PLC1 and H2O2 Mediated the Allelopathic Effect of Oridonin on Root Growth in Arabidopsis thaliana

Oridonin is a diterpenoid isolated from medicinal herb Rabdosia rubescens (Hemsl.) Hara (Lamiaceae), which has an allelopathic effect on plants. Phospholipase C (PLC1) and hydrogen peroxide (H2O2) are involved in many biotic or abiotic stress responses. Using the 16-day-old seedlings of Arabidopsis thaliana ecotype (WT) and PLC1-deficient mutant (plc1) as materials (treated with 10 mu M or 60 mu M oridonin for 72 h), the effect of oridonin on root growth regulating by PLC1 and H2O2 was investigated. The results showed that the promoting of root growth was about 6.9% at 10 mu mol L-1 oridonin and the inhibiting of root growth was about 19.73% at 60 mu mol L-1 oridonin in WT, the inhibiting of root growth was about 10.5% and 41.2% at 10 mol L-1 and 60 mol L-1 oridonin, respectively, in plc1. The expression of ARR1, ARR12, and AHK3 was promoted at low concentrations of oridonin and inhibited at high concentrations in WT, whereas the expression of ARR1 and ARR12 was inhibited with the increase of oridonin concentration in plc1. This suggested that PLC1 was involved in the root growth regulation of oridonin. H2O2 was promoted by oridonin with concentration dependence pattern in root cells. Oridonin increased the activity of antioxidant enzymes in both WT and plc1, but the activity of antioxidant enzymes in plc1 was lower than WT. This indicated that PLC1 involved in the activation of antioxidant enzymes promoted by the oridonin. Exogenous CaCl2 facilitated the accumulation of H2O2 in both WT and plc1. And the H2O2 of WT was obviously higher than that of plc1. The root growth of WT was inhibited by CaCl2 with the increase of oridonin. However, there is no effect of CaCl2 on the root growth in plc1. This reflected that PLC1 positively involved in the regulation of Ca2+ on the H2O2 and the inhibition effect of Ca2+ on the root growth under oridonin treatment. PA promoted the H2O2 and suppressed the root growth under oridonin treatment in both WT and plc1. In plc1, PA facilitated the root growth with no oridonin and inhibited the root growth with the increase of oridonin. This reflected that PLC1 positively regulated the promotion effect of PA on the root growth under high oridonin treatment. PLC1 mediated oridonin (10 and 60 mol L-1) to regulate H2O2 levels in A. thaliana seedlings, thereby regulating root tip cell morphology and mitosis. These results demonstrated that PLC1 mediated the low-promotion and high-inhibition effect of oridonin on the root growth in A. thaliana by regulating the concentrations of Ca2+ and PA, and further affecting the intracellular H2O2 level.