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Abstract Diketopiperazines (DKPs) are chemically and functionally diverse cyclic dipeptides associated primarily with microbes. Few DKPs have been reported from plants and animals; the best characterized is cyclo(His-Pro), found in the mammalian central nervous system, where it arises from the proteolytic cleavage of a thyrotropin-releasing tripeptide hormone. Herein, we report the identification of cyclo(His-Pro) in Arabidopsis (Arabidopsis thaliana), where its levels increase upon abiotic stress conditions, including high salt, heat, and cold. To screen for potential protein targets, we used isothermal shift assays, which examine changes in protein-melting stability upon ligand binding. Among the identified proteins, we focused on the glycolytic enzyme, cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPC1). Binding between the GAPC1 protein and cyclo(His-Pro) was validated using nano-differential scanning fluorimetry and microscale thermophoresis, and we could further demonstrate that cyclo(His-Pro) inhibits GAPC1 activity with an IC50 of ∼200 μm. This inhibition was conserved in human GAPDH. Inhibition of glyceraldehyde-3-phosphate dehydrogenase activity has previously been reported to reroute carbon from glycolysis toward the pentose phosphate pathway. Accordingly, cyclo(His-Pro) supplementation augmented NADPH levels, increasing the NADPH/NADP+ ratio. Phenotypic screening revealed that plants supplemented with cyclo(His-Pro) were more tolerant to high-salt stress, as manifested by higher biomass, which we show is dependent on GAPC1/2. Our work reports the identification and functional characterization of cyclo(His-Pro) as a modulator of glyceraldehyde-3-phosphate dehydrogenase in plants. 

Abstract Biomolecular condensates are membraneless organelle-like structures that can concentrate molecules and often form through liquid-liquid phase separation. Biomolecular condensate assembly is tightly regulated by developmental and environmental cues. Although research on biomolecular condensates has intensified in the past 10 years, our current understanding of the molecular mechanisms and components underlying their formation remains in its infancy, especially in plants. However, recent studies have shown that the formation of biomolecular condensates may be central to plant acclimation to stress conditions. Here, we describe the mechanism, regulation, and properties of stress-related condensates in plants, focusing on stress granules and processing bodies, two of the most well-characterized biomolecular condensates. In this regard, we showcase the proteomes of stress granules and processing bodies, in an attempt to suggest methods for elucidating the composition and function of biomolecular condensates. Finally, we discuss how biomolecular condensates modulate stress responses and how they might be used as targets for biotechnological efforts to improve stress tolerance. 

Abstract L-serine (Ser) and L-glycine (Gly) are critically important for the overall functioning of primary metabolism. We investigated the interaction of the phosphorylated pathway of Ser biosynthesis (PPSB) with the photorespiration-associated glycolate pathway of Ser biosynthesis (GPSB) using Arabidopsis thaliana PPSB-deficient lines, GPSB-deficient mutants, and crosses of PPSB with GPSB mutants. PPSB-deficient lines mainly showed retarded primary root growth. Mutation of the photorespiratory enzyme Ser-hydroxymethyltransferase 1 (SHMT1) in a PPSB-deficient background resumed primary root growth and induced a change in the plant metabolic pattern between roots and shoots. Grafting experiments demonstrated that metabolic changes in shoots were responsible for the changes in double mutant development. PPSB disruption led to a reduction in nitrogen (N) and sulfur (S) contents in shoots and a general transcriptional response to nutrient deficiency. Disruption of SHMT1 boosted the Gly flux out of the photorespiratory cycle, which increased the levels of the one-carbon (1C) metabolite 5,10-methylene-tetrahydrofolate and S-adenosylmethionine. Furthermore, disrupting SHMT1 reverted the transcriptional response to N and S deprivation and increased N and S contents in shoots of PPSB-deficient lines. Our work provides genetic evidence of the biological relevance of the Ser–Gly–1C metabolic network in N and S metabolism and in interorgan metabolic homeostasis. 

Tremendous chemical diversity is the hallmark of plants and is supported by highly complex biochemical machinery. Plant metabolic enzymes originated and were transferred from eukaryotic and prokaryotic ancestors and further diversified by the unprecedented rates of gene duplication and functionalization experienced in land plants. Unlike microbes, which have frequent horizontal gene transfer events and multiple inputs of energy and organic carbon, land plants predominantly rely on organic carbon generated from CO 2 and have experienced very few, if any, gene transfers during their recent evolutionary history. As such, plant metabolic networks have evolved in a stepwise manner and on existing networks under various evolutionary constraints. This review aims to take a broader view of plant metabolic evolution and lay a framework to further explore underlying evolutionary mechanisms of the complex metabolic network. Understanding the underlying metabolic and genetic constraints is also an empirical prerequisite for rational engineering and redesigning of plant metabolic pathways. Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

Growing knowledge about crop domestication, combined with increasingly powerful gene-editing toolkits, sets the stage for the continual domestication of crop wild relatives and other lesser-known plant species.