Differential regulation of phenylalanine ammonia lyase activity and protein level by light in tomato seedlings.

Red light, acting via phytochrome, stimulates phenylalanine ammonia lyase (PAL) activity in cotyledons and hypocotyls of tomato seedlings. The time course of photoinduction of PAL activity has a peak level at 4 h after which activity declines significantly. In tomato seedlings PAL activity comprised of three isoforms and light stimulated activity of all three isoforms. A polyclonal antibody raised against PAL purified from tomato leaves recognized PAL protein belonging to PAL-II and PAL-III isoforms. The mode of increase in PAL activity was investigated by immunochemical techniques. The photostimulated increase in PAL activity appeared to be dependent on de novo synthesis of protein and nucleic acid. However, inhibition of protein phosphatase activity blocked increase in PAL activity without affecting the increase in PAL protein levels. The results indicate that in addition to de novo synthesis, the photostimulation of PAL activity likely requires dephosphorylation by a type 2C protein phosphatase.