Plastocyanin-dependent photoreduction of NADP by agranal chloroplasts from maize

Abstract

It is well known that isolated chloroplasts containing grana can carry out the photoreduction of NADP coupled to the evolution of oxygen. In contrast, it has been recently shown that higher plant chloroplasts containing either no grana or a few small grana (‘agranal’ chloroplasts), such as those found in the bundle-sheath cells of leaves of certain Cq plants including maize, sugar cane and Sorghum, have a different photosynthetic electron transfer pathway. Isolated ‘agranal’ chloroplasts are unable to photoreduce NADP [ l-31 even though photosystems I and II are active [2,3] as the two photosystems do not appear to be linked by a membrane-bound electron transfer pathway. Evidence that photosystems I and II are in fact linked in the intact bundle-sheath cells [3] has led us to postulate the existence of a soluble linking protein(s) which is lost during isolation of the chloroplasts. In this paper we show that the copper-protein, plastocyanin, functions as a linking protein to permit photoreduction of NADP by ‘agranal’ chloroplasts isolated from bundle-sheath cells of maize. The rates of NADP photoreduction obtained in this reconstituted system are comparable with those obtained from grana-containing mesophyll chloroplasts.