Expression data from epidermis specific phytochrome B or global phytochrome B line comparing to phyB-9 mutant.

Seedling hypocotyls display negative gravitropism in the dark but agravitropism in the light. The Arabidopsis thaliana pif quadruple mutant (pifQ), which lacks four PHYTOCHROME-INTERACTING FACTORS (PIFs), is agravitropic in the dark. Endodermis-specific expression of PIF1 rescues gravitropism in pifQ mutant seedlings. Since phytochromes induce light responses by inhibiting PIFs and the COP1-SPA ubiquitin E3 ligase complex in the nucleus, we asked whether phyB can cell autonomously inhibit hypocotyl negative gravitropism in the endodermis. We found that while epidermis-specific expression of PHYB rescues hypocotyl negative gravitropism and all other phyB mutant phenotypes, endodermis-specific expression of PHYB does not. Epidermal phyB induces the phosphorylation and degradation of endodermal PIFs in response to red light. This induces a global gene expression pattern similar to that induced by red light treatment of seedlings expressing PHYB under the control of its own endogenous promoter. Our results imply that epidermal phyB generates an unidentified mobile signal that travels to the endodermis where it promotes PIF degradation and inhibits hypocotyl negative gravitropism. ML1pro:PHYBGFP/phyB-9 and PHYBpro:PHYBGFP/phyB-9 and phyB-9 seeds were grown in continuous red condition (11.4 μmol·m−2·s−1) and then, sampled for RNA extraction. Total 4 biological replicates were used for the microarray

幼苗下胚轴在黑暗中表现为负向重性（negative gravitropism），而在光照条件下则表现为无向重性（agravitropism）。拟南芥（Arabidopsis thaliana）PIF四突变体（pifQ）缺失四种光敏色素互作因子（PHYTOCHROME-INTERACTING FACTORS，PIFs），该突变体在黑暗中同样表现出无向重性。通过内皮层特异性表达PIF1，可恢复pifQ突变体幼苗的向重性功能。鉴于光敏色素通过在细胞核内抑制PIFs与COP1-SPA泛素E3连接酶复合物（COP1-SPA ubiquitin E3 ligase complex）来介导光信号响应，本研究探究了光敏色素B（phyB）能否在内皮层中以细胞自主性方式抑制下胚轴负向重性。我们发现，尽管在表皮中特异性表达PHYB可恢复下胚轴负向重性及phyB突变体的所有其他表型，但在内皮层特异性表达PHYB则无此效果。表皮中的phyB可响应红光信号，诱导内皮层PIFs的磷酸化与降解。该过程所诱导的全基因组基因表达谱，与在内源PHYB启动子调控下表达PHYB的幼苗经红光处理后的表达谱高度相似。本研究结果表明，表皮中的phyB可产生一种尚未被鉴定的可移动信号分子，该信号转运至内皮层后，可促进PIFs降解并抑制下胚轴负向重性。将ML1启动子驱动PHYB-GFP融合基因的phyB-9突变体（ML1pro:PHYBGFP/phyB-9）、内源PHYB启动子驱动PHYB-GFP融合基因的phyB-9突变体（PHYBpro:PHYBGFP/phyB-9）以及纯合phyB-9突变体的种子，在持续红光条件（11.4 μmol·m⁻²·s⁻¹）下培养，随后取样用于RNA提取。本实验共设置4次生物学重复，用于基因芯片（microarray）实验。