Unearthing the Rhizosphere
About 1% of all flowering plants are parasitic. Witchweeds (Striga spp.) and other parasitic plants are a major constraint on grain production. The survival of obligate parasites, such as Striga asiatica, is dependent on successful host detection and attachment. Host attachment occurs via a specialized organ known as the haustorium. Such developmental transitions are driven by host derived signals (or xenognosins) in a variety of parasites. Ongoing work in the Striga project explores the origin of these xenognosins as well as the mechanisms by which they are perceived. Such studies explore organogenesis in plants as well as the evolutionary divergence between the parasitic and non-parasitic paradigms. These studies may also improve crop regulation, reducing parasitic weeds's agricultural impact.
Right: S. asiatica in its parasitic glory
The haustorium is characterized by swelling of the root tip and the formation of root hairs. P-benzoquinones induce haustorium formation with a redox range between 0 to -250mV, suggesting oxidation and reduction are required for organogenesis.
Left: SEM of haustorium in S. asiatica, attached to a corn root. From: Parasitic Weeds in Agriculture
Following germination. S. asiatica seedlings begin exploiting the host roots's innate defenses by reacting with its peroxidases and phenols to generate haustorial inducing quinones. Given the role of reactive oxygen species (ROS) as secondary messengers, however, this seemed an unlikely mechanism for generating signals. Our studies (Andrew Palmer, John Keyes, and Lizhi Liang) have established that hydrogen peroxide production is down-regulated by the perception of the very signals which they help synthesize. We are currently mapping out regulation of this oxidants production at the the enzymatic and genetic levels.
Right: Day old S. asiatica seedlings stained with the cell permeable H2O2 reactive dye H2-DCFDA. Over 15 minutes fluorescence accumulates in untreated seedlings (top). Seedlings treated for 2hr with the haustorial inducing quinone DMBQ (2,6-dimethoxybenzoquinone) shows little to no fluorescence accumulation.
Agrobacterium tumefaciens, a soil-borne α-proteobacterium, can transfer DNA from a resident tumor inducing (Ti) plasmid into eukaryotes where the transferred DNA (T-DNA) integrates into the host genome. It is the only organism known to routinely engage in lateral gene transfer between kingdoms. In Agrobacterium, virulent bacteria recognize signals produced at a host wound, phenols, monosaccharides, and low pH, as cues inducing expression of the Ti-encoded virulence (vir) genes. The vir gene products, among other functions, are necessary for processing and transporting T-DNA from the bacterium to the eukaryote. Aindrila Mukhopadhyay and Rong Gao employed biochemical and molecular genetic methods to develop map the signal transduction process, Justin Maresh has revised signal perception models, and Fang Fang is attempting to transfer the DNA transfer machinery to heterologous hosts.