A phytotoxin Solanapyrone-A downregulates calcium-dependent protein kinase activity in potato.

We previously demonstrated that alternaric acid, a host-specific toxin produced by the plant pathogenic fungus Alternaria solani, in the presence of Ca(2+) and Mg(2+), stimulated in vitro phosphorylation of His-tagged calcium-dependent protein kinase 2 from potato cultivar Rishiri (RiCDPK2). Herein, we report that Solanapyrone-A (SpA), a non-host-specific toxin produced by A. solani, inhibited the phosphorylation of RiCDPK2 in the presence of Ca(2+) and Mg(2+). However, SpA stimulated RiCDPK2 phosphorylation in the absence of these cations. Based on the current findings, we suggest that RiCDPK2 may mediate SpA-induced signaling independent of Ca(2+) and Mg(2+), leading to a compatible interaction between potato and A. solani.

Alternaric acid stimulates phosphorylation of His-tagged RiCDPK2, a calcium-dependent protein kinase in potato plants.

Calcium-dependent protein kinases (CDPK) are an essential component of plant defense mechanisms against pathogens. We investigated the effect of alternaric acid, a host-specific toxin produced by the plant fungal pathogen Alternaria solani (Pleosporaceae), on a putative plasma membrane and cytosolic kinase RiCDPK2 of potato (Solanum tuberosum) and on hypersensitive cell death of host potato cells. Alternaric acid, in the presence of Ca²âº and Mg²âº, stimulated in vitro phosphorylation of His-tagged RiCDPK2, a Ca²âº-dependent protein kinase found in potato plants. We concluded that Ca²âº and Mg²âº play an important role in the interaction between alternaric acid and RiCDPK2. Based on our observations, alternaric acid regulates RiCDPK2 kinase during the infection process in an interaction between host and A. solani, leading to the inhibition of hypersensitive cell death in the host. We suggest that alternaric acid is a primary determinant by which A. solani stimulates CDPK activity in the host, suppressing hypersensitive cell death.

The role of vacuole in plant cell death.

Almost all plant cells have large vacuoles that contain both hydrolytic enzymes and a variety of defense proteins. Plants use vacuoles and vacuolar contents for programmed cell death (PCD) in two different ways: for a destructive way and for a non-destructive way. Destruction is caused by vacuolar membrane collapse, followed by the release of vacuolar hydrolytic enzymes into the cytosol, resulting in rapid and direct cell death. The destructive way is effective in the digestion of viruses proliferating in the cytosol, in susceptible cell death induced by fungal toxins, and in developmental cell death to generate integuments (seed coats) and tracheary elements. On the other hand, the non-destructive way involves fusion of the vacuolar and the plasma membrane, which allows vacuolar defense proteins to be discharged into the extracellular space where the bacteria proliferate. Membrane fusion, which is normally suppressed, was triggered in a proteasome-dependent manner. Intriguingly, both ways use enzymes with caspase-like activity; the membrane-fusion system uses proteasome subunit PBA1 with caspase-3-like activity, and the vacuolar-collapse system uses vacuolar processing enzyme (VPE) with caspase-1-like activity. This review summarizes two different ways of vacuole-mediated PCD and discusses how plants use them to attack pathogens that invade unexpectedly.

A cellular suicide strategy of plants: vacuole-mediated cell death.

Programmed cell death (PCD) occurs in animals and plants under various stresses and during development. Recently, vacuolar processing enzyme (VPE) was identified as an executioner of plant PCD. VPE is a cysteine protease that cleaves a peptide bond at the C-terminal side of asparagine and aspartic acid. VPE exhibited enzymatic properties similar to that of a caspase, which is a cysteine protease that mediates the PCD pathway in animals, although there is limited sequence identity between the two enzymes. VPE and caspase-1 share several structural properties: the catalytic dyads and three amino acids forming the substrate pockets (Asp pocket) are conserved between VPE and caspase-1. In contrast to such similarities, subcellular localizations of these proteases are completely different from each other. VPE is localized in the vacuoles, while caspases are localized in the cytosol. VPE functions as a key molecule of plant PCD through disrupting the vacuole in pathogenesis and development. Cell death triggered by vacuolar collapse is unique to plants and has not been seen in animals. Plants might have evolved a VPE-mediated vacuolar system as a cellular suicide strategy.