Chloroplast-encoded chlB gene from Pinus thunbergii promotes root and early chlorophyll pigment development in Nicotiana tabaccum

Evolution of light-independent protochlorophyllide oxidoreductase

The nonhomologous enzymes, the light-independent protochlorophyllide reductase (DPOR) and the light-dependent protochlorophyllide oxidoreductase (LPOR), catalyze the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) in the penultimate step of biosynthesis of chlorophyll (Chl) required for photosynthetic light absorption and energy conversion. The two enzymes differ with respect to the requirement of light for catalysis and oxygen sensitivity. DPOR and LPOR initially evolved in the ancestral prokaryotic genome perhaps at different times. DPOR originated in the anoxygenic environment of the Earth from nitrogenase-like enzyme of methanogenic archaea. Due to the transition from anoxygenic to oxygenic photosynthesis in the prokaryote, the DPOR was mostly inactivated in the daytime by photosynthetic O₂ leading to the evolution of oxygen-insensitive LPOR that could function in the light. The primary endosymbiotic event transferred the DPOR and LPOR genes to the eukaryotic phototroph; the DPOR remained in the genome of the ancestor that turned into the plastid, whereas LPOR was transferred to the host nuclear genome. From an evolutionary point of view, several compelling theories that explain the disappearance of DPOR from several species cutting across different phyla are as follows: (i) pressure of the oxygenic environment; (ii) change in the light conditions and temperature; and (iii) lineage-specific gene losses, RNA editing, and nonsynonymous substitution. Certain primary amino acid sequence and the physiochemical properties of the ChlL subunit of DPOR have similarity with that of LPOR suggesting a convergence of these two enzymes in certain evolutionary event. The newly obtained sequence data from different phototrophs will further enhance the width of the phylogenetic information on DPOR.

Chlorophyll a fluorescence and transcriptome reveal the toxicological effects of bisphenol A on an invasive cyanobacterium, Cylindrospermopsis raciborskii

Bisphenol A has attracted worldwide attention due to its harmful effects on humans, animals and plants. In this study, the toxicological effects of BPA on Cylindrospermopsis raciborskii were assessed based on chlorophyll a fluorescence and transcriptome analyses. The results showed that the growth of C. raciborskii was significantly inhibited when BPA exceeded 0.1 mg L^-1. A marked rise of phase J was observed at a concentration greater than 0.1 mg L^-1, while a K phase appeared at 20 mg L^-1. The chlorophyll a fluorescence parameters of RC/CS₀, F₀, φ_P0, φ_E0, and ψ₀, underwent a significant decline under all treatments of BPA, whereas a significant increase in both V_J and M₀ occurred under all concentrations of BPA. Additionally, ABS/RC and DIo/RC markedly increased at 10 mg L^-1 and 20 mg L^-1. The transcriptome analysis revealed that the genes of photosynthesis, including psbA, psbB, psbC, psbD, apcA, apcB, cpcA, and cpcB, as well as those of chlorophyll and carotenoid biosynthesis, namely hemN, acsF, chlL, chlN, chlP, crtB, pds, were all down-regulated. Moreover, BPA also inhibited the oxidative phosphorylation, glycolysis/gluconeogenesis, citrate cycle (TCA cycle), and fatty acid metabolism in C. raciborskii. Taken together, these results suggest BPA can negatively affect the expression of multiple genes and the vital energy metabolism process to arrest the growth and photosynthesis of C. raciborskii.

Ectomycorrhizal fungi reduce the light compensation point and promote carbon fixation of Pinus thunbergii seedlings to adapt to shade environments

We examined the effects of three ectomycorrhizal (ECM) symbionts on the growth and photosynthesis capacity of Japanese black pine (Pinus thunbergii) seedlings and estimated physiological and photosynthetic parameters such as the light compensation point (LCP), biomass, and phosphorus (Pi) concentration of P. thunbergii seedlings. Through this investigation, we documented a new role of ectomycorrhizal (ECM) fungi: enhancement of the survival and competitiveness of P. thunbergii seedlings under low-light condition by reducing the LCP of seedlings. At a CO2 concentration of 400 ppm, the LCP of seedlings with ECM inoculations was 40-70 μmol photons m-2 s-1, significantly lower than that of non-mycorrhizal (NM) seedlings (200 μmol photons m-2 s-1). In addition, photosynthetic carbon fixation (Pn) increased with light intensity and CO2 level, and the Pn of ECM seedlings was significantly higher than that of NM seedlings; Pisolithus sp. (Pt)- and Laccaria amethystea (La)-mycorrhizal seedlings had significantly lower Pn than Cenococcum geophilum (Cg)-mycorrhizal seedlings. However, La-mycorrhizal seedlings exhibited the highest fresh weight, relative water content (RWC), and the lowest LCP in the mycorrhizal group. Concomitantly, ECM seedlings showed significantly increased chlorophyll content of needles and higher Pi concentrations compared to NM seedlings. Overall, ECM symbionts promoted growth and photosynthesis while reducing the LCP of P. thunbergii seedlings. These findings indicate that ECM fungi can enhance the survival and competitiveness of host seedlings under low light.

The Effect of Two Amino acid Residue Substitutions via RNA Editing on Dark-operative Protochlorophyllide Oxidoreductase in the Black Pine Chloroplasts

Dark-operative protochlorophyllide oxidoreductase (DPOR) is a key enzyme to produce chlorophyll in the dark. Among photosynthetic eukaryotes, all three subunits chlL, chlN, and chlB are encoded by plastid genomes. In some gymnosperms, two codons of chlB mRNA are changed by RNA editing to codons encoding evolutionarily conserved amino acid residues. However, the effect of these substitutions on DPOR activity remains unknown. We first prepared cyanobacterial ChlB variants with amino acid substitution(s) to mimic ChlB translated from pre-edited mRNA. Their activities were evaluated by measuring chlorophyll content of dark-grown transformants of a chlB-lacking mutant of the cyanobacterium Leptolyngbya boryana that was complemented with pre-edited mimic chlB variants. The chlorophyll content of the transformant cells expressing the ChlB variant from the fully pre-edited mRNA was only one-fourth of the control cells. Co-purification experiments of ChlB with Strep-ChlN suggested that a stable complex with ChlN is greatly impaired in the substituted ChlB variant. We then confirmed that RNA editing efficiency was markedly greater in the dark than in the light in cotyledons of the black pine Pinus thunbergii. These results indicate that RNA editing on chlB mRNA is important to maintain appropriate DPOR activity in black pine chloroplasts.