Study On Plant Responses To Gravistimulation

Current studies are focused on the understanding of the mechanisms by which hormones transport can be regulated to allow changes in direction of transport. The final question that remains unanswered is how the lateral distribution of these hormones stimulates growth across the plant tissue. Studies on the GA-induced loosening of cell wall and water transport would help answer the question. In order to further understand the mechanisms of the plant gravtropic curvature, SSH was used to clone these genes which induced by gravistimulation in rice sheath bottom. Because microgravity experiments contribute to investigating the mechanisms of the plant gravtropic curvature, we found gibberellins mediated the degradation of starch grains induced by simulated microgravity in carrot callus cells. Our main results of these research aspects are as following: (1) The direct effect of microgravity on plant is the gravisensor (amyloplast). We used the clinostat to simulate the biological effects of microgravity. The induction of starch grain degradation by treatment of clinostating was found in carrot callus cells. To understand the mechanism of this induction, we detected the activities of gibberellins(GAs), total amylase and a-amylase. The increment of their activities induced by clinostating is synergetic. The increment of a-amylase activity was the main element in the increment of total amylase activity during clinostating. Carrot callus cells cultured on sucrose free medium (MS0) were further used as experiment materials in order to avoid the inhibition of expression of a-amylase genes by sugar molecules. a-amylase activity in carrot callus cells cultured on sugar free medium was increased in more degree by clinostat treatment than that in callus cells on sugar supplied medium. So sucrose can whittle the effect of clinostat treatment exerting on a-amylase. Further more, ancymidol, the inhibitor of the biosynthesis of GA could remarkably repress the increment of a-amylase activity. The synchronization of the degradation of amyloplasts, the increment of a-amylase activity and the change of GA, deduce that clinostat treatment can stimulate the raise of GAs, which induces the increment of the expression of a-amylase genes, then triggers the degradation of starch grains. As a result, plant can produce more active energy to respond to clinostat stimulus. (2) In the present work, we investgated the mechanisms of the rice asymmetric growth at the curvature site (1 cm length above rice roots). The seedlings were gravstimulate by rotating their pots through 900 in the absence of light, and the bending leaf sheath bottom (1 cm length above roots) were cut into the top and bottom halves (top leaf sheath segment half, tLSS; bottom leaf sheath segment half, bLSS) by bisection with a razor blade. We not only detected the asymmetric distribution of auxin, but also that of gibberellins (GAs) in rice LSS following gravistimulation. Gravistimulation induced the transient accumulation of greater amounts of both IAA and GA in the bLSS. OsGA3ox1, a gene of active GAs synthesis, was differentially induced by gravistimulation. Externally applied GA3 could recover the gravitropic curvature of rice LSS inhibited by 2,3,5-triodobenzoic acid (TIBA), an inhibitor of auxin transport. The expression of xyloglucan endotransglycosylase gene (XET) was differentially induced in the bLSS of gravistimulated rice seedlings and up-regulated by exogenous IAA and GA3 also. Both ancymidol, an inhibitor of GAs synthesis, and TIBA decreased the gradient of XET expression. These data suggest that the gradient distribution of auxin affected by gravistimulation induced a gradient of GAs via asymmetric expression of OsGA3ox1 in rice LSS, and hence caused the asymmetric expression of XET. Cell wall loosening in the curvature site of the LSS caused by the expression of XET would contribute to gravitropic growth. The expression of the RWC3 (rice water channel protein) gene as a response to gravistimulation was investigated in transgenic seedlings of rice (Oryza sativa L.) carrying the ?-glucuronidase (GUS) reporter gene linked to the promoter of the rice RWC3 gene. In LSS maintained parallel to the ground, GUS activity on the bLSS was greater compared to that on the tLSS, indicating an effect of gravity on the functioning of the RWC3 promoter. In the same LSS the osmotic potential on the bLSS was found to be less than that on the tLSS. HgCl2 and phloretin, two inhibitors of aquaporins, both significantly decreased the gravitropic curvature of transgenic rice LSS, indicating that RWC3 plays an important role in asymmetric growth during plant gravitropism. Both larger amount of auxins and gibberellins (GAs) were found to be asymmetrically distributed in the LSS maintained horizontally. Externally applied GA3 could increase the GUS activity in the bLSS. Ancymidol, a GA synthesis inhibitor, had the opposite effect on LSS, both decreasing curvature and GUS activity in the bLSS. These...