Abstract
Biosynthesis and accumulation of flavonolignans in plants are influenced by different environmental conditions. Biosynthesis and accumulation of silymarin in milk thistle (Silybum marianum L.) were studied under drought stress with respect to the antioxidant defense system at the physiological and gene expression level. The results revealed a reduction in leaf chlorophyll, ascorbic acid, and glutathione contents. In contrast, H2O2, proline, and antioxidative enzyme activities, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), and glutathione reductase (GR), were increased. These results confirmed that milk thistle undergoes oxidative stress under drought stress. Furthermore, transcription levels of APX, SOD, CAT, 1-Cys-Prx, and PrxQ were significantly increased in milk thistle under drought stress. Overall this suggests that protection against reactive oxygen species and peroxidation reactions in milk thistle are provided by enzymatic and non-enzymatic antioxidants. Flavonolignans from milk thistle seeds after different drought treatments were quantified by high-performance liquid chromatography (HPLC) and showed that severe drought stress enhanced the accumulation of silymarin and its components compared with seeds from the control (100% water capacity). Silybin is the major silymarin component and the most bioactive ingredient of the milk thistle extract. Silybin accumulation was the highest among all silymarin components in seeds obtained from drought-stressed plants. The expression of the chalcone synthase (CHS) genes (CHS1, CHS2, and CHS3), which are associated with the silybin biosynthetic pathway, was also increased during drought stress. These results indicated that milk thistle exhibits tolerance to drought stress and that seed derived from severe drought-stressed plants had higher levels of silymarin.
Highlights
Milk thistle (Silybum marianum (L.) Gaertn, Asteraceae) is an annual herbaceous plant, which is widespread in temperate and Mediterranean climatic regions [1]
A drought stress effect on the growth of milk thistle was evident at 75% water capacity, as indicated by the significant reduction of fresh weight (FW) and dry matter (DM) at 50% water capacity treatment compared with the well-watered plants (Figure 1A,B)
Total chlorophyll content of milk thistle leaves decreased under drought stress at 50% and 75% water capacity treatments (Figure 1C)
Summary
Milk thistle (Silybum marianum (L.) Gaertn, Asteraceae) is an annual herbaceous plant, which is widespread in temperate and Mediterranean climatic regions [1]. Drought stress results in the generation of reactive oxygen species (ROS), which have negative consequences at a cellular level: damage of membrane, DNA, lipids, and amino acids and limiting the activities of several enzymes [11]. To minimize these damaging effects, plants activate antioxidative guard systems involving both enzymatic and non-enzymatic antioxidants. The non-enzymatic antioxidants are very important because they include phenolics and flavonoids, which defend plants from excessive ROS damage [14] and certain of these metabolites have specific medicinal properties that have been shown to be useful in the treatment of various human diseases and illnesses [14]
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