Munronin O, a potential activator for plant resistance
Introduction
Tobacco mosaic virus (TMV) is a well-studied plant virus worldwide and causes significant crop loss. This virus infects >400 plant species of 36 families, including tobacco, tomato, potato, pepper, cucumber, and other plants [1]. Viral infection is extremely difficult to control under field conditions. Ningnanmycin is the most effective plant virus inhibitor and is used to prevent TMV disease; however, this antiviral is not widely used in crops due to photosensitivity and difficulty in applying the product [2]. Thus far, no chemical treatment can completely inhibit TMV once it has infected a plant [3]. Thus, experts face the challenge of fully protecting plants from TMV infection.
Resistance induced by activators, including salicylic acid (SA) and its derivatives [[4], [5], [6]], benzo[d][1,2,3]-thiadiazole-7-carboxylic acid (CGA210007) and its derivatives [7,8], and jasmonic acid (JA) [9] can provide defense against pathogens and these activators have been widely used agriculturally. Plant metabolic products are highly efficient and environment-friendly and involve unique modes of action [10,11]. Thus, these metabolic products are considered a rich source of plant activators that help protect plants from TMV infection. Some metabolic products exhibit good inhibitory activities against TMV, and these metabolic products include seco-pregnane steroids [12], cinchnaglycoside C [13], 7-deoxy-trans -dihydronarciclasine [14], 3-acetonyl-3-hydroxyoxindole [15], β-carbolines [16], quassinoids [17], limonoids [18], and eudesmanolides [19]. This property has been widely exploited in China to prevent the spread of plant viral diseases. Therefore, finding antiviral agents from natural products to protect plants has proven useful.
Up till now, only limited compounds with obvious activity inducing system acquired resistance (SAR) in plants have been reported, and the agricultural applications of plant inducers are far from being developed. Limonoids are a class of triterpenoids that exhibit wide range of bioactivities including anti-virus, antifeedant and have attracted a great deal of interesting for their potential in agriculture application [[20], [21], [22], [23], [24], [25]]. The present work investigated plant defense response mechanisms of compound 1; such mechanisms include defense-related enzyme activity, salicylic acid content, expression of deence-related gene and TMV CP gene. To the best of our knowledge, this study first demonstrated that compound 1 can enhance resistance against TMV as potential inducers of plant systemic resistance induction.
Section snippets
Antiviral biological assay
Nicotiana tabacum cv. K326 and Nicotiana tabacum L. plants were cultivated in a greenhouse. N. tabacum cv. K326 was used to determine systemic TMV infection and N. tabacum L. was used as local lesion host when the plants grew to the 5–6 leaf stage. TMV was purified by Gooding method [26], and in vivo actions of compounds were determined through a reported technique [27]. The commercial antiviral agent ningnanmycin was used as a positive control. Measurements were performed in triplicate.
Plant growth and compound treatments
N.
Antiviral activity against TMV in vivo
Antiviral activities of compounds 1–8 (Fig. 1) against TMV were tested and are reported in Table 1 [30,31]. Some compounds exhibited remarkable antiviral activities against TMV at 32 μg/mL. 1, 7, and 8 exhibited superior inactivation activities with values of 57.4%, 94.1% and 93.3%, respectively, compared with ningnanmycin (58.9%). 1, 7, and 8 exhibited significantly greater protective activities against TMV with values of 81.0%, 55.4% and 66.7%, respectively, than ningnanmycin (62.8%). 7 and 8
Disscusion
To further confirm the anti-TMV activities of compounds 1–8, the relative content of TMV in inoculated and systematic leaves was measured. With the treatment on lower leaves by compounds 1, the CP bands of newly grown leaves were undetected. It was possible that compounds 1 induced systemic acquired resistance in the plants to inhibit virus accumulation and movement. Among the tested compound 1 showed higher induction activities on both treated leaves and newly grown leaves. The physiological
Conclusion
In summary, eight limonoids, containing different substituent groups at C-17 were isolated from the whole plant of Munronia henryi Harms. In vivo studies indicated that compounds 1, 7, and 8 exhibited better curative, protective, and inactivating activities than that of ningnanmycin, especially compound 1, whose ethynyl moiety at C-17 exhibited the best protective activity among these compounds. This protective ability was associated with potentiation of defensive enzyme activity, salicylic
Acknowledgment
The work was supported financially by the National Natural Science Foundation of China (No. 31160374), the Scientific and Technological Planning Project of Guizhou (No. QKH-JC [2017]1141), and initial funding of PhD (No. YBH-J [2016-7]).
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