Red/far-red and blue light-responsive regions of maize rbcS-m3 are active in bundle sheath and mesophyll cells, respectively.

Leaves of the C4 plant maize have two major types of photosynthetic cells: a ring of five large bundle sheath cells (BSC) surrounds each vascular bundle and smaller mesophyll cells (MC) lie between the cylinders of bundle sheath cells. The enzyme ribulose bisphosphate carboxylase/oxygenase is encoded by nuclear rbcS and chloroplast rbcL genes. It is not present in MC but is abundant in adjacent BSC of green leaves. As reported previously, the separate regions of rbcS-m3, which are required for stimulating transcription of the gene in BSC and for suppressing expression of reporter genes in MC, were identified by an in situ expression assay; expression was not suppressed in MC until after leaves of dark-grown seedlings had been illuminated for 24 h. Now we have found that transient expression of rbcS-m3 reporter genes is stimulated in BSC via a red/far-red reversible phytochrome photoperception and signal transduction system but that blue light is required for suppressing rbcS-m3 reporter gene expression in MC. Blue light is also required for the suppression system to develop in MC. Thus, the maize gene rbcS-m3 contains certain sequences that are responsive to a phytochrome photoperception and signal transduction system and other regions that respond to a UVA/blue light photoperception and signal transduction system. Various models of "coaction" of plant photoreceptors have been advanced; these observations show the basis for one type of coaction.

Stimulation of the blue light phototropic receptor NPH1 causes a transient increase in cytosolic Ca2+

Blue light regulates plant growth and development, and three photoreceptors, CRY1, CRY2, and NPH1, have been identified. The transduction pathways of these receptors are poorly understood. Transgenic plants containing aequorin have been used to dissect the involvement of these three receptors in the regulation of intracellular Ca2+. Pulses of blue light induce cytosolic Ca2+ transients lasting about 80 s in Arabidopsis and tobacco seedlings. Use of organelle-targeted aequorins shows that Ca2+ increases are limited to the cytoplasm. Blue light treatment of cry1, cry2, and nph1 mutants showed that NPH1, which regulates phototropism, is largely responsible for the Ca2+ transient. The spectral response of the Ca2+ transient is similar to that of phototropism, supporting NPH1 involvement. Furthermore, known interactions between red and blue light and between successive blue light pulses on phototropic sensitivity are mirrored in the blue light control of cytosolic Ca2+ in these seedlings. Our observations raise the possibility that physiological responses regulated by NPH1, such as phototropism, may be transduced through cytosolic Ca2+.

Red Bell Pepper Chromoplasts Exhibit in Vitro Import Competency and Membrane Targeting of Passenger Proteins from the Thylakoidal Sec and ΔpH Pathways but Not the Chloroplast Signal Recognition Particle Pathway1

Chloroplast to chromoplast development involves new synthesis and plastid localization of nuclear-encoded proteins, as well as changes in the organization of internal plastid membrane compartments. We have demonstrated that isolated red bell pepper (Capsicum annuum) chromoplasts contain the 75-kD component of the chloroplast outer envelope translocon (Toc75) and are capable of importing chloroplast precursors in an ATP-dependent fashion, indicating a functional general import apparatus. The isolated chromoplasts were able to further localize the 33- and 17-kD subunits of the photosystem II O2-evolution complex (OE33 and OE17, respectively), lumen-targeted precursors that utilize the thylakoidal Sec and ΔpH pathways, respectively, to the lumen of an internal membrane compartment. Chromoplasts contained the thylakoid Sec component protein, cpSecA, at levels comparable to chloroplasts. Routing of OE17 to the lumen was abolished by ionophores, suggesting that routing is dependent on a transmembrane ΔpH. The chloroplast signal recognition particle pathway precursor major photosystem II light-harvesting chlorophyll a/b protein failed to associate with chromoplast membranes and instead accumulated in the stroma following import. The Pftf (plastid fusion/translocation factor), a chromoplast protein, integrated into the internal membranes of chromoplasts during in vitro assays, and immunoblot analysis indicated that endogenous plastid fusion/translocation factor was also an integral membrane protein of chromoplasts. These data demonstrate that the internal membranes of chromoplasts are functional with respect to protein translocation on the thylakoid Sec and ΔpH pathways.

The Interaction between Cold and Light Controls the Expression of the Cold-Regulated Barley Gene cor14b and the Accumulation of the Corresponding Protein1

We report the expression of the barley (Hordeum vulgare L.) COR (cold-regulated) gene cor14b (formerly pt59) and the accumulation of its chloroplast-localized protein product. A polyclonal antibody raised against the cor14b-encoded protein detected two chloroplast COR proteins: COR14a and COR14b. N-terminal sequencing of COR14a and expression of cor14b in Arabidopsis plants showed that COR14a is not encoded by the cor14b sequence, but it shared homology with the wheat (Triticum aestivum L.) WCS19 COR protein. The expression of cor14b was strongly impaired in the barley albino mutant an, suggesting the involvement of a plastidial factor in the control of gene expression. Low-level accumulation of COR14b was induced by cold treatment in etiolated plants, although cor14b expression and protein accumulation were enhanced after a short light pulse. Light quality was a determining factor in regulating gene expression: red or blue but not far-red or green light pulses were able to promote COR14b accumulation in etiolated plants, suggesting that phytochrome and blue light photoreceptors may be involved in the control of cor14b gene expression. Maximum accumulation of COR14b was reached only when plants were grown and/or hardened under the standard photoperiod. The effect of light on the COR14b stability was demonstrated by using transgenic Arabidopsis. These plants constitutively expressed cor14b mRNAs regardless of temperature and light conditions; nevertheless, green plants accumulated about twice as much COR14b protein as etiolated plants.

ΔpH-Dependent Photosystem II Fluorescence Quenching Induced by Saturating, Multiturnover Pulses in Red Algae1

We have previously shown that in the red alga Rhodella violacea, exposure to continuous low intensities of light 2 (green light) or near-saturating intensities of white light induces a ΔpH-dependent PSII fluorescence quenching. In this article we further characterize this fluorescence quenching by using white, saturating, multiturnover pulses. Even though the pulses are necessary to induce the ΔpH and the quenching, the development of the latter occurred in darkness and required several tens of seconds. In darkness or in the light in the presence of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, the dissipation of the quenching was very slow (more than 15 min) due to a low consumption of the ΔpH, which corresponds to an inactive ATP synthase. In contrast, under far-red illumination or in the presence of 3-(3,4-dichlorophenyl)-1,1′-dimethylurea (only in light), the fluorescence quenching relaxed in a few seconds. The presence of N,N′-dicyclohexyl carbodiimide hindered this relaxation. We propose that the quenching relaxation is related to the consumption of ΔpH by ATP synthase, which remains active under conditions favoring pseudolinear and cyclic electron transfer.

Influence of light on DNA content of Helianthus annuus Linnaeus.

Mean nuclear 2C DNA content (C equaling haploid DNA per nucleus) of the first leaf of the sunflower, Helianthus annuus L., is influenced by the quality and the quantity of light. Seedlings of two inbred lines, RHA 299 and RHA 271 were germinated and grown in controlled environmental conditions. Lighting was adjusted to provide different combinations of photon flux densities and red to far red (R:FR) ratios. At R:FR = 5.8 and photon flux densities of 170 mumol.m-2.s-1, 200 mumol.m-2.s-1, and 230 mumol.m-2.s-1, DNA content remained high and relatively constant (x = 6.97 pg for RHA 271 and x = 7.32 pg for RHA 299). When the photon flux density range (R:FR = 5.8) was elevated to 350 mumol.m-2.s-1, 410 mumol.m-2.s-1, and 470 mumol.m-2.s-1, mean DNA content was reduced to 6.23 pg (RHA 271) and 6.46 pg (RHA 299). At R:FR = 1.5, mean DNA content was consistently high (7.2-7.9 pg) only at the lowest photon flux density of 170 mumol.m-2.s-1. Significant decreases in DNA content (< or = 12%) were observed at photon flux densities of 200 mumol.m-2.s-1 and 230 mumol.m-2.s-1. At the higher photon flux densities (350 mumol.m-2.s-1, 410 mumol.m-2.s-1, and 470 mumol.m-2.s-1) and R:RF = 1.5, the plants had extremely low DNA contents (mean x = 3.36 pg for RHA 271 and 3.41 pg for RHA 299) and high between-plant variance. The instability of DNA content, particularly for plants grown under light that is far red rich, suggests that phytochromes may be involved in regulating DNA content of the sunflower.