Phytotoxic Effects of Leukamenin E (an ent-kaurene diterpenoid) on Root Growth and Root Hair Development in Lactuca sativa L. seedlings

Leukamenin E, an ent-kaurene diterpenoid isolated from Isodon racemosa (Hemsl) Hara, showed phytotoxic effects on root growth and root hair development of lettuce seedlings (Lactuca sativa L.). Lower concentrations (10 μM) of leukamenin E did not affect root growth, but at concentrations higher than 50 μM, the rate was inhibited. The influence of leukamenin E on root growth rate was closely correlated with alterations in the mitotic index. A low incidence of aberrant mitosis image was observed when lettuce roots were treated with higher concentrations (100 and 200 μM) of leukamenin E. This suggests that inhibition of root growth may be due to inhibition of cell division. All tested concentrations of the diterpenoid (10 μM or more) inhibited root hair development in a dose-dependent manner. At a concentration of 80 μM, leukamenin E completely blocked root hair initiation. Application of Ag⁺—an ethylene action inhibitor—to lettuce seedlings inhibited root hair elongation similar to the diterpenoid. Enhanced root hair length was stimulated by exogenous ethephon—an ethylene-releasing agent—and could be reversed by addition of leukamenin E. This suggests that leukamenin E may act as a potential ethylene action antagonist in the inhibition of lettuce root hair development. We conclude that leukamenin E may curb root hair development by interfering with ethylene action at concentrations above 10 μM and inhibits root growth via inhibition of cell division at concentrations above 50 μM. Lan Ding and Linlin Qi contributed equally to this work.