Phytoecdysteroid turnover in spinach: Long-term stability supports a plantdefense hypothesis

Using short (8-day) and long-term (28-day) experiments, we examined the sta bility of 10-hydroxyecdysone (20E) and the dominant phytosterols synthesize d from a pulse of [2-C-14]mevalonic acid ([C-14]MVA) in hydroponically grow n spinach (Spinacia oleracea). In the short-term experiment, plant dry mass and shoot 20E pools steadily increased. Root uptake of [C-14]MVA resulted in the stable incorporation of C-14 radiolabel into whole plant 20E pools, with no significant changes over time. Levels of free and saponifiable phyt osterols increased in the shoots while C-14-labeled shoot phytosterols rema ined constant. Unexpectedly, both C-14-labeled and unlabeled pools of root phytosterols decreased over time. In the long-term experiment. plant dry ma ss and shoot 20E levels increased over time, while total C-14-labeled 20E p ools remained constant. Both root and shoot phytosterol pools increased ove r rime while the C-14 incorporation in these pools remained constant. Toget her these experiments indicate that 20E in spinach is metabolically stabile and thus shares this characteristic with plant terpenoids of known defensi ve function. While little is known about phytosterol turnover in plants, ou r results suggest that phytosterols can indeed exist in a very dynamic stat e but may also be stable over time.