Deficiency in the Phosphorylated Pathway of Serine Biosynthesis Perturbs Sulfur Assimilation

Anoman AD, Flores-Tornero M, Benstein RM, et al. Deficiency in the Phosphorylated Pathway of Serine Biosynthesis Perturbs Sulfur Assimilation. PLANT PHYSIOLOGY. 2019;180(1):153-170.Although the plant Phosphorylated Pathway of L-Ser Biosynthesis (PPSB) is essential for embryo and pollen development, and for root growth, its metabolic implications have not been fully investigated. A transcriptomics analysis of Arabidopsis (Arabidopsis thaliana) PPSB-deficient mutants at night, when PPSB activity is thought to be more important, suggested interaction with the sulfate assimilation process. Because sulfate assimilation occurs mainly in the light, we also investigated it in PPSB-deficient lines in the day. Key genes in the sulfate starvation response, such as the adenosine 5'phosphosulfate reductase genes, along with sulfate transporters, especially those involved in sulfate translocation in the plant, were induced in the PPSB-deficient lines. However, sulfate content was not reduced in these lines as compared with wild-type plants; besides the glutathione (GSH) steady-state levels in roots of PPSB-deficient lines were even higher than in wild type. This suggested that PPSB deficiency perturbs the sulfate assimilation process between tissues/organs. Alteration of thiol distribution in leaves from different developmental stages, and between aerial parts and roots in plants with reduced PPSB activity, provided evidence supporting this idea. Diminished PPSB activity caused an enhanced flux of S-35 into thiol biosynthesis, especially in roots. GSH turnover also accelerated in the PPSB-deficient lines, supporting the notion that not only biosynthesis, but also transport and allocation, of thiols were perturbed in the PPSB mutants. Our results suggest that PPSB is required for sulfide assimilation in specific heterotrophic tissues and that a lack of PPSB activity perturbs sulfur homeostasis between photosynthetic and nonphotosynthetic tissues

Comparative analyses of angiosperm secretomes identify apoplastic pollen tube functions and novel secreted peptides

Key message Analyses of secretomes of in vitro grown pollen tubes from Amborella, maize and tobacco identified many components of processes associated with the cell wall, signaling and metabolism as well as novel small secreted peptides. Abstract Flowering plants (angiosperms) generate pollen grains that germinate on the stigma and produce tubes to transport their sperm cells cargo deep into the maternal reproductive tissues toward the ovules for a double fertilization process. During their journey, pollen tubes secrete many proteins (secreted proteome or secretome) required, for example, for communication with the maternal reproductive tissues, to build a solid own cell wall that withstands their high turgor pressure while softening simultaneously maternal cell wall tissue. The composition and species specificity or family specificity of the pollen tube secretome is poorly understood. Here, we provide a suitable method to obtain the pollen tube secretome from in vitro grown pollen tubes of the basal angiosperm Amborella trichopoda (Amborella) and the Poaceae model maize. The previously published secretome of tobacco pollen tubes was used as an example of eudicotyledonous plants in this comparative study. The secretome of the three species is each strongly different compared to the respective protein composition of pollen grains and tubes. In Amborella and maize, about 40% proteins are secreted by the conventional “classic” pathway and 30% by unconventional pathways. The latter pathway is expanded in tobacco. Proteins enriched in the secretome are especially involved in functions associated with the cell wall, cell surface, energy and lipid metabolism, proteolysis and redox processes. Expansins, pectin methylesterase inhibitors and RALFs are enriched in maize, while tobacco secretes many proteins involved, for example, in proteolysis and signaling. While the majority of proteins detected in the secretome occur also in pollen grains and pollen tubes, and correlate in the number of mapped peptides with relative gene expression levels, some novel secreted small proteins were identified. Moreover, the identification of secreted proteins containing pro peptides indicates that these are processed in the apoplast. In conclusion, we provide a proteome resource from three distinct angiosperm clades that can be utilized among others to study the localization, abundance and processing of known secreted proteins and help to identify novel pollen tube secreted proteins for functional studies

The phosphorylated pathway of serine biosynthesis links plant growth with nitrogen metabolism

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 N-15-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 glutamine synthetase/glutamine oxoglutarate aminotransferase (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

The appearance of plant organs mediated the explosive radiation of land plants, which shaped the biosphere and allowed the establishment of terrestrial animal life. The evolution of organs and immobile gametes required the coordinated acquisition of novel gene functions, the co-option of existing genes and the development of novel regulatory programmes. However, no large-scale analyses of genomic and transcriptomic data have been performed for land plants. To remedy this, we generated gene expression atlases for various organs and gametes of ten plant species comprising bryophytes, vascular plants, gymnosperms and flowering plants. A comparative analysis of the atlases identified hundreds of organ- and gamete-specific orthogroups and revealed that most of the specific transcriptomes are significantly conserved. Interestingly, our results suggest that co-option of existing genes is the main mechanism for evolving new organs. In contrast to female gametes, male gametes showed a high number and conservation of specific genes, which indicates that male reproduction is highly specialized. The expression atlas capturing pollen development revealed numerous transcription factors and kinases essential for pollen biogenesis and function