A fluorescent compound in oat root plasma membrane

Abstract

Homogenates of 7‐day‐old oat (Avena sativa L. cv. Brighton) roots were highly fluorescent (excitation and emission maxima around 360 and 440 nm, respectively). Less than 1/10 as much fluorescence per g fresh weight was found in oat shoots or in wheat (Triticum aestivum L. cv. Drabant) roots or shoots. Most of the fluorescence of oat roots was found in the soluble fraction (150 000g supernatant). However, some could be detected in the plasma membrane fraction (excitation and emission maxima 365 and 417 nm, respectively), which contained a 3‐fold higher fluorescence per mg protein than the homogenate.Growth of oat or wheat in a medium containing, 10‐−5M scopoletin (6‐methoxy‐7‐hy‐droxy coumarin), a fluorescent compound previously reported to be present in both wheat and oat roots, caused the disappearance of scopoletin from the medium (proportional to the amount of roots) and the appearance of increased fluorescence in the root homogenates but not in the shoot homogenates. In both oat and wheat roots ail of the extra fluorescence was recovered in the soluble fraction and at least in wheat it consisted of unconverted scopoletin. The concentration of scopoletin in wheat roots grown in 10‐−5M scopoletin was around 50 nmol (g fresh weight)−1, or about five times the concentration in the growth medium. Scopoletin in the growth medium (10‐−5M) or in the assays (up to 10‐−4M) did not affect Mg2+‐, Mg2++K+‐ or Ca2+‐ATPase activities in wheat or oat roots.The fluorescence properties of the oat plasma membrane were different from those of authentic scopoletin. Either the surroundings modify the fluorescence of membrane‐associated scopoletin or the endogenous fluorescent compound is not scopoletin but a glycoside‐derivative of scopoletin or some completely unrelated compound.