Plastid Anionic Lipids Are Essential for the Development of Both Photosynthetic and Non-Photosynthetic Organs in Arabidopsis thaliana

A novel strategy of bloom forming cyanobacteria Microcystis sp. in response to phosphorus deficiency: Using non-phosphorus lipids substitute phospholipids

Despite significant reductions in phosphorus (P) loads, lakes still experience cyanobacterial blooms. Little is known regarding cellular P regulation in response to P deficiency in widely distributed bloom causing species such as Microcystis. In this study, we investigated changes in P containing and non-P lipids contents and their ratios concomitantly with the determinations of expression levels of genes encoding these lipids in cultural and field Microcystis samples. In the culture, the content of phosphatidylglycerol (PG) decreased from 2.1 μg g-1 in P replete control to 1.2 μg g-1 in P-deficient treatment, while non-P lipids, like sulfoquinovosyldiacylglycerol (SQDG) and monogalactosyldiacylglycerol (MGDG), increased dramatically from 13.6 μg g-1 to 142.3 μg g-1, and from 0.9 μg g-1 to 16.74 μg g-1, respectively. The expression of the MGDG synthesis gene, mgdE, also increased under low P conditions. Significant positive relationships between soluble reactive phosphorus (SRP) and ratios of P-containing lipids (PG) to non-P lipids, including SQDG, MGDG and digalactosyldiacylglycerol (DGDG) (P < 0.05) were observed in the field investigations. Both cultural and field data indicated that Microcystis sp. might increase non-P lipids proportion to lower P demand when suffering from P deficiency. Furthermore, despite lipid remodeling, photosynthetic activity remained stable, as indicated by comparable chlorophyll fluorescence and Fv/Fm ratios among cultural treatments. These findings suggested that Microcystis sp. may dominate in P-limited environments by substituting glycolipids and sulfolipids for phospholipids to reduce P demand without compromising the photosynthetic activity. This effective strategy in response to P deficiency meant a stricter P reduction threshold is needed in terms of Microcystis bloom control.

Hydrogen sulphide alleviates root growth inhibition induced by phosphate starvation

Phosphorus (P) has crucial roles in plant growth and development. Hydrogen sulphide (H2S) has multiple functions in plants, particularly having the ability to promote tolerance to a variety of adversity stresses. However, it is unclear whether H2S has a function when plants suffer Pi-deficiency stress. DES1, encoding L-cysteine desulfhydrase1, is a crucial source of H2S in Arabidopsis thaliana by catalysing the substrate L-cysteine. Under phosphate starvation, the des1 mutant had a significantly shorter primary root length than the wild-type Col-0, and exogenous application of H2S donor NaHS could compensate for the root growth-sensitive phenotype. In contrast, the transgenic lines DES1ox overexpressing DES1 exhibited less sensitivity to phosphate starvation in terms of longer roots compared to the Col-0. These results demonstrate that H2S is involved in the regulation of Arabidopsis root growth under phosphate starvation. Moreover, using quantitative real-time polymerase chain reaction experiments to analyse the changes in genes induced by phosphate starvation in des1 mutant and Col-0, we screened to find that the expression of the Sulfoquinovosyl diacylglycerol 1 (SQD1) gene was significantly downregulated in the des1 mutant. Consistently, exogenous H2S significantly promoted SQD1 expression levels in roots of Col-0. Taken together, we demonstrate that DES1-mediated H2S participates in alleviating root growth inhibition by promoting the expression of SQD1 under Pi starvation.

Anionic lipids facilitate membrane development and protochlorophyllide biosynthesis in etioplasts

Dark-germinated angiosperm seedlings develop chloroplast precursors called etioplasts in cotyledon cells. Etioplasts develop lattice membrane structures called prolamellar bodies (PLBs), where the chlorophyll intermediate protochlorophyllide (Pchlide) forms a ternary complex with NADPH and light-dependent NADPH:Pchlide oxidoreductase (LPOR). The lipid bilayers of etioplast membranes are mainly composed of galactolipids, which play important roles in membrane-associated processes in etioplasts. Although etioplast membranes also contain 2 anionic lipids, phosphatidylglycerol (PG) and sulfoquinovosyldiacylglycerol (SQDG), their roles are unknown. To determine the roles of PG and SQDG in etioplast development, we characterized etiolated Arabidopsis (Arabidopsis thaliana) mutants deficient in PG and SQDG biosynthesis. A partial deficiency in PG biosynthesis loosened the lattice structure of PLBs and impaired the insertion of Mg2+ into protoporphyrin IX, leading to a substantial decrease in Pchlide content. Although a complete lack of SQDG biosynthesis did not notably affect PLB formation and Pchlide biosynthesis, lack of SQDG in addition to partial PG deficiency strongly impaired these processes. These results suggested that PG is required for PLB formation and Pchlide biosynthesis, whereas SQDG plays an auxiliary role in these processes. Notably, PG deficiency and lack of SQDG oppositely affected the dynamics of LPOR complexes after photoconversion, suggesting different involvements of PG and SQDG in LPOR complex organization. Our data demonstrate pleiotropic roles of anionic lipids in etioplast development.

Biosynthesis of phosphatidylglycerol in photosynthetic organisms

Phosphatidylglycerol (PG) is a unique phospholipid class with its indispensable role in photosynthesis and growth in land plants, algae, and cyanobacteria. PG is the only major phospholipid in the thylakoid membrane of cyanobacteria and plant chloroplasts and a main lipid component in photosynthetic protein-cofactor complexes such as photosystem I and photosystem II. In plants and algae, PG is also essential as a substrate for the biosynthesis of cardiolipin, which is a unique lipid present only in mitochondrial membranes and crucial for the functions of mitochondria. PG biosynthesis pathways in plants include three membranous organelles, plastids, mitochondria, and the endoplasmic reticulum in a complex manner. While the molecular biology underlying the role of PG in photosynthetic functions is well established, many enzymes responsible for the PG biosynthesis are only recently cloned and functionally characterized in the model plant species including Arabidopsis thaliana and Chlamydomonas reinhardtii and cyanobacteria such as Synechocystis sp. PCC 6803. The characterization of those enzymes helps understand not only the metabolic flow for PG production but also the crosstalk of biosynthesis pathways between PG and other lipids. This review aims to summarize recent advances in the understanding of the PG biosynthesis pathway and functions of involved enzymes.

Membrane composition and successful bioaugmentation. Studies of the interactions of model thylakoid and plasma cyanobacterial and bacterial membranes with fungal membrane-lytic enzyme Lecitase ultra

Cyanobacterial/bacterial consortia are frequently inoculated to soils to increase the soil fertility and to accelerate the biodegradation of organic pollutants. Moreover, such consortia can also be successfully applied in landfills especially for the biodegradation of plastic wastes. However, the bioaugmentation techniques turn out frequently inefficient due to the competition of the indigenous microorganisms attacking directly these inoculated or secreting to their surroundings cell wall and membrane-lytic enzymes. It can be hypothesized that the resistance of the microbial membrane to the enzymatic degradation is correlated with its lipid composition. To verify this hypothesis glycolipid and phospholipid Langmuir monolayers were applied as models of thylakoid and plasma cyanobacterial and bacterial membranes. Hybrid fungal enzyme Lecitase ultra joining the activity of lipase and phospholipase A1 was applied as the model of fungal membrane-lytic enzyme. It turned out that anionic thylakoid lipids sulfoquinovosyldiacylglycerol and phosphatidylglycerols were the main targets of Lecitase ultra in the model multicomponent thylakoid membranes. The resistance of the model plasma bacterial membranes to enzymatic degradation depended significantly to their composition. The resistance increased generally when the unsaturated lipids were exchanged to their saturated counterparts. However, most resistant turned out the membranes composed of unsaturated phosphatidylamine and saturated anionic phospholipids.

Impacts of phosphatidylglycerol on plastid gene expression and light induction of nuclear photosynthetic genes

Phosphatidylglycerol (PG) is the only major phospholipid in the thylakoid membrane of chloroplasts. PG is essential for photosynthesis, and loss of PG in Arabidopsis thaliana results in severe defects of growth and chloroplast development, with decreased chlorophyll accumulation, impaired thylakoid formation, and down-regulation of photosynthesis-associated genes encoded in nuclear and plastid genomes. However, how the absence of PG affects gene expression and plant growth remains unclear. To elucidate this mechanism, we investigated transcriptional profiles of a PG-deficient Arabidopsis mutant pgp1-2 under various light conditions. Microarray analysis demonstrated that reactive oxygen species (ROS)-responsive genes were up-regulated in pgp1-2. However, ROS production was not enhanced in the mutant even under strong light, indicating limited impacts of photooxidative stress on the defects of pgp1-2. Illumination to dark-adapted pgp1-2 triggered down-regulation of photosynthesis-associated nuclear-encoded genes (PhANGs), while plastid-encoded genes were constantly suppressed. Overexpression of GOLDEN2-LIKE1 (GLK1), a transcription factor gene regulating chloroplast development, in pgp1-2 up-regulated PhANGs but not plastid-encoded genes along with chlorophyll accumulation. Our data suggest a broad impact of PG biosynthesis on nuclear-encoded genes partially via GLK1 and a specific involvement of this lipid in plastid gene expression and plant development.