Rare sugars-monosaccharides that rarely exist in nature-are known to regulate diverse physiological responses in both plants and animals. With the advent of the discovery of "Izumoring," an in vitro enzymatic approach to the synthesis of all rare sugars, 1,2 research in this area has implicated rare sugars in many physiological conditions. These include but are not limited to the immunosuppressive activity in liver transplantation, 3 protection against liver ischemia reperfusion injury, 4,5 protection from reactive oxygen species (ROS) 6,7 and anticancer activity on different cancer cell lines. [8][9][10][11] Although not abundant, rare sugars-Dallose, D-psicose, D-allitol, L-galactose, tagatose or their derivatives-have been found in the tissues of higher plants as well. [12][13][14][15][16][17][18] It was found that D-psicose inhibits plant root growth via hexokinase-independent pathway; it also inhibits bacterial blight disease in rice.19,20 D-allose was found to inhibit the rice growth and prevent bacterial blight disease in rice as well. 21 Recently, Akimitsu's group, a pioneer in rare sugar research, through the use of powerful genetics study has shown that the rare sugar D-allose suppresses GA signaling pathway in rice. 22D-allose strongly inhibited GA mediated α-amylase induction in embryo-less rice half seeds, 22 implicating a negative role of D-allose in the GA pathway. Earlier, we have shown that as energy sources and structural components, sugars are the central regulators of plant growth and development. in addition to the abundant natural sugars in plants, more than 50 different kinds of rare sugars exist in nature, several of which show distinct roles in plant growth and development. recently, one of the rare sugars, D-allose, an epimer of D-glucose at C3, is found to suppress plant hormone gibberellin (Ga) signaling in rice. Scaffold protein raCK1a in the model plant arabidopsis is implicated in the Ga pathway as rack1a knockout mutants show insensitivity to Ga in Ga-induced seed germination. using genetic knockout lines and a reporter gene, the functional role of raCK1a in the D-allose pathway was investigated. it was found that the rack1a knockout seeds showed hypersensitivity to D-alloseinduced inhibition of seed germination, implicating a role for raCK1a in the D-allose mediated suppression of seed germination. on the other hand, a functional raCK1a in the background of the double knockout mutations in the other two raCK1 isoforms, rack1b/rack1c, showed significant resistance to the D-allose induced inhibition of seed germination. the collective results implicate the raCK1a in the D-allose mediated seed germination inhibition pathway. Elucidation of the rare sugar signaling mechanism will help to advance understanding of this less studied but important cellular signaling pathway. Abbreviations: GA, gibberellin; RACK1, receptor for activated C kinase 1; MS, murashige and skoog Arabidopsis scaffold protein RACK1A (Receptor for Activating C Kinase 1) positively regulates GA signaling pathway a...