STRUCTURAL AND FUNCTIONAL-PROPERTIES OF THE COLEOPTILE CHLOROPLAST - PHOTOSYNTHESIS AND PHOTOSENSORY TRANSDUCTION

Recent studies have shown that guard cell and coleoptile chloroplasts appear to be involved in blue light photoreception during blue light-d ependent stomatal opening and phototropic bending. The guard cell chlo roplast has been studied in detail but the coleoptile chloroplast is p oorly understood. The present study was aimed at the characterization of the corn coleoptile chloroplast, and its comparison with mesophyll and guard eel chloroplasts. Coleoptile chloroplasts operated the xanth ophyll cycle, and their zeaxanthin content tracked incident rates of s olar radiation throughout the day. Zeaxanthin formation was very sensi tive to low incident fluence rates, and saturated at around 800-1000 m u mol m(-2) s(-1). Zeaxanthin formation in corn mesophyll chloroplasts was insensitive to low fluence rates and saturated at around 1800 mu mol m(-2) s(-1). Quenching rates of chlorophyll a fluorescence transie nts from coleoptile chloroplasts induced by saturating fluence rates o f actinic red light increased as a function of zeaxanthin content. Thi s implies that zeaxanthin plays a photoprotective role in the coleopti le chloroplast. Addition of low fluence rates of blue light to saturat ing red light also increased quenching rates in a zeaxanthin-dependent fashion. This blue light response of the coleoptile chloroplast is an alogous to that of the guard cell chloroplast, and implicates these or ganelles in the sensory transduction of blue light. On a chlorophyll b asis, coleoptile chloroplasts had high rates of photosynthetic oxygen evolution and low rates of photosynthetic carbon fixation, as compared with mesophyll chloroplasts. In contrast with the uniform chloroplast distribution in the leaf, coleoptile chloroplasts were predominately found in the outer cell layers of the coleoptile cortex, and had large starch grains and a moderate amount of stacked grana and stroma lamel lae. Several key properties of the coleoptiIe chloroplast were differe nt from those of mesophyll chloroplasts and resembled those of guard c ell chloroplasts. We propose that the common properties of guard cell and coleoptile chloroplasts define a functional pattern characteristic of chloroplasts specialized in photosensory transduction.