Deoxyanthocyanidins

The 3-deoxyanthocyanidins are a class of phytoalexins found in sorghum. These compounds are so fungi-toxic that they are effective at femtogram levels (Snyder and Nicholson, 1990; Nicholson and Wood, 2001). The synthesis of these compounds is initiated on the endoplasmic reticulum. Compounds are then trafficked in subcellular inclusions. The inclusions appear similar to vesicles, but there is no evidence that membranes surround the inclusions (Snyder and Nicholson, 1990). Nielsen et al. (2004) recently summarized this defense response. The cytological response commences when clear, colorless inclusions (less than 0.1 ^m in diameter) accumulate in leaf cells under fungal attack. The inclusions eventually are seen as red bodies at the infection site. When the 3-deoxyanthocyanidins enter the apoplast, the host cell collapses. The phytoalexins then accumulate in the pathogen and cause its death. Excess phytoalexins are trapped in host cell walls at infection sites (Lo et al., 1998; 1999).

In the publication by Nielsen et al. (2004) images of the pigmented inclusions that contain the phytoalexins were prepared by confocal microscopy. This provided a three-dimensional perspective of inclusion body formation and visualization of the phytoalexins. A representation of deoxyanthocyanidin accumulation is shown in Figure 6-3 where inclusions begin to form by 5 to 8 hours after a fungal appressorium was formed by a hypha.

Figure 6-3. Cell-specific accumulation of 3-deoxyanthocyanidins in Sorghum bicolor in response to attempted fungal attack. (A) Illustration of changes in inclusion morphology in cells under fungal attack in response to formation of infectious structures 0-48 h after inoculation. (B) Site-specific accumulation of 3-deoxyanthocyanidins at site of incipient penetration, before host cell collapse 24 h after inoculation. (C) Illustration of site-specific trafficking (arrows) of inclusions in relation to position of fungal infectious structures. Reprinted from Phys. Mol. Plant Pathol., 65, Nielsen, K. A., Gottfredsen, C. H., Buch-Pedersen, M. J., Ammitzball, H., Mattsson, O., Duus, J. 0., and Nicholson, R. L., Antimicrobial flavonoid 3-deoxyanthocyanidins in Sorghum bicolor self-organize into spherical structures, 187-196, Copyright 2004, with permission from Elsevier.

Figure 6-3. Cell-specific accumulation of 3-deoxyanthocyanidins in Sorghum bicolor in response to attempted fungal attack. (A) Illustration of changes in inclusion morphology in cells under fungal attack in response to formation of infectious structures 0-48 h after inoculation. (B) Site-specific accumulation of 3-deoxyanthocyanidins at site of incipient penetration, before host cell collapse 24 h after inoculation. (C) Illustration of site-specific trafficking (arrows) of inclusions in relation to position of fungal infectious structures. Reprinted from Phys. Mol. Plant Pathol., 65, Nielsen, K. A., Gottfredsen, C. H., Buch-Pedersen, M. J., Ammitzball, H., Mattsson, O., Duus, J. 0., and Nicholson, R. L., Antimicrobial flavonoid 3-deoxyanthocyanidins in Sorghum bicolor self-organize into spherical structures, 187-196, Copyright 2004, with permission from Elsevier.

Note that the inclusions at this early time are not pigmented; rather they are colorless bodies that move through the cytoplasm toward the site of appressorial attack. Over time, the inclusions take on a yellow color and eventually become deep red in pigmentation. Inclusion size changes from less than 1 ^m to 20 ^m or even larger. Inclusions move to the penetration site and cluster in the area where the penetration peg has made physiological contact with the host cell. When the appressorium begins the process of penetration the inclusions burst, releasing their contents into the cytoplasm. The deoxyanthocyanidins kill the host cell itself and are taken up by the pathogen. This is possible because these deoxyanthocyanidins are soluble in both water and organic solvents. In this manner the pathogen is also killed and prevented from causing extensive damage and cell death of the host.

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