The unique features of starch metabolism in red algae

Red algae (Rhodophyceae) are photosynthetic eukaryotes that accumulate star ch granules outside of their plastids. The starch granules from red algae ( floridean starch) show structural similarities with higher plant starch gra nules bur lack amylose. Recent studies have indicated that the extra-plasti dic starch synthesis in red algae proceeds via a UDP glucose-selective alph a -glucan synthase, in analogy with the cytosolic pathway of glycogen synth esis in other eukaryotes. On the other hand, plastidic starch synthesis in green cells occurs selectively via ADP glucose in analogy with the pathway of glycogen synthesis in prokaryotes fi om which plastids have evolved. Giv en the emerging consensus of a monophyletic origin of plastids, it would ap pear that the capacity for starch synthesis selectively evolved from the al pha -glucan synthesizing machinery of the host ancestor and its endosymbion t in red algae and green algae, respectively. This implies the evolution of fundamentally different functional relationships between the different sub cellular compartments with regard to photosynthetic carbon metabolism in th ese organisms, It is suggested that the biochemical and molecular elucidati on of floridean starch synthesis offer new insights into the metabolic stra tegices of photosynthetic eukaryotes.