Photosynthetic membranes of higher plants contain specific nonphosphorous lipids like the sulfolipid sulfoquinovosyl diacylglycerol in addition to the ubiquitous phospholipid phosphatidylglycerol. In bacteria, an environmental factor that drastically affects thylakoid lipid composition appears to be the availability of phosphate. Accordingly, we discovered an increase in the relative amount of sulfolipid and a concomitant decrease in phosphatidylglycerol in Arabidopsis thaliana grown on medium with reduced amounts of phosphate, as well as in the pho1 mutant of A. thaliana deficient in phosphate transport. To investigate the molecular basis of the observed change in lipid composition, we isolated a cDNA of A. thaliana, designated SQD1, that encodes a protein involved in sulfolipid biosynthesis as suggested by three lines of evidence. First, the cDNA shows high sequence similarity to bacterial sqdB genes known to be essential for sulfolipid biosynthesis; second, the SQD1 gene product is imported into chloroplasts where sulfolipid biosynthesis takes place; and third, transgenic plants expressing SQD1 in antisense orientation show a reduction in sulfolipid content. In the pho1 mutant as well as in wild-type plants grown under reduced phosphate availability, increased amounts of SQD1 mRNA and SQD1 protein are detected, suggesting that the increase in sulfolipid content under phosphate limitation is the result of an increased expression of at least one gene required for sulfolipid biosynthesis in A. thaliana. It is suggested that a certain amount of anionic thylakoid lipid is maintained by substituting sulfolipid for phosphatidylglycerol under reduced phosphate availability.

Gene expression in response to Fe deficiency was analyzed in Arabidopsis roots and shoots through the use of a cDNA collection representing at least 6,000 individual gene sequences. Arabidopsis seedlings were grown 1, 3, and 7 d in the absence of Fe, and gene expression in roots and shoots was investigated. Following confirmation of data and normalization methods, expression of several sequences encoding enzymes known to be affected by Fe deficiency was investigated by microarray analysis. Confirmation of literature reports, particularly for changes in enzyme activity, was not always possible, but changes in gene expression could be confirmed. An expression analysis of genes in glycolysis, the tricarboxylic acid cycle, and oxidative pentose phosphate pathway revealed an induction of several enzymes within 3 d of Fe-deficient growth, indicating an increase in respiration in response to Fe deficiency. In roots, transcription of sequences corresponding to enzymes of anaerobic respiration was also induced, whereas in shoots, the induction of several genes in gluconeogenesis, starch degradation, and phloem loading was observed. Thus, it seemed likely that the energy demand in roots required for the Fe deficiency response exceeded the capacity of oxidative phosphorylation, and an increase in carbon import and anaerobic respiration were required to maintain metabolism.

The SQD1 enzyme is believed to be involved in the biosynthesis of the sulfoquinovosyl headgroup of plant sulfolipids, catalyzing the transfer of SO 3 ؊ to UDP-glucose. We have determined the structure of the complex of SQD1 from Arabidopsis thaliana with NAD ؉ and the putative substrate UDP-glucose at 1.6-Å resolution. Both bound ligands are completely buried within the binding cleft, along with an internal solvent cavity which is the likely binding site for the, as yet, unidentified sulfur-donor substrate. SQD1 is a member of the short-chain dehydrogenase͞reductase (SDR) family of enzymes, and its structure shows a conservation of the SDR catalytic residues. Among several highly conserved catalytic residues, Thr-145 forms unusually short hydrogen bonds with both susceptible hydroxyls of UDP-glucose. A His side chain may also be catalytically important in the sulfonation.T he sulfolipid sulfoquinovosyldiacylglycerol (SQDG) is common to all plants and many photosynthetic bacteria (1) and is found exclusively in the photosynthetic (thylakoid) membranes. SQDG-deficient null mutants from different photosynthetic bacteria exhibit a conditionally lethal phenotype under phosphate-limiting growth conditions (2, 3), suggesting that SQDG can substitute for phosphatidylglycerol under these conditions (1). This hypothesis also seems to be valid for plants such as Arabidopsis thaliana, for which concomitant changes in thylakoid membrane lipid composition and in the regulation of sulfolipid gene expression have been observed in response to phosphate starvation (4). Beyond its role in photosynthetic membranes, SQDG has been found to inhibit retroviral infections of mammalian cells and specifically the viral reverse transcriptase (5, 6).Genes essential for the biosynthesis of the SQDG were isolated from photosynthetic bacteria by genetic complementation of mutants (7,8). The amino acid sequence of the product of one of these genes, sqdB, is highly conserved among different organisms and a plant ortholog, SQD1, has been recently isolated from A. thaliana and expressed in Escherichia coli (4). The SQDB͞SQD1 proteins show modest sequence similarity to sugar-nucleotide enzymes (4), and this similarity has been recently exploited by Essigmann and coworkers (9) to predict the structure of the A. thaliana SQD1 protein. As they also demonstrated that SQD1 binds NAD ϩ and contains characteristic conserved Y-XXX-K and glycine-rich (G-XX-G-XX-G) sequence patterns, SQD1 appears to be a member of the shortchain dehydrogenase͞reductase (SDR) family (10-12).The sugar-nucleotide UDP-sulfoquinovose is thought to be the headgroup donor for SQDG biosynthesis (1), a conclusion supported by in situ labeling experiments with isolated chloroplasts and using synthetic UDP-sulfoquinovose (13,14) and by the discovery of UDP-sulfoquinovose in bacterial sulfolipid mutants and other organisms (15, 16). Pugh et al. (17) proposed a metabolic pathway for SQDG biosynthesis in which UDPglucose is first converted to a UDP-4-ketoglucose-5-ene intermediate, with ...

The pho1 mutant of Arabidopsis has been shown to respond to the phosphate deficiency in the leaves by decreasing the amount of phosphatidylglycerol (PG). PG is thought to be of crucial importance for the organization and function of the thylakoid membrane. This prompted us to ask what the consequences of the PG deficiency may be in the pho1 mutant when grown under low or high light. While in the wild-type, the lipid pattern was almost insensitive to changes in the growth light, PG was reduced to 45% under low light in the mutant, and it decreased further to 35% under high light. Concomitantly, sulfoquinovosyl diacylglycerol (SQDG) and to a lesser extent digalactosyl diacylglycerol (DGDG) increased. The SQDG increase correlated with increased amounts of the SQD1 protein, an indicator for an actively mediated process. Despite of alterations in the ultrastructure, mutant thylakoids showed virtually no effects on photosynthetic electron transfer, O2 evolution and excitation energy allocation to the reaction centers. Our results support the idea that PG deficiency can at least partially be compensated for by the anionic lipid SQDG and the not charged lipid DGDG. This seems to be an important strategy to maintain an optimal thylakoid lipid milieu for vital processes, such as photosynthesis, under a restricted phosphate availability.

Succinate dehydrogenase inhibitors (SDHIs) have played a crucial role in disease control to protect cereals as well as fruit and vegetables for more than a decade. Isoflucypram, the first representative of a newly installed subclass of SDHIs inside the Fungicide Resistance Action Committee (FRAC) family of complex II inhibitors, offers unparalleled long‐lasting efficacy against major foliar diseases in cereals. Herein we report the chemical optimization from early discovery towards isoflucypram and the first hypothesis of its altered binding mode in the ubiquinone binding site of succinate dehydrogenase. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.