Editing Starch (section)

===Biosynthesis===
Plants synthesize starch in two types of tissues. The first type is storage tissues, for example, cereal endosperm, and storage roots and stems such as cassava and potato. The second type is green tissue, for example, leaves, where many plant species synthesize transitory starch on a daily basis. In both tissue types, starch is synthesized in plastids (amyloplasts and chloroplasts).

The biochemical pathway involves conversion of [[glucose 1-phosphate]] to [[Adenosine diphosphate|ADP]]-glucose using the enzyme [[glucose-1-phosphate adenylyltransferase]]. This step requires energy in the form of [[Adenosine triphosphate|ATP]]. A number of [[starch synthase|starch synthases]] available in plastids then adds the ADP-glucose via α-1,4-[[glycosidic bond]] to a growing chain of glucose residues, liberating [[Adenosine diphosphate|ADP]]. The ADP-glucose is almost certainly added to the non-reducing end of the amylose polymer, as the UDP-glucose is added to the non-reducing end of glycogen during [[Glycogenesis|glycogen synthesis]].<ref>Nelson, D. (2013) Lehninger Principles of Biochemistry, 6th ed., W.H. Freeman and Company (p. 819)</ref> The small glucan chain, further agglomerate to form initials of starch granules. 

The biosynthesis and expansion of granules represent a complex molecular event that can be subdivided into four major steps, namely, granule initiation, coalescence of small granules,<ref>{{Cite journal |last1=Bürgy |first1=Léo |last2=Eicke |first2=Simona |last3=Kopp |first3=Christophe |last4=Jenny |first4=Camilla |last5=Lu |first5=Kuan Jen |last6=Escrig |first6=Stephane |last7=Meibom |first7=Anders |last8=Zeeman |first8=Samuel C. |date=2021-11-26 |title=Coalescence and directed anisotropic growth of starch granule initials in subdomains of Arabidopsis thaliana chloroplasts |journal=Nature Communications |language=en |volume=12 |issue=1 |page=6944 |doi=10.1038/s41467-021-27151-5 |issn=2041-1723 |pmc=8626487 |pmid=34836943|bibcode=2021NatCo..12.6944B }}</ref> phase transition, and expansion. Several proteins have been characterized for their involvement in each of these processes. For instance, a chloroplast membrane-associated protein, MFP1, determines the sites of granule initiation.<ref>{{Cite journal |last1=Sharma |first1=Mayank |last2=Abt |first2=Melanie R. |last3=Eicke |first3=Simona |last4=Ilse |first4=Theresa E. |last5=Liu |first5=Chun |last6=Lucas |first6=Miriam S. |last7=Pfister |first7=Barbara |last8=Zeeman |first8=Samuel C. |date=2024-01-16 |title=MFP1 defines the subchloroplast location of starch granule initiation |journal=Proceedings of the National Academy of Sciences |language=en |volume=121 |issue=3 |pages=e2309666121 |doi=10.1073/pnas.2309666121 |issn=0027-8424|doi-access=free |pmid=38190535 |pmc=10801857 |bibcode=2024PNAS..12109666S }}</ref> Another protein named PTST2 binds to small glucan chains and agglomerates to recruit starch synthase 4 (SS4).<ref>{{Cite journal |last1=Seung |first1=David |last2=Boudet |first2=Julien |last3=Monroe |first3=Jonathan |last4=Schreier |first4=Tina B. |last5=David |first5=Laure C. |last6=Abt |first6=Melanie |last7=Lu |first7=Kuan-Jen |last8=Zanella |first8=Martina |last9=Zeeman |first9=Samuel C. |date=July 2017 |title=Homologs of PROTEIN TARGETING TO STARCH Control Starch Granule Initiation in Arabidopsis Leaves |journal=The Plant Cell |language=en |volume=29 |issue=7 |pages=1657–1677 |doi=10.1105/tpc.17.00222 |issn=1040-4651 |pmc=5559754 |pmid=28684429}}</ref> Three other proteins, namely, PTST3, SS5, and MRC, are also known to be involved in the process of starch granule initiation.<ref>{{Cite journal |last1=Seung |first1=David |last2=Schreier |first2=Tina B. |last3=Bürgy |first3=Léo |last4=Eicke |first4=Simona |last5=Zeeman |first5=Samuel C. |date=July 2018 |title=Two Plastidial Coiled-Coil Proteins Are Essential for Normal Starch Granule Initiation in Arabidopsis |journal=The Plant Cell |language=en |volume=30 |issue=7 |pages=1523–1542 |doi=10.1105/tpc.18.00219 |issn=1040-4651 |pmc=6096604 |pmid=29866647}}</ref><ref>{{Cite journal |last1=Vandromme |first1=Camille |last2=Spriet |first2=Corentin |last3=Dauvillée |first3=David |last4=Courseaux |first4=Adeline |last5=Putaux |first5=Jean-Luc |last6=Wychowski |first6=Adeline |last7=Krzewinski |first7=Frédéric |last8=Facon |first8=Maud |last9=D'Hulst |first9=Christophe |last10=Wattebled |first10=Fabrice |date=January 2019 |title=PII1: a protein involved in starch initiation that determines granule number and size in Arabidopsis chloroplast |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15356 |journal=New Phytologist |language=en |volume=221 |issue=1 |pages=356–370 |doi=10.1111/nph.15356 |pmid=30055112 |bibcode=2019NewPh.221..356V |issn=0028-646X}}</ref><ref>{{Cite journal |last1=Abt |first1=Melanie R. |last2=Pfister |first2=Barbara |last3=Sharma |first3=Mayank |last4=Eicke |first4=Simona |last5=Bürgy |first5=Léo |last6=Neale |first6=Isabel |last7=Seung |first7=David |last8=Zeeman |first8=Samuel C. |date=August 2020 |title=STARCH SYNTHASE5, a Noncanonical Starch Synthase-Like Protein, Promotes Starch Granule Initiation in Arabidopsis |journal=The Plant Cell |language=en |volume=32 |issue=8 |pages=2543–2565 |doi=10.1105/tpc.19.00946 |issn=1040-4651 |pmc=7401018 |pmid=32471861}}</ref> Furthermore, two proteins named ESV and LESV play a role in the aqueous-to-crystalline phase transition of glucan chains.<ref>{{Cite journal |last1=Liu |first1=Chun |last2=Pfister |first2=Barbara |last3=Osman |first3=Rayan |last4=Ritter |first4=Maximilian |last5=Heutinck |first5=Arvid |last6=Sharma |first6=Mayank |last7=Eicke |first7=Simona |last8=Fischer-Stettler |first8=Michaela |last9=Seung |first9=David |last10=Bompard |first10=Coralie |last11=Abt |first11=Melanie R. |last12=Zeeman |first12=Samuel C. |date=2023-05-26 |title=LIKE EARLY STARVATION 1 and EARLY STARVATION 1 promote and stabilize amylopectin phase transition in starch biosynthesis |journal=Science Advances |language=en |volume=9 |issue=21 |pages=eadg7448 |doi=10.1126/sciadv.adg7448 |issn=2375-2548 |pmc=10219597 |pmid=37235646|bibcode=2023SciA....9G7448L }}</ref> Several catalytically active starch synthases, such as SS1, SS2, SS3, and GBSS, are critical for starch granule biosynthesis and play a catalytic role at each step of granule biogenesis and expansion.<ref>{{Cite journal |last1=Pfister |first1=Barbara |last2=Zeeman |first2=Samuel C. |date=July 2016 |title=Formation of starch in plant cells |journal=Cellular and Molecular Life Sciences |language=en |volume=73 |issue=14 |pages=2781–2807 |doi=10.1007/s00018-016-2250-x |issn=1420-682X |pmc=4919380 |pmid=27166931}}</ref>

In addition to above proteins, [[Starch branching enzyme|starch branching enzymes (BEs)]] introduces α-1,6-glycosidic bonds between the glucose chains, creating the branched amylopectin. The starch debranching enzyme (DBE) [[isoamylase]] removes some of these branches. Several [[isoform]]s of these enzymes exist, leading to a highly complex synthesis process.<ref>{{cite journal |doi=10.1021/bm000133c |title=The Biosynthesis of Starch Granules |date=2001 |last1=Smith |first1=Alison M. |journal=Biomacromolecules |volume=2 |issue=2 |pages=335–41 |pmid=11749190}}</ref>