3 ABSTRACT 49The regulated transport of mRNAs from the cell nucleus to the cytosol is a critical step 50 linking transcript synthesis and processing with translation. However, in plants, only few of 51 the factors that act in the mRNA export pathway have been functionally characterised. 52Flowering plant genomes encode several members of the ALY protein family, which function 53 as mRNA export factors in other organisms. Arabidopsis thaliana ALY1-4 are commonly 54 detected in root and leaf cells, but are differentially expressed in reproductive tissue. 55Moreover, the subnuclear distribution of ALY1/2 differs from that of ALY3/4. ALY1 binds 56 with higher affinity to ssRNA than dsRNA and ssDNA, and interacts preferentially with 5- THO rather than that of yeast (Yelina et al. 2010). Likewise, THO associates with UAP56, 99ALYs and MOS11 (the orthologue of CIP29) in Arabidopsis cells (Sørensen et al. 2017 (Germain et al. 2010;Lu et al. 2010;Pan et al. 2012;Xu et al. 2015; 114 Sørensen et al. 2017), but so far none of the ALY mRNA export adaptor candidates have been 115shown to function in nucleo-cytosolic transport of mRNAs in plants. 116In this study, we have systematically studied the Arabidopsis ALY proteins including their RESULTS 126 ALY proteins in Arabidopsis and other plants 127First, we compared the amino acid sequences of the four Arabidopsis ALY proteins (ALY1- sequence identity) as well as ALY3 and ALY4 (70% amino acid sequence identity) share a 137 high degree of sequence similarity, whereas the similarity of ALY1/2 versus ALY3/4 is 138 clearly lower (<42% amino acid sequence identity) (Supplemental Fig. S1B (Fig. 1A). ALY1 and truncated versions of the protein were expressed in 157 E. coli as 6xHis-GB1 fusion proteins, purified by two-step chromatography, and examined by 158 SDS-PAGE (Fig. 1B). For comparison we also used the unfused 6xHis-GB1 tag. The purified (Fig. 1C, Student's t-test, P < 0.05 and P < 0.001, respectively). The unfused 6xHis-GB1 165 tag did not exhibit affinity for ssRNA. To examine which domains of ALY1 contribute to the 166 RNA interactions, the binding to ssRNA of the different recombinant ALY1 versions was 167 measured (Fig. 1D (Fig. 3C, D). Therefore, the four ALY proteins and UAP56 228 are widely expressed in sporophytic cells of Arabidopsis plants. 229The subnuclear localisation of the ALY-GFP fusions was inspected in more detail by 230 CLSM in comparison to the UAP56-GFP and GFP-NLS controls. GFP-NLS (Antosch et al. immunofluorescence microscopy analysis (Kammel et al. 2013 (Table S1). In leaf cells, the nucleoplasmic distribution of 238 the ALY proteins appeared more heterogeneous than in root cells particularly for ALY4 that 239 partially localised to nucleoplasmic foci (Fig. 4B). Notably, the nucleolar enrichment of 253In view of the apparently ubiquitous expression of the four ALY-GFP proteins in 254 sporophytic cells, their occurrence was analysed in male and female gametophytes by CLSM. 255In mature pollen grains (Fig. 5A), the fluorescent signal of ALY1-...

Because it is the precursor for various essential cellular components, the amino acid serine is indispensable for every living organism. In plants, serine is synthesized by two major pathways: photorespiration and the phosphorylated pathway of serine biosynthesis (PPSB). However, the importance of these pathways in providing serine for plant development is not fully understood. In this study, we examine the relative contributions of photorespiration and PPSB to providing serine for growth and metabolism in the C3 model plant Arabidopsis thaliana. Our analyses of cell proliferation and elongation reveal that PPSB-derived serine is indispensable for plant growth and its loss cannot be compensated by photorespiratory serine biosynthesis. Using isotope labeling, we show that PPSB-deficiency impairs the synthesis of proteins and purine nucleotides in plants. Furthermore, deficiency in PPSB-mediated serine biosynthesis leads to a strong accumulation of metabolites related to nitrogen metabolism. This result corroborates 15N-isotope labeling in which we observed an increased enrichment in labeled amino acids in PPSB-deficient plants. Expression studies indicate that elevated ammonium uptake and higher GS/GOGAT activity causes this phenotype. Metabolic analyses further show that elevated nitrogen assimilation and reduced amino acid turnover into proteins and nucleotides are the most likely driving forces for changes in respiratory metabolism and amino acid catabolism in PPSB-deficient plants. Accordingly, we conclude that even though photorespiration generates high amounts of serine in plants, PPSB-derived serine is more important for plant growth and its deficiency triggers the induction of nitrogen assimilation, most likely as an amino acid starvation response.

Comparative transcriptomic analysis reveals conserved programmes underpinning organogenesis and reproduction in land plants Julca

This study characterizes the phosphorylated pathway of Ser biosynthesis (PPSB) in Arabidopsis thaliana by targeting phosphoserine phosphatase (PSP1), the last enzyme of the pathway. Lack of PSP1 activity delayed embryo development, leading to aborted embryos that could be classified as early curled cotyledons. The embryo-lethal phenotype of psp1 mutants could be complemented with PSP1 cDNA under the control of Pro35S (Pro35S:PSP1). However, this construct, which was poorly expressed in the anther tapetum, did not complement mutant fertility. Microspore development in psp1.1/psp1.1 Pro35S:PSP1 arrested at the polarized stage. The tapetum from these lines displayed delayed and irregular development. The expression of PSP1 in the tapetum at critical stages of microspore development suggests that PSP1 activity in this cell layer is essential in pollen development. In addition to embryo death and male sterility, conditional psp1 mutants displayed a shortroot phenotype, which was reverted in the presence of Ser. A metabolomic study demonstrated that the PPSB plays a crucial role in plant metabolism by affecting glycolysis, the tricarboxylic acid cycle, and the biosynthesis of amino acids. We provide evidence of the crucial role of the PPSB in embryo, pollen, and root development and suggest that this pathway is an important link connecting primary metabolism with development.

This work contributes to unraveling the role of the phosphorylated pathway of serine (Ser) biosynthesis in Arabidopsis (Arabidopsis thaliana) by functionally characterizing genes coding for the first enzyme of this pathway, 3-phosphoglycerate dehydrogenase (PGDH). We identified two Arabidopsis plastid-localized PGDH genes (3-PGDH and EMBRYO SAC DEVELOPMENT ARREST9[EDA9]) with a high percentage of amino acid identity with a previously identified PGDH. All three genes displayed a different expression pattern indicating that they are not functionally redundant. pgdh and 3-pgdh mutants presented no drastic visual phenotypes, but eda9 displayed delayed embryo development, leading to aborted embryos that could be classified as early curled cotyledons. The embryo-lethal phenotype of eda9 was complemented with an EDA9 complementary DNA under the control of a 35S promoter (Pro-35S:EDA9). However, this construct, which is poorly expressed in the anther tapetum, did not complement mutant fertility. Microspore development in eda9.1eda9.1 Pro-35S:EDA9 was arrested at the polarized stage. Pollen from these lines lacked tryphine in the interstices of the exine layer, displayed shrunken and collapsed forms, and were unable to germinate when cultured in vitro. A metabolomic analysis of PGDH mutant and overexpressing plants revealed that all three PGDH family genes can regulate Ser homeostasis, with PGDH being quantitatively the most important in the process of Ser biosynthesis at the whole-plant level. By contrast, the essential role of EDA9 could be related to its expression in very specific cell types. We demonstrate the crucial role of EDA9 in embryo and pollen development, suggesting that the phosphorylated pathway of Ser biosynthesis is an important link connecting primary metabolism with development.