Suggested models of starch biosynthesis in leaves.

<p>(A) The classic model of starch biosynthesis according to which (a) the starch biosynthetic process takes place exclusively in the chloroplast, segregated from the sucrose biosynthetic process taking place in the cytosol, and (b) AGP exclusively catalyzes the synthesis of ADPG. (B) Suggested “additional/alternative” model of starch biosynthesis wherein (a) ADPG is produced in the cytosol by enzyme(s) such as SuSy and then is transported to the chloroplast by the action of an ADPG translocator, and (b) pPGM and AGP play an important role in the scavenging of glucose units derived from starch breakdown. Starch to glucose conversion would involve the coordinated actions of amylases, isoamylase and disproportionating enzyme <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104997#pone.0104997-Asatsuma1" target="_blank">[21]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104997#pone.0104997-Streb3" target="_blank">[23]</a>. According to this interpretation of transitory starch biosynthesis starch accumulation in leaves is the result of the balance between <i>de novo</i> starch synthesis from ADPG entering the chloroplast and breakdown, and the efficiency by which starch breakdown products are recycled back to starch by means of pPGM and AGP. Thus, this view predicts that the recovery towards starch biosynthesis of the glucose units derived from the starch breakdown will be deficient in pPGM and AGP mutants, resulting in a parallel decline of starch accumulation and enhancement of soluble sugars content since starch breakdown derived products (especially glucose) will leak out the chloroplast through the very active glucose translocator <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104997#pone.0104997-Cho1" target="_blank">[24]</a>. The enzyme activities involved are numbered as follows: 1, 1′, fructose-1,6-bisphosphate aldolase; 2, 2′, fructose 1,6-bisphosphatase; 3, PPi:fructose-6-phosphate phosphotransferase; 4, 4′, PGI; 5, 5′, PGM; 6, UDPG pyrophosphorylase; 7, sucrose phosphate synthase; 8, sucrose-phosphate-phosphatase; 9, AGP; 10, SS; 11, starch phosphorylase; 12, SuSy; 13, plastidial hexokinase <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104997#pone.0104997-Olsson1" target="_blank">[25]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104997#pone.0104997-Giese1" target="_blank">[26]</a>. FBP: fructose bis-phosphate; UDPG: UDP-glucose.