Responses of the weed Bidens pilosa L. to exogenous application of the steroidal saponin protodioscin and plant growth regulators 24‐epibrassinolide, indol‐3‐acetic acid and abscisic acid

Abstract

The exogenous application of plant hormones and their analogues has been exploited to improve crop performance in the field. Protodioscin is a saponin whose steroidal moiety has some similarities to plant steroidal hormones, brassinosteroids. To test the possibility that protodioscin acts as an agonist or antagonist of brassinosteroids or other plant growth regulators, we compared responses of the weed species Bidens pilosa L. to treatment with protodioscin, brassinosteroids, auxins (IAA) and abscisic acid (ABA). Seeds were germinated and grown in agar containing protodioscin, dioscin, brassinolides, IAA and ABA. Root apex respiratory activity was measured with an oxygen electrode. Malondialdehyde (MDA) and antioxidant enzymes activities were assessed. Protodioscin at 48–240 μm inhibited growth of B. pilosa seedlings. The steroidal hormone 24‐epibrassinolide (0.1–5 μm) also inhibited growth of primary roots, but brassicasterol was inactive. IAA at higher concentrations (0.5–10.0 μm) strongly inhibited primary root length and fresh weight of stems. ABA inhibited all parameters of seedling growth and also seed germination. Respiratory activity of primary roots (KCN‐sensitive and KCN‐insensitive) was activated by protodioscin. IAA and ABA reduced KCN‐insensitive respiration. The content of MDA in primary roots increased only after protodioscin treatment. All assayed compounds increased APx and POD activity, with 24‐epibrassinolide being most active. The activity of CAT was stimulated by protodioscin and 24‐epibrassinolide. The results revealed that protodioscin was toxic to B. pilosa through a mechanism not related to plant growth regulator signalling. Protodioscin caused a disturbance in mitochondrial respiratory activity, which could be related to overproduction of ROS and consequent cell membrane damage.

The Steroid Saponin Protodioscin Modulates Arabidopsis thaliana Root Morphology Altering Auxin Homeostasis, Transport and Distribution

“…A similar value was found in our previous study with the weed I. grandifolia (54 µg mL −1 ) and D. insularis (34 µg mL −1 ) [51]. An ED 50 of 240 µM was found in B. pilosa by Mito et al, (2019). These values indicate that protodioscin has a higher inhibitory effect on weeds species when compared with the crop soybean.…”

“…The in vitro assay revealed the inhibitory effects of protodioscin on Arabidopsis root growth, confirming the higher phytotoxic potential of this saponin [ 49 , 50 , 51 ], compared to other natural molecules [ 53 ]. The inhibition of primary root elongation was even observed at low concentrations, with an ED 50 equal to 50 μM.…”

“…Despite reports on the phytotoxicity of protodioscin, its mechanisms of action have not yet been elucidated. A disturbance in mitochondrial respiratory activity and an oxidative stress condition was demonstrated in the primary roots of Bidens pilosa L. treated with protodioscin [ 50 ]. Recently, we demonstrated that the dicot Ipomoea grandifolia (Dammer) O’Donell and the monocot Digitaria insularis (L.) Fedde weed treated with protodioscin and butanolic extract of U. ruziziensis have drastic alterations in root morphology, including a reduction in the primary root length, the precocious appearance of lateral roots and reduction in root hairs [ 51 ].…”