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Arabidopsis bZIP28, an ER membrane-associated transcription factor, is activated in response to conditions that induce ER stress-adverse environmental conditions or exposure to ER stress agents such as tunicamycin and dithiothreitol. Upon stress treatment, bZIP28 exits the ER and moves to the Golgi, where it is proteolytically processed, releasing its transcriptional component, which relocates to the nucleus. In this study, we tracked the movement of GFP-tagged bZIP28 in an effort to understand its mobilization from the ER and release from the Golgi. We identified a small region in bZIP28 that is rich in dibasic amino acids and proximal to the transmembrane domain required for its movement from the ER. In response to ER stress, bZIP28 showed enhanced interaction with Sar1 and Sec12, components of the COPII machinery. We demonstrated that the dibasic amino acid-rich region in bZIP28 is involved in the interaction with Sar1. Upon migration to the Golgi, bZIP28 is proteolytically processed by proteases S1P and S2P. We found a putative helix-breaking residue in the transmembrane domain of bZIP28 to be crucial for its processing and liberation from Golgi bodies. Thus, in response to stress, bZIP28 moves from organelle to organelle by interaction of critical elements in the molecule with the transport and/or proteolytic machinery resident in the various organelles. [ABSTRACT FROM AUTHOR]

The actin cytoskeleton is involved in the transport and positioning of Golgi bodies, but the actin-based processes that determine the positioning and motility behavior of Golgi bodies are not well understood. In this work, we have studied the relationship between Golgi body motility behavior and actin organization in intercalary growing root epidermal cells during different developmental stages. We show that in these cells two distinct actin configurations are present, depending on the developmental stage. In small cells of the early root elongation zone, fine filamentous actin (F-actin) occupies the whole cell, including the cortex. In larger cells in the late elongation zone that have almost completed cell elongation, actin filament bundles are interspersed with areas containing this fine F-actin and areas without F-actin. Golgi bodies in areas with the fine F-actin exhibit a non-directional, wiggling type of motility. Golgi bodies in areas containing actin filament bundles move up to 7 μm s−1. Since the motility of Golgi bodies changes when they enter an area with a different actin configuration, we conclude that the type of movement depends on the actin organization and not on the individual organelle. Our results show that the positioning of Golgi bodies depends on the local actin organization. [ABSTRACT FROM AUTHOR]

Abstract: An early cellular reaction necessary for resistance expression of plant cells to pathogen attack is the migration of organelles towards the site of attempted pathogen ingress. We crossed lines expressing either Golgi bodies- or peroxisomal-targeted GFP with rar1-20, sgt1b, pad4-1 and npr1-2 mutants to test whether mutations in these signalling components impaired this cellular reaction. Defence-signalling-mutant-GFP lines and wild-type-GFP lines inoculated with Hyaloperonospora arabidopsidis isolates showed no observable differences. Thus, relocation of intracellular organelles is part of an innate, basal immune response that operates independently of RAR1, SGT1b, PAD4 and NPR1. The evolutionary implications of our results are discussed. [Copyright &y& Elsevier]