Abstract
Plant starch is the main energy contributor to the human diet. Its biosynthesis is catalyzed and regulated by co-ordinated actions of several enzymes. Recently, a factor termed Protein Targeting to Starch 1 (PTST1) was identified as being required for correct granule-bound starch synthase (GBSS) localization and demonstrated to be crucial for amylose synthesis in Arabidopsis. However, the function of its homologous protein in storage tissues (e.g. endosperm) is unknown. We identified a PTST1 homolog in barley and it was found to contain a crucial coiled-coil domain and carbohydrate-binding module. We demonstrated the interaction between PTST1 and GBSS1 by fluorescence resonance energy transfer (FRET) in barley endosperm. By tagging PTST1 with the fluorophore mCherry, we observed that it is localized in the stroma of barley endosperm amyloplasts. PTST1 overexpression in endosperm increased endogenous gbss1a gene expression and amylose content. Gbss1a and ptst1 mutants were generated using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-related protein 9 (Cas9)-based targeted mutagenesis. Homozygous gbss1a mutants showed a waxy phenotype. Grains of ptst1 mutants did not accumulate any starch. These grains dried out during the desiccation stage and were unable to germinate, suggesting that PTST1 is essential for development of starchy endosperm and viable grains.
Highlights
Starch biosynthesis in plants requires the co-ordinated actions of several enzymes, including ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), soluble starch synthase (SSS), starch branching enzyme, starch debranching enzyme, glucan water dikinase, and starch phosphorylase (Blennow et al, 2013).After converting glucose-1-phosphate to activated glucosyl donor ADP-glucose by AGPase, amylose and amylopectin are produced by the involvement of GBSS and SSS, respectively
In search of a Protein Targeting to Starch 1 (PTST1) protein in barley, we used BLASTp and, as entry, the amino acid sequence of AtPTST1.Though similar proteins were found in rice, maize, soybean, and other species, no similar protein was found in barley in the NCBI database
The candidate PTST1 in barley showed 94% similarity with wheat, 76% with maize and rice, and 54% with Arabidopsis PTST1, respectively (Fig. 1A).We further analyzed the domain structure of the candidate barley PTST1 using UniProt and the COILS/PCOILS www-service and found coiled-coil domains (Fig. 1B, right).They were similar in length to those found in AtPTST1 (Seung et al, 2015)
Summary
Starch biosynthesis in plants requires the co-ordinated actions of several enzymes, including ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), soluble starch synthase (SSS), starch branching enzyme, starch debranching enzyme, glucan water dikinase, and starch phosphorylase (Blennow et al, 2013).After converting glucose-1-phosphate to activated glucosyl donor ADP-glucose by AGPase, amylose and amylopectin are produced by the involvement of GBSS and SSS, respectively. Various isoforms of SS have been identified: GBSS, SSI, SSII, SSIII, and SSIV (Nakamura, 2002; James et al, 2003). Storage starch is accumulated in storage tissue including developing seeds and tubers, and serves as a long-term carbon store mainly for germination. Due to their different physiological functions, the mechanisms for starch biosynthesis in storage and transient tissues are significantly different. Even for storage tissues in cereals, the homologous genes involved in starch biosynthesis in different species may function in different ways.The same homologous mutant of ptst in barley, identified as fra, contains fractured starch granules rather than an altered number of starch granules (Saito et al, 2018). It is important to characterize the function of PTST homologs in different cereals and tissues
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
