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BackgroundPhotosynthetic oleaginous microalgae are considered promising feedstocks for biofuels. The marine microalga, Nannochloropsis oceanica, has been attracting ever-increasing interest because of its fast growth, high triacylglycerol (TAG) content, and available genome sequence and genetic tools. Diacylglycerol acyltransferase (DGAT) catalyzes the last and committed step of TAG biosynthesis in the acyl-CoA-dependent pathway. Previous studies have identified 13 putative DGAT-encoding genes in the genome of N. oceanica, but the functional role of DGAT genes, especially type-I DGAT (DGAT1), remains ambiguous.Results Nannochloropsis oceanica IMET1 possesses two DGAT1 genes: NoDGAT1A and NoDGAT1B. Functional complementation demonstrated the capability of NoDGAT1A rather than NoDGAT1B to restore TAG synthesis in a TAG-deficient yeast strain. In vitro DGAT assays revealed that NoDGAT1A preferred saturated/monounsaturated acyl-CoAs and eukaryotic diacylglycerols (DAGs) for TAG synthesis, while NoDGAT1B had no detectable enzymatic activity. Assisted with green fluorescence protein (GFP) fusion, fluorescence microscopy analysis indicated the localization of NoDGAT1A in the chloroplast endoplasmic reticulum (cER) of N. oceanica. NoDGAT1A knockdown caused ~25% decline in TAG content upon nitrogen depletion, accompanied by the reduced C16:0, C18:0, and C18:1 in TAG sn-1/sn-3 positions and C18:1 in the TAG sn-2 position. NoDGAT1A overexpression, on the other hand, led to ~39% increase in TAG content upon nitrogen depletion, accompanied by the enhanced C16:0 and C18:1 in the TAG sn-1/sn-3 positions and C18:1 in the TAG sn-2 position. Interestingly, NoDGAT1A overexpression also promoted TAG accumulation (by ~2.4-fold) under nitrogen-replete conditions without compromising cell growth, and TAG yield of the overexpression line reached 0.49 g L−1 at the end of a 10-day batch culture, 47% greater than that of the control line.ConclusionsTaken together, our work demonstrates the functional role of NoDGAT1A and sheds light on the underlying mechanism for the biosynthesis of various TAG species in N. oceanica. NoDGAT1A resides likely in cER and prefers to transfer C16 and C18 saturated/monounsaturated fatty acids to eukaryotic DAGs for TAG assembly. This work also provides insights into the rational genetic engineering of microalgae by manipulating rate-limiting enzymes such as DGAT to modulate TAG biosynthesis and fatty acid composition for biofuel production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0858-1) contains supplementary material, which is available to authorized users.

Melatonin regulates growth in many plants; however, the mechanism remains unclear. In this study, exogenous melatonin feeding resulted in both promotional (≤10 μm) and inhibitory (≥100 μm) effects on maize seedling growth. Initial analyses suggested positive correlations between the amount of melatonin and sucrose synthesis and hydrolysis-related gene expression, enzyme activities, and sucrose metabolites. However, assays of photosynthetic rate, hexokinase (HxK) activity, expression of photosynthetic marker genes, and HxK-related genes showed opposite effects under 10 μm (positive) and 100 μm (negative) melatonin treatments. Similarly, 10 μm melatonin accelerated starch catabolism at night, whereas 100 μm melatonin significantly decreased this process and led to starch accumulation in photosynthetic tissues. Furthermore, expression analysis of genes related to sucrose phloem loading resulted in a slight upregulation of sucrose transporters (SUT1 and SUT2) when seedlings were induced with 10 μm melatonin, while treatment with 100 μm melatonin resulted in significant downregulation of these sucrose transporter genes (SUT1 and SUT2), as well as tie-dyed2 (Tdy2) and sucrose export defective 1. Taken together, these results suggest that low doses of melatonin benefit maize seedling growth by promoting sugar metabolism, photosynthesis, and sucrose phloem loading. Conversely, high doses of melatonin inhibit seedling growth by inducing the excessive accumulation of sucrose, hexose and starch, suppressing photosynthesis and sucrose phloem loading.

Branching/tillering is an important parameter of plant architecture and is tightly regulated by both internal factors (such as plant hormones) and external factors (such as light conditions). How the various signaling pathways converge to coordinately regulate branching is not well understood. Here, we report that in Arabidopsis, FHY3 and FAR1, two homologous transcription factors essential for phytochrome A-mediated light signaling, and SMXL6/ SMXL7/SMXL8, three key repressors of the strigolactone (SL) signaling pathway, directly interact with SPL9 and SPL15 and suppress their transcriptional activation of BRC1, a key repressor of branching, thus promoting branching. In addition, FHY3 and FAR1 also directly up-regulate the expression of SMXL6 and SMXL7 to promote branching. Simulated shade treatment reduces the accumulation of FHY3 protein, leading to increased expression of BRC1 and reduced branching. Our results establish an integrated model of light and SL coordinately regulating BRC1 expression and branching through converging at the BRC1 promoter.

Increasing crop yield per unit of area can be achieved by increasing planting density. However, high-density planting could trigger shade avoidance responses, which cause exaggerated growth and increased susceptibility to various diseases. Previous studies have shown that the rapid elongation of plants under shade (i.e., reduced red to far-red ratios) is regulated by phytochromes and various phytohormones. However, the detailed molecular mechanisms governing the interaction among these signaling pathways are not well understood. Here, we report that loss-of-function mutants of FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and FAR-RED-IMPAIRED RESPONSE1 (FAR1), which encode two homologous transcription factors essential for phytochrome signaling, exhibit an exaggerated shade avoidance phenotype. We show that FHY3 and FAR1 repress plant growth through directly activating the expression of two atypical basic helix-loop-helix transcriptional cofactors, PHYTOCHROME RAPIDLY REGULATED1 (PAR1) and PAR2, and that this process is antagonized by a group of JASMONATE ZIM-DOMAIN proteins, key repressors of the jasmonic acid (JA) signaling pathway, through physical interactions. Furthermore, we show that FHY3 interacts with MYC2, a key transcriptional regulator of JA responses, coordinately regulating JA-responsive defense gene expression. Our results unveil a previously unrecognized mechanism whereby plants balance their growth and defense responses through convergence of the phytochrome signaling pathway and JA signaling pathway under shade conditions.