Abstract
Preharvest application of hormetic doses of ultraviolet-C (UV-C) generates beneficial effects in plants. In this study, within 1 week, four UV-C treatments of 0.4 kJ/m2 were applied to 3-week-old lettuce seedlings. The leaves were inoculated with a virulent strain of Xanthomonas campestris pv. vitians (Xcv) 48 h after the last UV-C application. The extent of the disease was tracked over time and a transcriptomic analysis was performed on lettuce leaf samples. Samples of lettuce leaves, from both control and treated groups, were taken at two different times corresponding to T2, 48 h after the last UV-C treatment and T3, 24 h after inoculation (i.e., 72 h after the last UV-C treatment). A significant decrease in disease severity between the UV-C treated lettuce and the control was observed on days 4, 8, and 14 after pathogen inoculation. Data from the transcriptomic study revealed, that in response to the effect of UV-C alone and/or UV-C + Xcv, a total of 3828 genes were differentially regulated with fold change (|log2-FC|) > 1.5 and false discovery rate (FDR) < 0.05. Among these, of the 2270 genes of known function 1556 were upregulated and 714 were downregulated. A total of 10 candidate genes were verified by qPCR and were generally consistent with the transcriptomic results. The differentially expressed genes observed in lettuce under the conditions of the present study were associated with 14 different biological processes in the plant. These genes are involved in a series of metabolic pathways associated with the ability of lettuce treated with hormetic doses of UV-C to resume normal growth and to defend themselves against potential stressors. The results indicate that the hormetic dose of UV-C applied preharvest on lettuce in this study, can be considered as an eustress that does not interfere with the ability of the treated plants to carry on a set of key physiological processes namely: homeostasis, growth and defense.
Highlights
Plants live in a complex and constantly changing environment, where they continuously interact with biotic and abiotic factors (Foyer et al, 2016)
This work confirms that a UV-C treatment of 1.6 kJ/m2 applied as described by Nicolas et al (2020) falls within the hormetic dose range conferring a protective effect on lettuce against bacterial leaf spot (BLS)
We show that the early responses of lettuce treated with UV-C followed by Xanthomonas campestris pv. vitians (Xcv) inoculation led to a change in the transcriptome, which likely translated into a return to homeostasis, resumption of normal growth and the activation of defense mechanisms
Summary
Plants live in a complex and constantly changing environment, where they continuously interact with biotic and abiotic factors (Foyer et al, 2016). These factors can be the cause of severe economic losses in edible plants such as lettuce, one of the most widely consumed leafy vegetables in the world (Moraes et al, 2018). To obtain the expected biological response, pesticides are used excessively and repeatedly, resulting in enormous costs and posing a serious threat to the environment and human health (Konatu and Jardim, 2018). New and effective biological approaches to improve the control of BLS are urgently needed
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