On the Inside

摘要

The growth rates of lily (Lilium formosanum) pollen tubes oscillate with a period of 20 to 50 s during polarized tip growth. Many physiological processes underlying polarized tip growth also oscillate with the same period as growth but not with the same phase and amplitude. Among the oscillatory processes, most attention has been directed toward ions. Cl– efflux mirrors growth rate, whereas changes in intracellular and extracellular Ca2+, as well as extracellular H+ and K+, occur after growth. In searching for events that anticipate and potentially lead growth, changes in metabolism are of special interest since energy production and the processes that drive the synthesis of new cell wall material must at some level precede and anticipate the changes in growth rate. The high energy status and reducing power of NADH and NADPH collectively [NAD(P)H] drive many key biosynthetic reactions and ATP production. Moreover, since NAD(P)H but not NAD(P)+ possesses an endogenous fluorescence, it is possible to detect the reduced form in living cells. Cárdenas et al. (pp. 1460–1468) have examined changes in NAD(P)H during oscillatory pollen tube growth. They report that NAD(P)H oscillates with the same period as growth but not the same phase. The strongest signal resides 20 to 40 µm behind the apex where mitochondria accumulate. The authors suggest that the changes in fluorescence reflect an oscillation between the reduced (peaks) and oxidized (troughs) states of NAD(P)H, and that an increase in the oxidized state [NAD(P)+] may be coupled to the synthesis of ATP. The troughs in NAD(P)H appear to anticipate changes in growth rate and, thus, might be key players in polarized tip growth.