Design, Synthesis, Antibacterial Activity, and Mechanism of Novel Mesoionic Compounds Based on Natural Pyrazole Isolated from an Endophytic Fungus Colletotrichum gloeosporioides

Xanthomonas oryzae pv. oryzicola (Xoo) is a type of bacteria that
causes bacterial
leaf blight disease in rice plants. This disease is substantially
harmful, and the current prevention and control measures are facing
challenges. This study has investigated the effectiveness of the control
activity that the endophytic fungus NS7 fermented from Dendrobium candidum possessed against Xoo. Twenty-eight novel mesoionic compounds were designed and synthesized
based on the natural compound D. These compounds displayed
moderate to excellent anti-Xoo activity in
vitro. Notably, compound 24 exhibited prominent
anti-Xoo activity in vitro with
an EC50 value of 40.3 mg/L, which was better than that
of the positive control thiodiazole copper (TC)(71.2 mg/L) and the
lead compound D (108.1 mg/L). In vivo pot experiments on Xoo showed that compound 24 exhibited protective and curative activities of 39.4 and
30.4%, respectively, which were better than those of TC (35.7 and
28.8%, respectively). Further, a preliminary mechanism study indicated
that compound 24 could enhance the activity of defense
enzymes to improve the ability for anti-Xoo. Meanwhile,
compound 24 could also regulate the carbon fixation in
photosynthetic organisms, which might be related to the enhanced immune
function of rice. This study offers a new strategy for discovering
antibacterial agents based on natural products.