ABI4 downregulates expression of the sodium transporter HKT1;1 in Arabidopsis roots and affects salt tolerance

A plant's ability to cope with salt stress is highly correlated with their ability to reduce the accumulation of sodium ions in the shoot. Arabidopsis mutants affected in the ABSCISIC ACID INSENSITIVE (ABI) 4 gene display increased salt tolerance, whereas ABI4‐overexpressors are hypersensitive to salinity from seed germination to late vegetative developmental stages. In this study we demonstrate that abi4 mutant plants accumulate lower levels of sodium ions and higher levels of proline than wild‐type plants following salt stress. We show higher HKT1;1 expression in abi4 mutant plants and lower levels of expression in ABI4‐overexpressing plants, resulting in reduced accumulation of sodium ions in the shoot of abi4 mutants. HKT1;1 encodes a sodium transporter which is known to unload sodium ions from the root xylem stream into the xylem parenchyma stele cells. We have shown recently that ABI4 is expressed in the root stele at various developmental stages and that it plays a key role in determining root architecture. Thus ABI4 and HKT1;1 are expressed in the same cells, which suggests the possibility of direct binding of ABI4 to the HKT1;1 promoter. In planta chromatin immunoprecipitation and in vitro electrophoresis mobility shift assays demonstrated that ABI4 binds two highly related sites within the HKT1;1 promoter. These sites, GC(C/G)GCTT(T), termed ABI4‐binding element (ABE), have also been identified in other ABI4‐repressed genes. We therefore suggest that ABI4 is a major modulator of root development and function.     

Syntaxin 4 mediates endosome recycling for lytic granule exocytosis in cytotoxic T‐lymphocytes

Adaptive and innate immunity utilize the perforin‐killing pathway to eliminate virus‐infected or cancer cells. Cytotoxic T‐lymphocytes (CTLs) and natural killer cells mediate this process by releasing toxic proteins at the contact area with target cells known as immunological synapse (IS). Formation of a stable IS and exocytosis of toxic proteins requires persistent fusion of Rab11a recycling endosomes with the plasma membrane (PM) that may assure the delivery of key effector proteins. Despite the importance of the recycling endosomal compartment, the membrane fusion proteins that control this process at the IS remain elusive. Here, by performing knockdown experiments we found that syntaxin 4 (STX4) is necessary for cytotoxic activity and CD107a degranulation against target cells in a similar fashion to syntaxin 11, which is involved in lytic granule (LG) exocytosis and immunodeficiency when it is mutated. Using total internal reflection fluorescent microscopy we identified that STX4 mediates fusion of EGFP‐Rab11a vesicles at the IS. Immunoprecipitation experiments in lysates of activated CTLs indicate that endogenous STX4 may drive this fusion step by interacting with cognate proteins: Munc18‐3/SNAP23/VAMP7 and/or VAMP8. These results reveal the role of STX4 in mediating fusion of Rab11a endosomes upstream of lytic granules (LGs) exocytosis and further demonstrate the importance of this pathway in controlling CTL‐mediated cytotoxicity.      

Phytoglobins in the nuclei,cytoplasm and chloroplasts modulate nitric oxide signaling and interact with abscisic acid

Symbiotic hemoglobins provide O₂ to N₂‐fixing bacteria within legume nodules, but the functions of non‐symbiotic hemoglobins or phytoglobins (Glbs) are much less defined. Immunolabeling combined with confocal microscopy of the Glbs tagged at the C‐terminus with green fluorescent protein was used to determine their subcellular localizations in Arabidopsis and Lotus japonicus. Recombinant proteins were used to examine nitric oxide (NO) scavenging in vitro and transgenic plants to show S‐nitrosylation and other in vivo interactions with NO and abscisic acid (ABA) responses. We found that Glbs occur in the nuclei, chloroplasts and amyloplasts of both model plants, and also in the cytoplasm of Arabidopsis cells. The proteins show similar NO dioxygenase activities in vitro, are nitrosylated in Cys residues in vivo, and scavenge NO in the stomatal cells. The Cys/Ser mutation does not affect NO dioxygenase activity, and S‐nitrosylation does not significantly consume NO. We demonstrate an interaction between Glbs and ABA on several grounds: Glb1 and Glb2 scavenge NO produced in stomatal guard cells following ABA supply; plants overexpressing Glb1 show higher constitutive expression of the ABA responsive genes Responsive to ABA (RAB18), Responsive to Dehydration (RD29A) and Highly ABA‐Induced 2 (HAI2), and are more tolerant to dehydration; and ABA strongly upregulates class 1 Glbs. We conclude that Glbs modulate NO and interact with ABA in crucial physiological processes such as the plant's response to dessication.     

十字花科植物ABI4序列演化与转录激活活性分析

转录因子ABI4参与植物激素ABA的绝大多数生物学功能, 它不仅是ABA和GA在植物体内含量平衡的核心调控因子, 同时还连接ABA与植物细胞内多个信号通路。利用拟南芥(Arabidopsis thaliana) ABI4序列在十字花科其它19种植物(除拟南芥外)中检索得到27个同源基因, 通过序列多态性分析、系统发生重建、染色体共线性分析和转录激活活性比较, 探讨了该基因在十字花科植物中的演化历史。结果表明, ABI4蛋白质序列和结构在十字花科植物中具有较高的保守性, 暗示了其功能的重要性; 单独的ABI4蛋白并不具备显著的转录激活活性, 说明其生物学功能的具体分子机制可能相对复杂, 需要进一步探讨。