Circadian expression of the light-harvesting protein of Photosystem II in etiolated bean leaves following a single red light pulse: Coordination with the capacity of the plant to form chlorophyll and the thylakoid-bound protease

Comparison of transcriptome under red and blue light culture of Saccharina japonica (Phaeophyceae)

Saccharina japonica is one of the most important economic seaweeds. Several aspects such as photosynthesis in Saccharina lives are affected by blue light, the predominant light spectrum in the habitat. In this study, transcriptome profiling of S. japonica by next generation sequencing technology generated 55,102 qualified transcripts and 40.5 % transcripts were assigned to functional annotation. Expression of a large proportion of genes has been previously reported to be regulated by blue light, taking dark as control. However, by comparison among white, blue and red light, the significantly differentially expressed gene tags (DEGs) accounted for only 6.75 % of the identified sequences. It indicated that light-regulated gene expression in kelps is not a specific blue-light response. Unexpectedly, red light had more extensive effects on the transcriptomic activity than blue light did, since the most (68.4 %) DEGs were red light-regulated and only 17.5 % were specifically regulated by blue light. Some of the DEGs with the highest mRNA levels under blue light are not blue light-upregulated but red light-downregulated. The extensive regulation on gene expression under red light together with the abundant presence of phytochrome-like protein gene tags in S. japonica indicated their significant roles in the lives of brown algae. By highlighting the photosynthetic metabolism, blue light is more efficient than red light in triggering the pigment biosynthesis, light reaction and carbon fixation, revealing a molecular basis for rapid growth of kelps, since most of the time blue light is predominant in their habitat.

Etioplast and etio-chloroplast formation under natural conditions: the dark side of chlorophyll biosynthesis in angiosperms

Chloroplast development is usually regarded as proceeding from proplastids. However, direct or indirect conversion pathways have been described in the literature, the latter involving the etioplast or the etio-chloroplast stages. Etioplasts are characterized by the absence of chlorophylls (Chl-s) and the presence of a unique inner membrane network, the prolamellar body (PLB), whereas etio-chloroplasts contain Chl-s and small PLBs interconnected with chloroplast thylakoids. As etioplast development requires growth in darkness for several days, this stage is generally regarded as a nonnatural pathway of chloroplast development occurring only under laboratory conditions. In this article, we have reviewed the data in favor of the involvement of etioplasts and etio-chloroplasts as intermediary stage(s) in chloroplast formation under natural conditions, the molecular aspects of PLB formation and we propose a dynamic model for its regulation.

Implications of a developmental-stage-dependent thylakoid-bound protease in the stabilization of the light-harvesting pigment-protein complex serving photosystem II during thylakoid biogenesis in red kidney bean

Intact etioplasts of bean (Phaseolus vulgaris) plants exhibit proteolytic activity against the exogenously added apoprotein of the light-harvesting pigment-protein complex serving photosystem II (LHCII) that increases as etiolation is prolonged. The activity increases in the membrane fraction but not in the stroma, where it remains low and constant and is mainly directed against LHCII and protochlorophyllide oxidoreductase. The thylakoid proteolytic activity, which is low in etioplasts of 6-d-old etiolated plants, increases in plants pretreated with a pulse of light or exposed to intermittent-light (ImL) cycles, but decreases during prolonged exposure to continuous light, coincident with chlorophyll (Chl) accumulation. To distinguish between the control of Chl and/or development on proteolytic activity, we used plants exposed to ImL cycles of varying dark-phase durations. In ImL plants exposed to an equal number of ImL cycles with short or long dark intervals (i.e. equal Chl accumulation but different developmental stage) proteolytic activity increased with the duration of the dark phase. In plants exposed to ImL for equal durations to such light-dark cycles (i.e. different Chl accumulation but same developmental stage) the proteolytic activity was similar. These results suggest that the protease, which is free to act under limited Chl accumulation, is dependent on the developmental stage of the chloroplast, and give a clue as to why plants in ImL with short dark intervals contain LHCII, whereas those with long dark intervals possess only photosystem-unit cores and lack LHCII.

Thylakoid-bound proteolytic activity against LHC II apoprotein in bean

Triton X-100 solubilized thylakoids, isolated from Phaseolus vulgaris chloroplasts, degrade endogenous or exogenously added LHC II. The degradation, as monitored by immunodetection of the remaining LHC II after incubation at 37°C, is activated by Mg(++) and inhibited by pCMB, EDTA, PMSF and benzamidine; the activity under high light conditions parallels chlorophyll photooxidation. The thylakoid-bound proteolytic activity is under phytochrome control. Etiolated plants pretreated by a white light pulse, and kept in the dark thereafter, show enhanced proteolytic activity, which follows rhythmical oscillations. On the other hand, chloramphenicol pretreatment of etiolated plants, prior to their transfer to continuous light, reduces the proteolytic activity against LHC II. The results suggest that the degradation involves a serine type protease, which depends on SH group(s), coded by the plastid genome; the protease action on LHC II is regulated by Mg(++), phytochrome, the biological clock and chlorophyll accumulation in the thylakoid. The stroma lamellar fraction, separated from French press disrupted chloroplasts, exhibits higher activity towards exogenous LHC II than the grana fraction. The stroma of intact chloroplasts exhibits also high proteolytic activity, which is drastically reduced when the lysis medium is supplemented with cations. This suggests that the protease is bound mainly on stroma lamellae and peripheral granal membranes, its association to the membranes being possibly under cation control.