Structural and functional properties of the coleoptile chloroplast: Photosynthesis and photosensory transduction

Abstract

Recent studies have shown that guard cell and coleoptile chloroplasts appear to be involved in blue light photoreception during blue light-dependent stomatal opening and phototropic bending. The guard cell chloroplast has been studied in detail but the coleoptile chloroplast is poorly understood. The present study was aimed at the characterization of the corn coleoptile chloroplast, and its comparison with mesophyll and guard cell chloroplasts. Coleoptile chloroplasts operated the xanthophyll cycle, and their zeaxanthin content tracked incident rates of solar radiation throughout the day. Zeaxanthin formation was very sensitive to low incident fluence rates, and saturated at around 800-1000 μmol m(-2) s(-1). Zeaxanthin formation in corn mesophyll chloroplasts was insensitive to low fluence rates and saturated at around 1800 μmol m(-2) s(-1). Quenching rates of chlorophyll a fluorescence transients from coleoptile chloroplasts induced by saturating fluence rates of actinic red light increased as a function of zeaxanthin content. This implies that zeaxanthin plays a photoprotective role in the coleoptile chloroplast. Addition of low fluence rates of blue light to saturating red light also increased quenching rates in a zeaxanthin-dependent fashion. This blue light response of the coleoptile chloroplast is analogous to that of the guard cell chloroplast, and implicates these organelles in the sensory transduction of blue light. On a chlorophyll basis, 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 lamellae. Several key properties of the coleoptile chloroplast were different from those of mesophyll chloroplasts and resembled those of guard cell chloroplasts. We propose that the common properties of guard cell and coleoptile chloroplasts define a functional pattern characteristic of chloroplasts specialized in photosensory transduction.

The combined treatments of brassinolide and zeaxanthin better alleviate oxidative damage and improve hypocotyl length, biomass, and the quality of radish sprouts stored at low temperature

“…1 A–D). Similar to our results, zeaxanthin has been found to participate in photosensory transduction and protect the chloroplasts in the coleoptile ( Zhu, Zeiger, & Zeiger, 1995 ). Tang et al (2021) found that exogenous zeaxanthin pretreatment maintained a higher photosystem process and fresh weight of pepper seedlings than the control during low temperatures.…”

Close correspondence between the action spectra for the blue light responses of the guard cell and coleoptile chloroplasts, and the spectra for blue light-dependent stomatal opening and coleoptile phototropism.

“…Light-grown corn coleoptiles have functional chloroplasts (19) and show chlorophyll a fluorescence transients similar to those obtained from guard cell chloroplasts (data not shown). We investigated whether blue light can enhance the red light-stimulated fluorescence quenching from coleoptile chloroplasts, in a response homologous to that observed with guard cell chloroplasts.…”

“…A recent study has characterized photosynthetic properties of corn coleoptile chloroplasts which contrast with those from corn mesophyll and are strikingly similar to the properties of guard cell chloroplasts (19). These properties include high rates of photosystem II-dependent oxygen evolution, low rates of carbon fixation, a large starch content, and a xanthophyll cycle which is highly sensitive to low fluence rates of incident radiation.…”

Red light activates a chloroplast‐dependent ion uptake mechanism for stomatal opening under reduced CO2 concentrations in Vicia spp.

“…In addition to its role as an environmental sensor (Outlaw et al. , 1981), emerging data point to the guard cell chloroplast as an evolutionary repository specialized for integrated signal transduction to regulate stomatal aperture in a variety of developmental, environmental and physiological circumstances (Zhu et al. , 1995; Quinones et al.…”