Sulfolipid Biosynthesis and Function in Plants

A leaf-level spectral library to support high throughput plant phenotyping: Predictive accuracy and model transfer

Leaf-level hyperspectral reflectance has become an effective tool for high-throughput phenotyping of plant leaf traits due to its rapid, low-cost, multi-sensing, and non-destructive nature. However, collecting samples for model calibration can still be expensive; and models show poor transferability among different datasets. This study had three specific objectives: (i) assemble a large library of leaf hyperspectral data (n=2460) from maize and sorghum, (ii) evaluate two machine-learning approaches to estimate nine leaf properties (chlorophyll, thickness, water content, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur), and (iii) investigate the usefulness of this spectral library for predicting external datasets (n=445) including soybean and camelina using extra-weighted spiking. Internal cross-validation showed satisfactory performance of the spectral library to estimate all nine traits (average R 2 0.688), with Partial Least Squares Regression outperforming Deep Neural Network models. Models calibrated solely using the spectral library showed degraded performance on external datasets (average R 2 0.159 for camelina, 0.337 for soybean). Models improved significantly when a small portion of external samples (n=20) was added to the library via extra-weighted spiking (average R 2 0.574 for camelina, 0.536 for soybean). The leaf-level spectral library greatly benefits plant physiological and biochemical phenotyping; whereas extra-weight spiking improves model transferability and extends its utility.

Induced accumulation of glucuronosyldiacylglycerol in tomato and soybean under phosphorus deprivation

Glucuronosyldiacylglycerol (GlcADG) is a plant glycolipid that accumulates in Arabidopsis and rice in response to phosphorus (P) starvation. It has been suggested that GlcADG functions to mitigate the stress induced by P depletion. Biosynthesis of GlcADG requires sulfolipid (SQDG) synthase, which is coded for in plant genomes. This indicates the possibility that GlcADG may be a general constituent of membrane lipids in plants. In this study, we investigated the SQDG synthases found in the genomes of higher plants, ferns, mosses, algae and cyanobacteria. In addition, we analyzed GlcADG accumulation, and the expression of SQDG synthase homologs in tomato and soybean plants grown under P-limited conditions. LC-MS analysis of lipids from these plants confirmed that GlcADG accumulated during P deprivation, as previously observed in Arabidopsis and rice. We also observed upregulation of SQDG synthase transcripts in these plants during P deprivation. These data suggest that GlcADG is present not only in model plants, but also in various other plant species, and that this lipid molecule performs an important physiological function as a mitigator of P-deprivation stress in plants.

Diversity and distribution of a key sulpholipid biosynthetic gene in marine microbial assemblages

Sulphoquinovosyldiacylglycerols (SQDG) are polar sulphur-containing membrane lipids, whose presence has been related to a microbial strategy to adapt to phosphate deprivation. In this study, we have targeted the sqdB gene coding the uridine 5'-diphosphate-sulphoquinovose (UDP-SQ) synthase involved in the SQDG biosynthetic pathway to assess potential microbial sources of SQDGs in the marine environment. The phylogeny of the sqdB-coding protein reveals two distinct clusters: one including green algae, higher plants and cyanobacteria, and another one comprising mainly non-photosynthetic bacteria, as well as other cyanobacteria and algal groups. Evolutionary analysis suggests that the appearance of UDP-SQ synthase occurred twice in cyanobacterial evolution, and one of those branches led to the diversification of the protein in members of the phylum Proteobacteria. A search of homologues of sqdB-proteins in marine metagenomes strongly suggested the presence of heterotrophic bacteria potential SQDG producers. Application of newly developed sqdB gene primers in the marine environment revealed a high diversity of sequences affiliated to cyanobacteria and Proteobacteria in microbial mats, while in North Sea surface water, most of the detected sqdB genes were attributed to the cyanobacterium Synechococcus sp. Lipid analysis revealed that specific SQDGs were characteristic of microbial mat depth, suggesting that SQDG lipids are associated with specific producers.

A new class of plant lipid is essential for protection against phosphorus depletion

Phosphorus supply is a major factor responsible for reduced crop yields. As a result, plants utilize various adaptive mechanisms against phosphorus depletion, including lipid remodelling. Here we report the involvement of a novel plant lipid, glucuronosyldiacylglycerol, against phosphorus depletion. Lipidomic analysis of Arabidopsis plants cultured in phosphorus-depleted conditions revealed inducible accumulation of glucuronosyldiacylglycerol. Investigation using a series of sulfolipid sulfoquinovosyldiacylglycerol synthesis-deficient mutants of Arabidopsis determined that the biosynthesis of glucuronosyldiacylglycerol shares the pathway of sulfoquinovosyldiacylglycerol synthesis in chloroplasts. Under phosphorus-depleted conditions, the Arabidopsis sqd2 mutant, which does not accumulate either sulfoquinovosyldiacylglycerol or glucuronosyldiacylglycerol, was the most severely damaged of three sulfoquinovosyldiacylglycerol-deficient mutants. As glucuronosyldiacylglycerol is still present in the other two mutants, this result indicates that glucuronosyldiacylglycerol has a role in the protection of plants against phosphorus limitation stress. Glucuronosyldiacylglycerol was also found in rice, and its concentration increased significantly following phosphorus limitation, suggesting a shared physiological significance of this novel lipid against phosphorus depletion in plants.

Phosphorus supply is one of the major factors responsible for reduced crop yields. Here Okazaki et al. use untargeted lipidomics to elucidate the biosynthetic pathway of a novel plant lipid, glucuronosyldiacylglycerol, which is essential for the protection of plants against phosphorus depletion.

Fatty acid neutral losses observed in tandem mass spectrometry with collision-induced dissociation allows regiochemical assignment of sulfoquinovosyl-diacylglycerols

A full characterization of sulfoquinovosyldiacylglycerols (SQDGs) in the lipid extract of spinach leaves has been achieved using liquid chromatography/electrospray ionization-linear quadrupole ion trap mass spectrometry (MS). Low-energy collision-induced dissociation tandem MS (MS/MS) of the deprotonated species [M - H](-) was exploited for a detailed study of sulfolipid fragmentation. Losses of neutral fatty acids from the acyl side chains (i.e. [M - H - RCOOH](-)) were found to prevail over ketene losses ([M - H - R'CHCO](-)) or carboxylates of long-chain fatty acids ([RCOO](-)), as expected for gas-phase acidity of SQDG ions. A new concerted mechanism for RCOOH elimination, based on a charge-remote fragmentation, is proposed. The preferential loss of a fatty acids molecule from the sn-1 position (i.e. [M - H - R(1)COOH](-)) of the glycerol backbone, most likely due to kinetic control of the gas-phase fragmentation process, was exploited for the regiochemical assignment of the investigated sulfolipids. As a result, 24 SQDGs were detected and identified in the lipid extract of spinach leaves, their number and variety being unprecedented in the field of plant sulfolipids. Moreover, the prevailing presence of a palmitic acyl chain (16:0) on the glycerol sn-2 position of spinach SQDGs suggests a prokaryotic or chloroplastic path as the main route for their biosynthesis.

NONPHOSPHORUS LIPIDS IN PERIPHYTON REFLECT AVAILABLE NUTRIENTS IN THE FLORIDA EVERGLADES, USA(1)

Algal and plant production of nonphosphorus lipids in place of phospholipids is a physiological response to low phosphorus (P) availability. This response has been shown in culture and in marine plankton studies, but examples from freshwater algae remain minimal. Herein, we analyzed the nutrient contents and lipid composition of periphyton communities across the Florida Everglades ecosystem. We hypothesized that in phosphate-poor areas, periphyton in high- and low-sulfate waters would vary the proportion of sulfolipids (SLs) and betaine lipids (BLs), respectively. In phosphate-enriched areas, periphyton would produce more phospholipids (PLs). We observed that at low-P sites, PLs were a minor lipid component. In cyanobacteria-dominated periphyton where sulfate was abundant, BLs were only slightly more abundant than SLs. However, in the low-P, low-sulfate area, periphyton were comprised to a greater degree green algae and diatoms, and BLs represented the majority of the total lipids. Even in a P-rich area, PLs were a small component of periphyton lipid profiles. Despite the phosphorus limitations of the Everglades, periphyton can develop tremendous biomass. Our results suggest a physiological response by periphyton to oligotrophic conditions whereby periphyton increase abundances of nonphosphorus lipids and have reduced proportions of PLs.