Ethylene signaling mediates host invasion by parasitic plants

Parasitic plants form a specialized organ, a haustorium, to invade host tissues and acquire water and nutrients. To understand the molecular mechanism of haustorium development, we performed a forward genetics screening to isolate mutants exhibiting haustorial defects in the model parasitic plant Phtheirospermum japonicum. We isolated two mutants that show prolonged and sometimes aberrant meristematic activity in the haustorium apex, resulting in severe defects on host invasion. Whole genome sequencing revealed that the two mutants respectively have point mutations in homologs of ETHYLENE RECEPTOR 1 (ETR1) and ETHYLENE INSENSITIVE 2 (EIN2), signaling components in response to the gaseous phytohormone ethylene. Application of the ethylene signaling inhibitors also caused similar haustorial defects, indicating that ethylene signaling regulates cell proliferation and differentiation of parasite cells. Importantly, genetic disruption of host ethylene production also perturbs parasite invasion. We propose that parasitic plants utilize ethylene as a signal to invade host roots.

Methods
Whole genome sequence of a facultative parasitic plant Phtheirospermum japonicum (ecotype Okayama) was analysed by illumina paired-end and mate-pair sequencing, together with PacBio long read sequencing. The draft genome was assembly using allpaths-lg and PBJelly, and annotated using MakerP pipeline using P. japonicum transcriptome.