was less at the systemic site than the inoculated site. In contrast, symptoms’ expression and viral RNA accumulation were decreased at the inoculated site at low (20 and 15 °C) temperatures, no viral symptoms were observed at the systemic site (15 °C); in addition to viral RNA accumulation suppression at this site. GBNV infection at the inoculated site induced the higher accumulation of H₂O₂ followed by the induction of cell death at higher temperatures (30 and 25 °C) than the lower (20 and 15 °C) temperatures.

Conclusion

This study suggests that viral RNA accumulation parallels with the H₂O₂ production and induction of cell death by GBNV infection in cowpea plants is temperature dependent.

HR-Mediated Defense Response is Overcome at High Temperatures in Capsicum Species

Resistance to Tomato spotted wilt virus isolated from paprika (TSWV-Pap) was overcome at high temperatures (30 ± 2°C) in both accessions of Capsicum annuum S3669 (Hana Seed Company) and C. chinense PI15225 (AVRDC Vegetable Genetic Resources). S3669 and PI15225, which carrying the Tsw gene, were mechanically inoculated with TSWV-Pap, and then maintained in growth chambers at temperatures ranging from 15 ± 2°C to 30 ± 2°C (in 5°C increments). Seven days post inoculation (dpi), a hypersensitivity reaction (HR) was induced in inoculated leaves of PI152225 and S3669 plants maintained at 25°C ± 2°C. Meanwhile, necrotic spots were formed in upper leaves of 33% of PI15225 plants maintained at 30 ± 2°C, while systemic mottle symptoms developed in 50% of S3669 plants inoculated. By 15 dpi, 25% of S3669 plants had recovered from systemic mottling induced at 30 ± 2°C. These results demonstrated that resistance to TSWV-Pap can be overcome at higher temperatures in both C. chinense and C. annuum. This is the first study reporting the determination of temperatures at which TSWV resistance is overcome in a C. annuum genetic resource expressing the Tsw gene. Our results indicated that TSWV resistance shown from pepper plants possess the Tsw gene could be overcome at high temperature. Thus, breeders should conduct evaluation of TSWV resistance in pepper cultivars at higher temperature than 30°C (constant temperature).

Effects of Temperature on Systemic Infection and Symptom Expression of Turnip mosaic virus in Chinese cabbage (Brassica campestris)

Using the Chinese cabbage (Brassica campestris) cultivar 'Chun-goang' as a host and turnip mosaic virus (TuMV) as a pathogen, we studied the effects of ambient temperature (13°C, 18°C, 23°C, 28°C and 33°C) on disease intensity and the speed of systemic infection. The optimal temperature for symptom expression of TuMV was 18-28°C. However, symptoms of viral infection were initiated at 23-28°C and 6 days post infection (dpi). Plants maintained at 33°C were systemically infected as early as 6 dpi and remained symptomless until 12 or 22 dpi, depending on growth stage at the time of inoculation. It took 45 days for infection of plants grown at 13°C. Quantitative real-time polymerase chain reaction (q-PCR) results showed that the accumulation of virus coat protein was greater in plants grown at 23-28°C. The speed of systemic infection increased linearly with rising ambient temperature, up to 23°C. The zero-infection temperature was 10.1°C. To study the effects of abruptly elevated temperatures on systemic infection, plants inoculated with TuMV were maintained at 10°C for 20 d; transferred to a growth chamber at temperatures of 13°C, 18°C, 23°C, 28°C, or 33°C for 1, 2, or 3 d; and then moved back to 10°C. The numbers of plants infected increased as duration of exposure to higher temperatures and dpi increased.

Effect of plant age at inoculation on expression of genetic resistance to tomato yellow leaf curl virus

To determine the effects of plant age on the expression of genetic resistance to tomato yellow leaf curl virus (TYLCV), six TYLCV-resistant and two susceptible tomato varieties were inoculated at 14, 28 or 45 days after sowing (DAS). Inoculation at 14 and 28 DAS was performed in the greenhouse, and the plants were transplanted to the field at 30 DAS. Inoculation at 45 DAS was performed in the field, by covering the target plants with polypropylene ("Agril") sheets and releasing viruliferous whiteflies under them. Resistance was assayed mainly by comparing yield components of inoculated plants to those of control, non-inoculated plants of the same variety. Symptom severity and plant height were also followed. Plant age at inoculation had no effect on disease-severity scores of the susceptible varieties, and little or no effect on those of the resistant varieties. In contrast, plant age at inoculation had a significant effect on the yield of all varieties tested. All varieties suffered a significant yield reduction due to inoculation with TYLCV; the lowest yield was produced by plants inoculated at 14 DAS. A smaller TYLCV-induced yield reduction (yield increase of 50 to 100%, depending on the variety's resistance level), was achieved following inoculation at 28 DAS. A further reduction in yield loss (yield increase of 30 to 40%) was achieved following inoculation at 45 DAS. Our results clearly demonstrate the occurrence of age-related (or mature-plant) resistance in tomato plants to TYLCV.

Rhizobacteria-Mediated Growth Promotion of Tomato Leads to Protection Against Cucumber mosaic virus

ABSTRACT We evaluated combinations of two strains of plant growth-promoting rhizobacteria (PGPR) formulated with the carrier chitosan for the ability to induce growth promotion of tomato plants and resistance to infection by Cucumber mosaic virus (CMV). Each PGPR combination included GB03 (Bacillus subtilis) and one of the following PGPR strains: SE34 (B. pumilus), IN937a (B. amyloliquefaciens), IN937b (B. subtilis), INR7 (B. pumilus), or T4 (B. pumilus). The PGPR combinations formulated with chitosan are referred to as biopreparations. Tomato plants treated with each of the biopreparations appeared phenotypically and developmentally similar to nonbacterized control plants that were 10 days older (referred to as the older control). When plants were challenged with CMV, all plants in the biopreparation treatments and the older control treatment had significantly greater height, fresh weight, and flower and fruit numbers than that of plants in the CMV-inoculated same age control treatment. CMV disease severity ratings were significantly lower for biopreparation-treated and older control tomato plants than for that of same age control plants at 14 and 28 days postinoculation (dpi). CMV accumulation in young noninoculated leaves was significantly less for all biopreparation-treated plants and those in the older control than for the same age control plants at 14 dpi and for four of the five biopreparation treatments at 28 dpi. In those tomato plants shown to be infected, the amount of CMV in noninoculated leaves was significantly lower for three of the biopreparation treatments and the older control treatment at 14 dpi and biopreparation G/INR7 treatment at 28 dpi when compared with the control treatment. These data show that treatment of tomato plants with biopreparations results in significant enhancement of growth and protection against infection by CMV.

A CAPS marker to assist selection of tomato spotted wilt virus (TSWV) resistance in pepper

The hypersensitive resistance to tomato spotted wilt virus (TSWV) in pepper is determined by a single dominant gene (resistant allele: Tsw) in several Capsicum chinense genotypes. In order to facilitate the selection for this resistance, four RAPD (among 250 10-mer primers tested) were found linked to the Tsw locus using the bulked segregant analysis and 153 F2 individuals. A close RAPD marker was converted into a codominant cleaved amplified polymorphic sequence (CAPS) using specific PCR primers and restriction enzymes. This CAPS marker is tightly linked to Tsw (0.9 ± 0.6 cM) and is helpful for marker-assisted selection in a wide range of genetic intercrosses. Key words: Capsicum sp., PCR marker, bulked segregant analysis, Tospovirus, resistance gene.