Dependence of starch synthase activity from maize leaves on light, pH and redox state

The starch synthase activity of bundle sheath cells of maize leaves is very low in darkened plants and increases on illumination. In order to find an explanation of these facts we subjected leaf strips, isolated bundle sheath cells and enzymic preparations to pH-changes and to oxidizing or reducing substances. We found that the increase of the activity produced by light could also be caused in the leaf strips by adding malate to the suspending fluid in the dark. On the contrary, with isolated bundle sheath cells no increase with light or with the addition of malate could be observed. There was a decrease of activity due to acidification or to the addition of dichlorophenol-indophenol (DCPIP). The same effect could be observed with enzymic preparations as with the isolated bundle sheath cells, whereas the decrease of activity was higher by acidification of samples coming from darkened plants than for those enzymes prepared from illuminated plants. Also other substances inhibited the synthase activity: benzoquinone, DOPA, caffeic acid, TMPD or Fe (CN6) 3- and PCMB, while only DTE could increase it. After preincubation with PCMB the enzyme activity could be restored with DTE, not so for the preincubation with DCPIP. With an enzymic preparation that contained intact organelles the increase of activity could also be produced by NADH. Thus we follow that the decrease of synthase activity in the darkened plants is due to acidification and to the presence of oxidizing substances, too.