Gravitropic response is a control of growth direction with respect to thegravity vector. Gravity sensing process starts with downward movement ofamyloplasts in statocytes and subsequent physiological signaling is presumed toregulate auxin transport. Putative signaling components, Arabidopsis LAZY1(AtLAZY1) and ALTERED RESPONSE TO GRAVITY 1 (ARG1) are involved innegative gravitropism (upward bending) of Arabidopsis shoots. However, little isknown about relationships between them. In this study, I examined geneticinteraction between AtLAZY1 and ARG1 using various mutants of Arabidopsis. Thedouble mutant atlazy1 arg1 showed synergistic loss of stem gravitropism, implicatingthat there are multiple pathways of gravitropic signaling. Auxin transporter genesPIN-FORMED 3 (PIN3), PIN4 and PIN7 did not genetically interact with AtLAZY1. Iisolated a novel mutant dsl1 that suppressed atlazy1 defect. dsl1 did notsuppressed defects of sgr2 and eal1 that were mutants of amyloplast sedimentation,suggesting that dsl1 specifically suppresses the AtLAZY1-dependent signalingpathway. dsl1 encoded ASL5/LBD12 transcription factor and microarray analysesrevealed that secondary cell wall deposition was significantly upregulated in dsl1,while expression of auxin signaling components was not altered. Moreover, I foundthat impaired secondary cellulose biosynthesis and inhibition of cellulose biosynthesisby isoxaben treatment attenuated stem gravitropism. These results indicate that cellwall biogenesis plays an important role in AtLAZY1-dependent gravitropism ofArabidopsis stems.To further investigate how AtLAZY1 functioned in gravitropism, we used fluorescence microscopy to examine the subcellular localization of AtLAZY1 and itstruncated proteins fused to GFP in tobacco leaves. I found that AtLAZY1 localized tothe plasma membrane through the C-terminal region, indicating that the putativetransmembrane domain in the N-terminal half is not required for its localization. Next,I took a biochemical approach to investigate the membrane association of AtLAZY1.Transiently expressed AtLAZY1 in transgenic Arabidopsis was fractionated in aninsoluble fraction that contained membranous compartments. The AtLAZY1 proteinwas solubilized by a non-ionic detergent or at a high pH condition, suggesting thatAtLAZY1 is a peripheral membrane protein. I also found that when expressed intobacco the C-terminal part of AtLAZY1 co-localized with microtubules. Amicrotubule binding assay showed that the C-terminal half of AtLAZY1, whichlocalized to the plasma membrane, interacted with microtubules in vitro. Theseresults suggest that AtLAZY1 may function with microtubules at the periphery of theplasma membrane in the gravity signaling process. (主査) 准教授 綿引 雅昭, 教授 加藤 敦之, 教授 山口 淳二, 特任教授 山本 興太朗 生命科学院（生命科学専攻）