1.4.11
CHEMICAL INDUCTION OF PR ENZYMES AND FUNGAL RESISTANCE IN CARICA PAPAYA

J ZHU1, S SCHENCK1, H ALBERT2 and PH MOORE2

1Hawaii Agriculture Research Center, 99-193 Aiea Heights Drive, Aiea, Hawaii 96701, USA; 2USDA-ARS, Tropical Fruits, Vegetables and Sugarcane Research Unit, 99-193 Aiea Heights Drive, Aiea, Hawaii 96701, USA

Background and objectives
Plants contain an array of defence genes, including pathogenesis-related (PR) genes that are activated by a gene-for-gene interaction with a specific avirulent pathogen. This induced defence response is systemic, and is termed systemic acquired resistance (SAR). SAR is transmitted throughout the plant by signal chemicals, can be long-lasting and has potential for replacing pesticides to protect crops against pathogen attacks. Salicylic acid (SA), a candidate plant signal for SAR, can activate SAR in the absence of a pathogen when it is applied exogenously to plants. Two analogues of SA, 2,6-dichloro-isonicotinic acid (INA) and benzol (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH), also induce SAR, are less toxic than SA when applied to plants, and have produced resistance to a broad spectrum of diseases in both dicotyledenous and monocotyledonous plants: tobacco [1, 2], wheat [3], and Arabidopsis [4]. The tropical fruit papaya, Carica papaya, is susceptible to several fungal diseases that are controlled in the field with pesticide applications. We are exploring the possibility of using SA- or BTH-induced SAR as an alternative approach to control the root and fruit rot diseases caused by Phytophthora palmivora.

Results and conclusions
Young papaya plants pretreated with 15 mM SA showed 35-50% fewer lesions and smaller infected areas 2 weeks after inoculation with P. palmivora than similar plants without SA treatment. BTH increased papaya plant resistance to P. palmivora at 1.0 mM. BTH at this concentration exhibited slight toxicity to the papaya seedlings but gave 100% survival against P. palmivora, whereas control plants treated with water showed over 70% mortality 5 days after inoculation. Studies on the effectiveness of a range of concentrations of BTH are reported. Enzyme activities of the pathogenesis-related proteins chitinase and beta-1,3-glucanase increased more than six-fold following BTH treatment, indicating that BTH is acting as a chemical inducer of SAR in papaya.

References
1. L Friedrich, K Lawton, W Ruess et al., 1996. Plant Journal 10, 61-70.
2. ER Ward, SJ Uknes, SC Williams et al., 1991. Plant Cell 3, 1085-1094.
3. J Gorlach, S Volrath, G Knauf-Beiter et al., 1996. Plant Cell 8, 629-43.
4. KA Lawton, L Friedrich, M Hunt et al., 1996. Plant Journal 10, 71-82.