Figure 610

Natural and synthetic haustorium inductors and inhibitors.

Recently, a chemical model to explain the formation of haustorium in the presence of benzoquinones has been reported, based on the Redox properties of these compounds.24 This model explains the release of quinones from the host through oxidation of pectins, phenyl- propanoid esters decorating the pectins, and the syringic acid that are present in the surface of the root cell walls. The enzymes catalyzing this reaction are cell wall peroxidases from the host and need the presence of hydrogen peroxide (H2O2) as co-factor. This compound is widely present in nature (specially in plant tissues) and accumulates in the parasite root tip. The quinones generated from oxidation have been characterized as benzo-1,4-quinone (BQ, 77), methoxybenzo-1,4-quinone (MBQ, 78) and DMBQ (79). However, not all quinones are able to induce haustorium formation. In fact, and after assaying a wide variety of benzo- and naphtoquinones only those compounds, which can be easily reduced and oxidized are active. Those compounds having half-reduction redox potential out of the range -250 to 0 mV ("active window") are inactive.44

According to these findings, the model proposes as the active compound the semiquinone intermediate that reversibly binds the active site. Those electropositive compounds lying to the right of the "active window" (e.g., tetrafluorobenzo-1,4-quinone, TFBQ, 82), will easily accept one electron. However, the next re-oxidizing step will be thermodynamically restricted, and the quinone will irreversibly remain bonded to the active site, thus inhibiting germination. In the opposite case, compounds in the left side of the window (mostly naftoquinones like juglone 84) will be too difficult to half-reduce under the environmental conditions; semiquinones will not be produced, and the haustorial phase will not be induced. This is further proved when the cyclopropyl-p-benzoquinone (83) is assayed along with DMBQ. In this case, the semiquinone intermediate suffers a ring-opening rearrangement and the resulting carbon radical irreversibly binds the active site, thus inhibiting the haustorium formation induced by the DMBQ.

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