Abstract:

The study was conducted to study the effect of sugar metabolism on the resistance of tomato to bacterial leaf spot disease (Pst DC3000) and the physiological and biochemical mechanisms possibly involved, which could benefit the breeding of new tomato varieties with high resistance to Pst DC3000. To get leaves with different characteristics in sugar metabolism,tomato leaves were sampled at two different time (8:00 AM and 6:00 PM), which were referred to as morning-sampled leaves and evening-sampled leaves, respectively. These two kinds of leaves were in vitro inoculated with Pst DC3000 and then tested at 0, 24 and 48 h after inoculation for several indicators, including Pst DC3000 resistance, bacterial density, the content of soluble sugar and starch, invertase activities, the content of salicylic acid (SA) and jasmonic acid (JA), cell death incidence and H₂O₂ accumulation. It was found that, as compared with the morning-sampled leaves, the evening-sampled leaves showed higher resistance to Pst DC3000 as revealed by less incidence of disease lesion and cell death, and lower bacterial density in leaves (P≤0.01). Furthermore, evening-sampled leaves had higher starch content (0-48 h), glucose content (0 h and 24 h), and fructose content (24 h and 48 h). However, there was no significant difference in sucrose content between two kinds of leaves. In addition, the measurement of three kinds of invertase activities showed that evening-sampled leaves had lower cell wall invertase (CWIN) activity at 0 h and 48 h after inoculation, but significantly (P≤0.05) higher cytoplasmic invertase (CIN) activities at 24 h and 48 h after inoculation than morning-sampled leaves. Notably, there was no significant difference in vacuolar invertase (VIN) activity between two kinds of leaves, Finally, as compared with the morning-sampled leaves, evening-sampled leaves showed significantly (P≤0.05)higher content of free SA and JA, and simultaneously less H₂O₂ accumulation at 48 h after inoculation. In conclusion, as compared with the morning-sampled leaves, evening-sampled leaves showed higher starch and hexose content, CIN activity, but lower CWIN activity. These differences in sugar metabolism may contribute to higher SA and JA content and less H₂O₂ accumulation and cell death in evening-sampled leaves, and ultimatly enhance its resistance to Pst DC3000.

Key words: tomato, bacterial leaf spot, cell wall invertase, cytoplasmic invertase, sugar content