An extra sugar protects: Many microbial pathogens produce proteins that are toxic to the cells that they are targeting. Broad-leaved plants are susceptible to NLP (necrosis and ethylene-inducing peptide 1–like protein) toxins. Lenarčič et al. identified the receptors for NLP toxins to be GIPC (glycosylinositol phosphorylceramide) sphingolipids (see the Perspective by Van den Ackerveken). Their findings reveal why these toxins only attack broad-leaved plants (so-called eudicots): If the sphingolipid carries just two hexoses, as is the case for eudicots, the toxin binds and causes cell lysis. But in monocots with sphingolipids that have three hexoses, the toxin is ineffective. Science , this issue p. 1431 ; see also p. 1383

Chloroplast membranes with their unique lipid composition are crucial for photosynthesis. Maintenance of the chloroplast membranes requires finely tuned lipid anabolic and catabolic reactions. Despite the presence of a large number of predicted lipid-degrading enzymes in the chloroplasts, their biological functions remain largely unknown. Recently, we described PLASTID LIPASE1 (PLIP1), a plastid phospholipase A1 that contributes to seed oil biosynthesis. The Arabidopsis thaliana genome encodes two putative PLIP1 paralogs, which we designated PLIP2 and PLIP3. PLIP2 and PLIP3 are also present in the chloroplasts, but likely with different subplastid locations. In vitro analysis indicated that both are glycerolipid A1 lipases. In vivo, PLIP2 prefers monogalactosyldiacylglycerol as substrate and PLIP3 phosphatidylglycerol. Overexpression of PLIP2 or PLIP3 severely reduced plant growth and led to accumulation of the bioactive form of jasmonate and related oxylipins. Genetically blocking jasmonate perception restored the growth of the PLIP2/3 overexpressing plants. The expression of PLIP2 and PLIP3, but not PLIP1 was induced by abscisic acid (ABA), and plip1 plip2 plip3 triple mutants exhibited compromised oxylipin biosynthesis in response to ABA. The plip triple mutants also showed hypersensitivity to ABA. We propose that PLIP2 and PLIP3 provide a mechanistic link between ABA-mediated abiotic stress responses and oxylipin signaling.

Photosynthetic microalgae are considered a viable and sustainable resource for biofuel feedstocks, because they can produce higher biomass per land area than plants and can be grown on non-arable land. Among many microalgae considered for biofuel production, Nannochloropsis oceanica (CCMP1779) is particularly promising, because following nutrient deprivation it produces very high amounts of triacylglycerols (TAG). The committed step in TAG synthesis is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT). Remarkably, a total of 13 putative DGAT-encoding genes have been previously identified in CCMP1779 but most have not yet been studied in detail. Based on their expression profile, six out of 12 type-2 DGAT-encoding genes (NoDGTT1-NoDGTT6) were chosen for their possible role in TAG biosynthesis and the respective cDNAs were expressed in a TAG synthesis-deficient mutant of yeast. Yeast expressing NoDGTT5 accumulated TAG to the highest level. Over-expression of NoDGTT5 in CCMP1779 grown in N-replete medium resulted in levels of TAG normally observed only after N deprivation. Reduced growth rates accompanied NoDGTT5 over-expression in CCMP1779. Constitutive expression of NoDGTT5 in Arabidopsis thaliana was accompanied by increased TAG content in seeds and leaves. A broad substrate specificity for NoDGTT5 was revealed, with preference for unsaturated acyl groups. Furthermore, NoDGTT5 was able to successfully rescue the Arabidopsis tag1-1 mutant by restoring the TAG content in seeds. Taken together, our results identified NoDGTT5 as the most promising gene for the engineering of TAG synthesis in multiple hosts among the 13 DGAT-encoding genes of N. oceanica CCMP1779. Consequently, this study demonstrates the potential of NoDGTT5 as a tool for enhancing the energy density in biomass by increasing TAG content in transgenic crops used for biofuel production.

The marine microalgae Nannochloropsis oceanica (CCMP1779) is a prolific producer of oil and is considered a viable and sustainable resource for biofuel feedstocks. Nitrogen (N) availability has a strong impact on the physiological status and metabolism of microalgal cells, but the exact nature of this response is poorly understood. To fill this gap we performed transcriptomic profiling combined with cellular and molecular analyses of N. oceanica CCMP1779 during the transition from quiescence to autotrophy. N deprivation-induced quiescence was accompanied by a strong reorganization of the photosynthetic apparatus and changes in the lipid homeostasis, leading to accumulation of triacylglycerol. Cell cycle activation and re-establishment of photosynthetic activity observed in response to resupply of the growth medium with N were accompanied by a rapid degradation of triacylglycerol stored in lipid droplets (LDs). Besides observing LD translocation into vacuoles, we also provide evidence for direct interaction between the LD surface protein (NoLDSP) and AUTOPHAGY-RELATED8 (NoATG8) protein and show a role of microlipophagy in LD turnover in N. oceanica CCMP1779. This knowledge is crucial not only for understanding the fundamental mechanisms controlling the cellular energy homeostasis in microalgal cells but also for development of efficient strategies to achieve higher algal biomass and better microalgal lipid productivity.

Although microalgal biofuels have potential advantages over conventional fossil fuels, high production costs limit their application in the market. We developed bio-flocculation and incubation methods for the marine alga, Nannochloropsis oceanica CCMP1779, and the oleaginous fungus, Mortierella elongata AG77, resulting in increased oil productivity. By growing separately and then combining the cells, the M. elongata mycelium could efficiently capture N. oceanica due to an intricate cellular interaction between the two species leading to bio-flocculation. Use of a high-salt culture medium induced accumulation of triacylglycerol (TAG) and enhanced the contents of polyunsaturated fatty acids (PUFAs) including arachidonic acid and docosahexaenoic acid in M. elongata. To increase TAG productivity in the alga, we developed an effective, reduced nitrogen-supply regime based on ammonium in environmental photobioreactors. Under optimized conditions, N. oceanica produced high levels of TAG that could be indirectly monitored by following chlorophyll content. Combining N. oceanica and M. elongata to initiate bio-flocculation yielded high levels of TAG and total fatty acids, with ~ 15 and 22% of total dry weight (DW), respectively, as well as high levels of PUFAs. Genetic engineering of N. oceanica for higher TAG content in nutrient-replete medium was accomplished by overexpressing DGTT5, a gene encoding the type II acyl-CoA:diacylglycerol acyltransferase 5. Combined with bio-flocculation, this approach led to increased production of TAG under nutrient-replete conditions (~ 10% of DW) compared to the wild type (~ 6% of DW). The combined use of M. elongata and N. oceanica with available genomes and genetic engineering tools for both species opens up new avenues to improve biofuel productivity and allows for the engineering of polyunsaturated fatty acids.