Arabidopsis trigalactosyldiacylglycerol1 mutants reveal a critical role for phosphtidylcholine remodeling in lipid homeostasis.

Lipid remodeling plays a critical role in plant response to abiotic stress and metabolic perturbations. Key steps in this process involve modifications of phosphatidylcholine (PC) acyl chains mediated by lysophosphatidylcholine: acyl-CoA acyltransferases (LPCATs) and phosphatidylcholine: diacylglycerol cholinephosphotransferase (ROD1). To assess their importance in lipid homeostasis, we took advantage of the trigalactosyldiacylglycerol1 (tgd1) mutant that exhibits marked increases in fatty acid synthesis and fatty acid flux through PC due to a block in inter-organelle lipid trafficking. Here, we showed that the increased fatty acid synthesis in tgd1 is due to posttranslational activation of the plastidic acetyl-coenzyme A carboxylase. Genetic analysis showed that knockout of LPCAT1 and 2 resulted in a lethal phenotype in tgd1. In addition, plants homozygous for lpcat2 and heterozygous for lpcat1 in the tgd1 background showed reduced levels of PC and triacylglycerols (TAG) and alterations in their fatty acid profiles. We further showed that disruption of ROD1 in tgd1 resulted in changes in fatty acid composition of PC and TAG, decreased leaf TAG content and reduced seedling growth. Together, our results reveal a critical role of LPCATs and ROD1 in maintaining cellular lipid homeostasis under conditions, in which fatty acid production largely exceeds the cellular demand for membrane lipid synthesis.

The outstanding capacity of Prasiola antarctica to thrive in contrasting harsh environments relies on the constitutive protection of thylakoids and on morphological plasticity.

The determination of physiological tolerance ranges of photosynthetic species and of the biochemical mechanisms underneath are fundamental to identify target processes and metabolites that will inspire enhanced plant management and production for the future. In this context, the terrestrial green algae within the genus Prasiola represent ideal models due to their success in harsh environments (polar tundras) and their extraordinary ecological plasticity. Here we focus on the outstanding Prasiola antarctica and compare two natural populations living in very contrasting microenvironments in Antarctica: the dry sandy substrate of a beach and the rocky bed of an ephemeral freshwater stream. Specifically, we assessed their photosynthetic performance at different temperatures, reporting for the first time gnsd values in algae and changes in thylakoid metabolites in response to extreme desiccation. Stream population showed lower α-tocopherol content and thicker cell walls and thus, lower gnsd and photosynthesis. Both populations had high temperatures for optimal photosynthesis (around +20°C) and strong constitutive tolerance to freezing and desiccation. This tolerance seems to be related to the high constitutive levels of xanthophylls and of the cylindrical lipids di- and tri-galactosyldiacylglycerol in thylakoids, very likely related to the effective protection and stability of membranes. Overall, P. antarctica shows a complex battery of constitutive and plastic protective mechanisms that enable it to thrive under harsh conditions and to acclimate to very contrasting microenvironments, respectively. Some of these anatomical and biochemical adaptations may partially limit photosynthesis, but this has a great potential to rise in a context of increasing temperature.

SENSITIVE TO FREEZING2 is crucial for growth of Marchantia polymorpha under acidic conditions.

Land plants have evolved many systems to adapt to a wide range of environmental stresses. In seed plants, oligogalactolipid synthesis is involved in tolerance to freezing and dehydration, but it has not been analyzed in non-vascular plants. Here we analyzed trigalactosyldiacylglycerol (TGDG) synthesis in Marchantia polymorpha. TGDG is synthesized by galactolipid: galactolipid galactosyltransferase [GGGT; SENSITIVE TO FREEZING2 (SFR2) in Arabidopsis]. We analyzed the subcellular localization and GGGT activity of two M. polymorpha SFR2 homologs (MpGGGT1 and MpGGGT2, each as a GFP-fusion protein) using a transient expression system in Nicotiana benthamiana leaves and found that MpGGGT1-GFP localized in the chloroplast envelope membrane. We produced mutants Mpgggt1 and Mpgggt2 and found that TGDG did not accumulate in Mpgggt1 upon treatment of the thallus with acetic acid. Moreover, growth of Mpgggt1 mutants was impaired by acetic acid treatment. Microscopy revealed that the acetic acid treatment of M. polymorpha plants damaged intracellular membranes. The fact that the effect was similar for wild-type and Mpgggt1 plants suggested that MpGGGT has a role in recovery from damage. These results indicate that MpGGGT plays a crucial role in M. polymorpha growth under conditions of acid stress, which may have been encountered during the ancient terrestrial colonization of plants.

Trigalactosyldiacylglycerol 3 protein orthologs are required for basal disease resistance in Nicotiana benthamiana.

Phosphatidic acid plays an important role in Nicotiana benthamiana immune responses against phytopathogenic bacteria. We analyzed the contributions of endoplasmic reticulum-derived chloroplast phospholipids, including phosphatidic acid, to the resistance of N. benthamiana against Ralstonia solanacearum. Here, we focused on trigalactosyldiacylglycerol 3 (TGD3) protein as a candidate required for phosphatidic acid signaling. On the basis of Arabidopsis thaliana TGD3 sequences, we identified two putative TGD3 orthologs in the N. benthamiana genome, NbTGD3-1 and NbTGD3-2. To address the role of TGD3s in plant defense responses, we created double NbTGD3-silenced plants using virus-induced gene silencing. The NbTGD3-silenced plants showed a moderately reduced growth phenotype. Bacterial growth and the appearance of bacterial wilt disease were accelerated in NbTGD3-silenced plants, compared with control plants, challenged with R. solanacearum. The NbTGD3-silenced plants showed reduced both expression of allene oxide synthase that encoded jasmonic acid biosynthetic enzyme and NbPR-4, a marker gene for jasmonic acid signaling, after inoculation with R. solanacearum. Thus, NbTGD3-mediated endoplasmic reticulum-chloroplast lipid transport might be required for jasmonic acid signaling-mediated basal disease resistance in N. benthamiana.

Inventory and general analysis of the ATP-binding cassette (ABC) gene superfamily in maize (Zea mays L.).

The metabolic functions of ATP-binding cassette (or ABC) proteins, one of the largest families of proteins presented in all organisms, have been investigated in many protozoan, animal and plant species. To facilitate more systematic and complicated studies on maize ABC proteins in the future, we present the first complete inventory of these proteins, including 130 open reading frames (ORFs), and provide general descriptions of their classifications, basic structures, typical functions, evolution track analysis and expression profiles. The 130 ORFs were assigned to eight subfamilies based on their structures and homological features. Five of these subfamilies consist of 109 proteins, containing transmembrane domains (TM) performing as transporters. The rest three subfamilies contain 21 soluble proteins involved in various functions other than molecular transport. A comparison of ABC proteins among nine selected species revealed either convergence or divergence in each of the ABC subfamilies. Generally, plant genomes contain far more ABC genes than animal genomes. The expression profiles and evolution track of each maize ABC gene were further investigated, the results of which could provide clues for analyzing their functions. Quantitative real-time polymerase chain reaction experiments (PCR) were conducted to detect induced expression in select ABC genes under several common stresses. This investigation provides valuable information for future research on stress tolerance in plants and potential strategies for enhancing maize production under stressful conditions.