A shot in the dark: how parasitic plants find host roots.

Parasitism, ranging from facultative to obligate and fromgeneral to very host specific, can be a great lifestyle, if you have the right adaptations. In the plant parasite family Orobanchaceae, which includes the agronomically destructive weed Striga, one key adaptation is the ability to sense the proximity of a host root. Host roots can be sensed either by direct contact or by perception of diffusible phenolic compounds from the host; the resulting signals induce the development of an invasive feeding structure, the haustorium (see figure). The haustorium links the two plants, providing a conduit for diversion of the water and nutrients from the host to the parasite. The haustorium-inducing factors (HIFs) sensed by the parasite include phenolic compounds derived from lignin, such as the quinone 2,6 dimethoxy-1,4-benzoquinone (DMBQ) (reviewed in Westwood et al., 2010). Indeed, phenolic molecules, and specifically quinones, appear to be key factors in haustorium induction and may require activation bya redoxmechanism.To examine redox activation of quinone signals in the facultative root parasite Triphysaria versicolor, Bandaranayake et al. (pages 1404–1419) looked at the functions of two quinone oxidoreductases (QR1 and QR2) that are rapidly transcriptionally induced in response toHIFs. They found that both transcripts increase in response to DMBQ, but onlyQR1 increases in response to rootproximity. Also, QR1 functions in haustorium induction or development, as RNA interference silencing of QR1 caused a strong reduction of haustorium formation in response to root proximity or HIFs. However, silencingofQR2didnot causea reduction in haustorium formation, indicating that the two enzymes have different functions. By purification and biochemical characterization of bacterially expressed QR1 and QR2, the authors further found that the different functions of these enzymes in haustorium induction are borne out by different enzymatic functions. Thus, QR1 acts as an NADPH-dependent quinone oxidoreductase that likely catalyzes a single-electron transfer reaction to produce a reactive radical quinone intermediate and reactive oxygen species, such as hydrogen peroxide (see figure). By contrast, QR2 likely catalyzes a two-electron transfer reaction that produces a nonreactive, detoxified product. Thus, although these two enzymes are induced in response to the same stimulus, they proved to have distinct enzymatic functions and biological roles in haustorium formation, one acting in redox activation of a quinone signal and the other acting in quinone detoxification, but both likely contributing to essential adaptations for parasitism.