The Gravitropism Of Rice Leaf Sheath Bases And Space Fight Experiment Of Rice And Arabidopsis

Gravitropism results from the asymmetric growth or curvature of plant organs in response to gravistimulation. Differential growth of plants in response to changes in the gravity vector requires a complex signal transduction cascade. Ca2+, proton, IP3, auxin, Gibberellin and ethylene involved in the process. Using the whole seedlings or root tips as material, many gravitropism-regulated genes in Arabidopsis are identified by microarray. Because there is asymmetric growth between upper side and lower side in the gravitropic bending, the function of these genes in the gravitropism is not clear. Using rice as material, we identified genes related with asymmetrical growth in the gravitropism by RT-PCR, SSH, Microarray, Realtime PCR, etc. The function of genes in the gravitropism are explored, related physiological research are done. In addition, we sent the dry seeds of rice and Arabidopsis to the space on board of a recoverable satellite, and did related experiments after landing.We find that an expansin gene (OsEXPA4) and a plasma membrane aquaporin (OsRWC3) are involved in gravitropic bending of rice leaf sheath bases. Expression of OsEXPA4 and OsRWC3 were induced in the lower side of rice leaf sheath bases during gravitropism. Expression of both of these genes was induced by GA3.β-glucuronidase (GUS) activity in transgenic rice expressing a GUS reporter gene under the control of the gibberellin (GA)-responsive OsRWC3 promoter was investigated during gravitropic curvature and found to be enhanced in the lower side of graviresponding leaf sheath bases. Ancymidol, a GA biosynthesis inhibitor, reduced the expression gradient of OsEXPA4 and OsRWC3. The osmotic potential was transiently lower on the lower side of gravistimulated leaf sheath bases, and the gravitropic curvature was inhibited by HgCl2 and phloretin, two known inhibitors of aquaporins. These data suggest that asymmetric GA redistribution following gravistimulation results in the localized expression of OsEXPA4 and OsRWC3, which in turn might bring about differential cell wall loosening, cell expansion and water influx via aquaporins, thus leading to gravitropic bending.In order to further study the molecular mechanism in the gravitropic bending, we identify 12 genes differently expressed between upper and lower halves of rice leaf sheath bases by combination of the suppressive subtractive hybridization (SSH) and cDNA microarray techniques. 11 genes specially expressed in the lower side of rice leaf sheath bases, which included Aldehyde dehydrogenase(ALDH1),Reversibly glycosylated polypeptide(RGP1), Sucrose synthase(SUS), Hexose transporter, Ribophorin II, Structural maintenace of chromosomes 3(SMC3), Uroporphyrinogen decarboxylase(UROD), Squalene synthase(SQS) and 3 unknown protein. Cystathionine gamma-synthase (CGS1) specially expressed in the upper side. In addion, IAA treatment could induce the expression of RGP1, SUS, UROD and SMC3. TIBA (an inhibitor of auxin transport) could abolish the differential expression of these genes in the gravitropism. These results suggested that asymmetrical auxin induced the asymmetrical expression of RGP1, SUS, UROD and SMC3 in gravitropic bending of rice leaf sheath beases. The hexose content of rice leaf sheath base was measured, and there was more hexose in the lower halves after 12h gravi-stimulation. The asymmetrical hexose content occurred later than that of SUS and Hexose transporter. It suggested that the higher hexose content in the lower side of rice leaf sheath bases results from the higher expression of SUS and Hexose transporter. These gravitropism-regulated genes involved in some important process: metabolism of cell wall substance, hexose synthesis and hexose transport, energy substance synthesis, metabolism of steroids, the methionine synthesis and protein translation, etc.Dry seeds of rice and Arabidopsis were carried to the space on board of a recoverable satellite for 18 days.Some related experiments were done after landing. There was one dwarfish lethal mutant in rice, and three mutants in Arabidopsis. In addtiton, the aberrance of one Arabidopsis plant occured in SP1, and resumed normal in SP2. In the three Arabidopsis mutants, one had the phenotype of corymb-like inflorescence, different from raceme of Arabidopsis wild type, so this mutant was nominated cli (Corymb-like inflorescence). Compared with wild type, cli mutant had curled leaf, circinal pod bottom, strong inflorescence stem, lower plant height. Compared with Arabidopsis wild type, dark treatment had less effect on growth of cli mutant's hypocotyls. In further study, we plan to clone the mutant gene in cli mutant by map-based cloning strategy.