Subtilase activity in intrusive cells mediates haustorium maturation in parasitic plants

Abstract

Parasitic plants that infect crops are devastating to agriculture throughout the world. These parasites develop a unique inducible organ called the haustorium that connects the vascular systems of the parasite and host to establish a flow of water and nutrients. Upon contact with the host, the haustorial epidermal cells at the interface with the host differentiate into specific cells called intrusive cells that grow endophytically towards the host vasculature. Following this, some of the intrusive cells re-differentiate to form a xylem bridge that connects the vasculatures of the parasite and host. Despite the prominent role of intrusive cells in host infection, the molecular mechanisms mediating parasitism in the intrusive cells remain poorly understood. In this study, we investigated differential gene expression in the intrusive cells of the facultative parasite Phtheirospermum japonicum in the family Orobanchaceae by RNA-Sequencing of laser-microdissected haustoria. We then used promoter analyses to identify genes that are specifically induced in intrusive cells, and promoter fusions with genes encoding fluorescent proteins to develop intrusive cell-specific markers. Four of the identified intrusive cell-specific genes encode subtilisin-like serine proteases (SBTs), whose biological functions in parasitic plants are unknown. Expression of SBT inhibitors in intrusive cells inhibited both intrusive cell and xylem bridge development and reduced auxin response levels adjacent to the area of xylem bridge development. Therefore, we propose that subtilase activity plays an important role in haustorium development in P. japonicum.

The Phtheirospermum japonicum isopentenyltransferase PjIPT1a regulates host cytokinin responses in Arabidopsis

“…We surveyed published Phtheirospermum transcriptomes for infection‐specific expression of IPTs and identified one, hereinafter referred to as PjIPT1a , that is upregulated in the course of infection and shows 25‐fold higher expression in intrusive cells relative to the rest of the haustorium (Fig. 1b) (Ogawa et al ., 2020). PjIPT1a expression increases until 3 d postinfection (dpi) and continues at a relatively high level for the later time points (Fig.…”

“…(a) Prenylation of ATP, ADP and AMP by isopentenyltransferases (IPTs) with di‐methylallyl pyrophosphate (DMAPP) to isopentenyladenosine‐5’‐triphosphate (IPRTP), isopentenyladenosine‐5'‐diphosphate (IPRDP) and isopentenyladenosine‐5'‐monophosphate (IPRMP) is the first step of cytokinin (CK) biosynthesis. (b) RNA‐sequencing data (originally published in Ogawa et al ., 2020) show increased expression of PjIPT1a in intrusive cells (purple) of Phtheirospermum japonicum ( Pj ) haustoria 5 d postinfection (dpi) of rice roots ( Oryza sativa , Os ). (c) Upregulation of PjIPT1a expression during infection of Arabidopsis ( At ) roots peaks with 61 CPM (counts per million mapped reads) at 3 dpi (RNA‐sequencing data from Kurotani et al ., 2020).…”

The Phtheirospermum japonicum isopentenyltransferase PjIPT1a regulates host cytokinin responses in Arabidopsis

“…We surveyed published Phtheirospermum transcriptomes for infection-specific expression of IPTs and identified one, hereinafter referred to as PjIPT1a , that is upregulated in the course of infection and shows 25-fold higher expression in intrusive cells relative to the rest of the haustorium (Fig. 1b ) (Ogawa et al , 2020). PjIPT1a expression increases until day three of the infection and continues at a relatively high level for the later time points (Fig.…”

“…(a) Prenylation of ATP, ADP and AMP by isopentenyltransferases (IPT) with di-methylallyl pyrophosphate (DMAPP) to isopentenyladenosine-5’-triphosphate (IPRTP), isopentenyladenosine-5’-diphosphate (IPRDP) and isopentenyladenosine-5’-monophosphate (IPRMP) is the first step of cytokinin (CK) biosynthesis. (b) RNA-Sequencing data (originally published in Ogawa et al, 2020) shows increased expression of PjIPT1a in intrusive cells of Phtheirospermum haustoria 5 days post-infection (dpi) of rice roots ( Oryza sativa , Os ). (c) Upregulation of PjIPT1a expression during infection of Arabidopsis ( At ) roots peaks with 61 CPM (counts per million mapped reads) at 3 dpi (RNA-Sequencing data from Kurotani et al, 2020).…”

Strigolactones are chemoattractants for host tropism in Orobanchaceae parasitic plants

“…are notoriously devastating invaders of major agricultural crops, leading to a multibillion-dollar economic loss annually 5,6 . Most obligate Orobanchaceae parasites have evolved three steps to complete invasion 2 : germination near the host root in response to host-derived stimulants such as strigolactones (SLs) 7 , active extension of the parasite's root to host roots 8,9 , and connection of the vasculature via formation of an invasive organ called a haustorium 2,6,[10][11][12] . Although many studies have focused on germination and haustorium development, the molecular basis for host tropism is largely unknown.…”