c HIGH TEMPERATURE EFFECT ON TOMATO PREDISPOSITION TO TSWV Mariel Mitidieri EEA INTA San Pedro. CC 43.CP 2930 San Pedro. Bs. As. Argentina. TE/FAX: (03329) 424074 E-mail:
[email protected]
Elvio Biderbost , Pilar Castellano and Edith Taleisnik IFFIVE -INTA. Camino 60 Cuadras km 5,5. Córdoba. Argentina. TE: (0351) 4973636
Keywords: Heat shock, virus, salicylic acid Abstract | ' | Tüinato spotted wilt is a inajor disease aííecting greenhouse cultivated tomato. Several i cnvironmental and plant factors can affect the severity of this disease, among them high j iemperatures. Salicylic acid participates in the physiological mechanisms leading to disease symptorn expression, and high temperaturas can modify its accumulation. The purpose of this work was to determine whether the exposure of tomato seedlings to high iemperatures, prior to infection, affects their response to TSWV, and if salicylic acid treatments can revert the heat stress effects. Tests were run in two tomato hybrids: one susceptible and the other carrying gene Sw5, resistant to TSWV. High temperatura prior lo mechanical inoculation increased the severity of TSWV infections. In the susceptible cultivar, more severa symptoms were observed when plants were stressed before inoculation, and yield was lower. In inoculated folióles of the resistant cultivar, heat stress increased the number of local lesions and absorbance valúes of ELISA tests. Exogenous salicylic acid applications to the susceptible cultivar prior to inoculation, resulted in less severe symptoms in non-lieated plants. Salicylic acid caused a reduction in virus titer at ihe beginning of infection in liealed plants, suggesting it is involved in the response of heat-stressed tomato plants to TSWV infections. I.
Introduction
Tomato spotted wilt is one of the most serious virus diseascs affccting tomato plants in Argentina, wherc it is caused by three tospoviruses: tomato spotted wilt (TSWV), tomato chlorotic spot virus (TCSV) and groundnut ring spot virus (GRSV) (De Avila, 1992; Boiteux et al., 1993; Dewey et al., 1994). Damage caused by this disease is importan! in terms of both yield losses and of the environmentál impact of phytosanitary control measures exerted against the main vector for these viruses, the thrips Frcuikliniellu occidentalis. The use of cultivars with genetic tolerance/resistance has proved to be an cfíective strategy to avoid the disease (Roselló et al., 1998) and gene Sw5 (Stevens et al., 1992) is the most common resistance source used by breeders (Roselló el ai., 1998).Virus disease symptom expression varíes among host species, and is influenced by plant age (Kaminska, 1993), culture and cnvironmental conditions (Best, 1968). In the specific case of tomato spotted wilt, it has been reported that increasing nitrogen fertilizer resulted in increased disease incidence in lettuce (Jordá et al., 1996), severe water stress reduced symptom expression in tomato (Córdoba et al., 1991), while high temperatures prior to infection increased symptoms in different hosts (Kassanis, 1952, Llamas-Llamas et al., 1998). In Argentina, high temperatura conditions are not uncomrnon in commercial greenhouses. Salicylic acid (SA) applications can induce resistance mechanisms against many plant pathogens (Chivasa et al., 1997) and can mitígate symptom expression, as scen in tobáceo inoculated with TMV (Raskin, 1992). Apparently, SA can interfere with virus accumulation at the point of inoculation and also inhibit virus movement out of inoculated tissue (Murphy el al., 1999). High temperatures may modify symptom expression, in association with a reduced accumulation of this compound (Raskiri, 1992,
Ite. 5ih IS Proitci. Culi. Milu WinterCIim. I Jv Ivnuíiidc/, M;uunc/. & C ¿islilla \,l.i Horl. 559, I S I I S 2001
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Yalpani el ai, 1991, Malamy el al., 1992). The purpose of this work was lo determine whether heat stress conditions prior to infection influence the severity of TSWV diseñe (and thus. tomato fruit production) and whether SA applications can alter the responde ti> this stress. 2.
Materials and Methods 2.1. Piant material and culture conditions
Two indeterminate commercial tomato hybrids were used, Bonanza (susceptible m TSWV) and other heterozygous for resistance gene Sw5. Seeds were sown in a gromili room, and seedlings bearing two leaves were transplanted individually to 1 L pots \viih sterilizcd gardcn soil. Plants were kept at 25 °C, under a 16~h photoperiod. When pliints had three expanded leaves, heat stress (40° C for 24 h) was applied to half of the plañís. Treated plants were then returned to 25 °C and all plants were mechanically inocúlala! with TSWV when leaf temperature in both groups was similar (approximately 1 hr aftcr the end of the heat treatment). Plants for yield assessment were treated as described abo', e ancl were transplanted, 15-20 days after inoeulation, to a greenhouse with insect protcction (wire mesh of 15x15 wires per cm in al i windows). In addition, thrips control was excrtcd with formetanate (200 g/hl). 2.2. Inoculum source, inoculation procedure and disease evaluation
SWV-infected Nicotiana rustica plants expressing characteristic syslemic discase symptoms and testing positive to ELISA were used as inoculum source. Leaves from such plants were homogenized in 0.01 N Na2?O4 buffer (1:7 W:V), and the resulling slurry was used for rub-inoculating tomato plants. Three folióles of the second and third leal were inoculated with the aid of Carborundum mesh 400 as abrader. Control plants weie mock-inoculated with healthy Nicotiana rustica extraéis. Symptom severity in llie susceptible cultivar plants was evaluated wilh a 10 point visual scale, as described ' \a et al. (1 healthy and dead plants, respectively. In the TSWV-resistant cultivar the numbcr oí lesions in inoculated folióles was counted daily. The presence of the virus was detecleci h\A (Clark concentration indirectly inferred from A41Q valúes. In the resistant cultivar, the infectivo capacity of ELISA-positive folióles was determined by using the same folióles te inocúlate Nicotiana rustica plants, which were later tested for TSWV infections In ELISA. All experiments were repeatecl at least 2 times with similar results, and data from one of the experiments are reported in tables and graphs. Results were subject to analysis of variance using the SAS statistical packagc, procedure GLM (Freund and Littel, 1981). 3.
Results and discussion 3.1. Effect of TSWV infection and heat stress during seedling growth on tomato yield and fruit quality of a susceptible cultivar
Fifteen days after inoculation, apical folióles from heat-treated plants of the susceptible cultivar had significantly higher (P 200 g), médium (gTj 150-200 g) and small (•| 100-150 g) fruits comprising marlcetable yield
(kg/plant) 6
cracked deformed small virus-affecte marketable
V
HS
HS + V
treatment
785
