Phytoecdysteroid Turnover in Spinach: Long-term Stability Supports a Plant Defense Hypothesis.

Using short (8-day) and long-term (28-day) experiments, we examined the stability of 20-hydroxyecdysone (20E) and the dominant phytosterols synthesized from a pulse of [2-¹⁴C]mevalonic acid ([¹⁴C]MVA) in hydroponically grown spinach ( Spinacia oleracea). In the short-term experiment, plant dry mass and shoot 20E pools steadily increased. Root uptake of [¹⁴C]MVA resulted in the stable incorporation of ¹⁴C radiolabel into whole plant 20E pools, with no significant changes over time. Levels of free and saponifiable phytosterols increased in the shoots while ¹⁴C-labeled shoot phytosterols remained constant. Unexpectedly, both ¹⁴C-labeled and unlabeled pools of root phytosterols decreased over time. In the long-term experiment, plant dry mass and shoot 20E levels increased over time, while total ¹⁴C-labeled 20E pools remained constant. Both root and shoot phytosterol pools increased over time while the ¹⁴C incorporation in these pools remained constant. Together these experiments indicate that 20E in spinach is metabolically stabile and thus shares this characteristic with plant terpenoids of known defensive function. While little is known about phytosterol turnover in plants, our results suggest that phytosterols can indeed exist in a very dynamic state but may also be stable over time. [ABSTRACT FROM AUTHOR]