Search

The voltage-dependent anion channels (VDACs), mitochondrial outer membrane components, are present in organisms from fungi to animals and plants. They are thought to function in the regulation of metabolite transport between mitochondria and the cytoplasm. Sufficient knowledge on plant VDACs has been accumulated, so that we can here summarize the current information. Then, the involvement of mitochondria in plant defense and cell death is overviewed. While, in mammals, it is suggested that VDAC, also known as a component of the permeability transition pore (PTP) complex formed in the junction site of mitochondrial outer and inner membrane, is a key player in mitochondria-mediated cell death, little is known about the role of plant VDACs in this process. We have shown that plant VDACs are involved in mitochondria-mediated cell death and in defense against a non-host pathogen. In light of the current findings, we discuss the role of the PTP complex and VDAC as its component in plant pathogen defense and cell death.

The voltage-dependent anion channel (VDAC) is a major outer mitochondrial membrane protein. It is well documented that VDAC plays an important role in apoptosis, a kind of programmed cell death, in mammalian systems. However, little is known about the role of the plant counterpart during the process of plant-specific cell death such as pathogen-induced hypersensitive response. To address this issue, we isolated three VDAC full-length cDNAs (NtVDAC1-3) from Nicotiana tabacum. The deduced products, NtVDACs, share 78-85% identity and retain the conserved eukaryotic mitochondrial porin signature distal to their C-terminal regions. Mitochondrial localization of three NtVDACs in plant cells was confirmed via a green fluorescent protein fusion method. Then, we addressed the main issue concerning pathogenesis relation. The N. benthamiana orthologues of NtVDACs were upregulated by challenge with the non-host pathogen Pseudomonas cichorii, but not after challenge with the virulent pathogen P. syringae pv. tabaci. Both the pharmaceutical inhibition of VDAC and silencing of NbVDACs genes compromised the non-host resistance against P. cichorii, suggesting the involvement of VDACs in defense against non-host pathogen. Involvement of NbVDACs in Bax-mediated cell death was also verified using a similar approach.

The multipass membrane protein ACD6 forms large protein complexes and is subject to constitutive ER-associated degradation. Salicylic acid signaling increases the size of ACD6 complexes and the plasma membrane levels of ACD6 and the ACD6-associated pattern receptor FLS2. ACD6 is necessary for promoting increased levels of FLS2 and its co-receptor BAK1 specifically at the plasma membrane during salicylic acid (SA) signaling, suggesting that ACD6 maturation and plasma membrane levels are tightly controlled as a mechanism for regulating these pattern receptors.ACCELERATED CELL DEATH6 (ACD6) is a multipass membrane protein with an ankyrin domain that acts in a positive feedback loop with the defense signal salicylic acid (SA). This study implemented biochemical approaches to infer changes in ACD6 complexes and localization. In addition to forming endoplasmic reticulum (ER)- and plasma membrane (PM)-localized complexes, ACD6 forms soluble complexes, where it is bound to cytosolic HSP70, ubiquitinated, and degraded via the proteasome. Thus, ACD6 constitutively undergoes ER-associated degradation. During SA signaling, the soluble ACD6 pool decreases, whereas the PM pool increases. Similarly, ACD6-1, an activated version of ACD6 that induces SA, is present at low levels in the soluble fraction and high levels in the PM. However, ACD6 variants with amino acid substitutions in the ankyrin domain form aberrant, inactive complexes, are induced by a SA agonist, but show no PM localization. SA signaling also increases the PM pools of FLAGELLIN SENSING2 (FLS2) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1). FLS2 forms complexes ACD6; both FLS2 and BAK1 require ACD6 for maximal accumulation at the PM in response to SA signaling. A plausible scenario is that SA increases the efficiency of productive folding and/or complex formation in the ER, such that ACD6, together with FLS2 and BAK1, reaches the cell surface to more effectively promote immune responses. [ABSTRACT FROM AUTHOR]