The bZIP protein VIP1 is involved in touch responses in Arabidopsis roots

Abstract

VIP1 is a bZIP transcription factor in Arabidopsis (Arabidopsis thaliana). VIP1 transiently accumulates in the nucleus when cells are exposed to hypoosmotic conditions, but its physiological relevance is unclear. This is possibly because Arabidopsis has approximately 10 close homologs of VIP1 and they function redundantly. To examine their physiological roles, transgenic plants overexpressing a repression domain-fused form of VIP1 (VIP1-SRDXox plants), in which the gene activation mediated by VIP1 is expected to be repressed, were generated. Because hypoosmotic stress can mimic mechanical stimuli (e.g. touch), the touchinduced root-waving phenotypes and gene expression patterns in those transgenic plants were examined. VIP1-SRDXox plants exhibited more severe root waving and lower expression of putative VIP1 target genes. The expression of the VIP1-green fluorescent protein (GFP) fusion protein partially suppressed the VIP1-SRDX-induced increase in root waving when expressed in the VIP1-SRDXox plants. These results suggest that VIP1 can suppress the touch-induced root waving. The VIP1-SRDX-induced increase in root waving was also suppressed when the synthetic auxin 2,4-dichlorophenoxy acetic acid or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, which is known to activate auxin biosynthesis, was present in the growth medium. Root cap cells with the auxin marker DR5rev::GFP were more abundant in the VIP1-SRDXox background than in the wild-type background. Auxin is transported via the root cap, and the conditions of outermost root cap layers were abnormal in VIP1-SRDXox plants. These results raise the possibility that VIP1 influences structures of the root cap and thereby regulates the local auxin responses in roots.

Fluctuations of the transcription factor ATML1 generate the pattern of giant cells in the Arabidopsis sepal

“…To see whether other genes also exhibit variable expression similarly to mCitrine-ATML1 in the developing sepal nuclei, we measured the expression of two fluorescently-tagged transcription factors, VIP1-mCitrine ( pVIP1::VIP1-mCitrine ) and AP2-2XYpet ( pAP2::AP2-2XYpet ), and the SEC24A transcriptional reporter ( pSEC24A::H2B-GFP ). VIP1 is a mechano-sensitive transcription factor that localizes to the nucleus upon hypo-osmotic treatment (Tian et al, 2004; Tsugama et al, 2016) and AP2 is a master regulator of floral organ identity that is expressed in sepals (Wollmann et al, 2010). SEC24A is a ubiquitously expressed CopII vesicle-coat protein that is involved in vesicle trafficking from the ER to the Golgi and has been previously reported to influence giant cell formation on the sepal (Qu et al, 2014).…”

“…The mCitrine marker was excited with a 514 nm laser and collected with a 519–564 filter. VIP1 is a bZIP transcripton factor that is cytoplasmically localized under stable conditions but will become nuclear localized upon hypoosmotic treatment (Tsugama et al, 2016). To nuclear localize VIP1, VIP1-mCitrine inflorescences were submerged in a hypoosmotic solution (H2O and 0.001% triton-X) for approximately 10 min prior to confocal imaging.…”

The Agrobacterium VirE2 effector interacts with multiple members of the Arabidopsis VIP1 protein family

“…The proposed involvement of the Arabidopsis bZIP protein VIP1 in Agrobacterium infection is consistent with its known roles in different types of biotic and abiotic stress (Djamei et al ., ; Pitzschke et al ., ; Tsugama et al ., ). Yet, a recent study has suggested that a loss‐of‐function mutant of VIP1 is still susceptible to infection (Shi et al ., ).…”

“…However, specific VirE2 proteins showed different, yet overlapping, specificities towards individual VIP1 homologues, potentially affecting the host range of the different bacterial species. This subset of VIP1 homologues shares somewhat redundant biological functions in uninfected cells (Tsugama et al, 2014(Tsugama et al, , 2016Van Leene et al, 2016), which suggests that this redundancy could also apply to their interactions with VirE2 during the infection process. Whereas, in nature, the host range of Agrobacterium is essentially limited to dicotyledonous plants, under laboratory conditions, many more eukaryotic species can be transformed by Agrobacterium, from monocotyledonous plants, to yeast and other fungi, to mammalian cultured cells (Lacroix et al, 2006).…”

“…One of the plant proteins that interacts with VirE2 is VIP1 (VirE2-interacting protein 1), a transcription factor of the bZIP family involved in the stress response (Djamei et al, 2007;Pitzschke et al, 2009) and the osmosensory and touch responses (Tsugama et al, 2014(Tsugama et al, , 2016. VIP1 has been suggested to act as a molecular adaptor between VirE2 and several host cell factors, such as importin a of the nuclear import pathway Tzfira et al, 2001Tzfira et al, , 2002, plant and bacterial F-box proteins, such as VirF and VBF, of the proteasomal degradation pathway Zaltsman et al, 2010Zaltsman et al, , 2013, and nucleosomal histones (Lacroix et al, 2008;Loyter et al, 2005).…”