Haustorium Inducing Factors for Parasitic Orobanchaceae

Abstract

Parasitic plants in the Orobanchaceae family include devastating weed species, such as Striga, Orobanche, and Phelipanche, which infest important crops and cause economic losses of over a billion US dollars worldwide, yet the molecular and cellular processes responsible for such parasitic relationships remain largely unknown. Parasitic species of the Orobanchaceae family form specialized invasion organs called haustoria on their roots to enable the invasion of host root tissues. The process of forming haustoria can be divided into two steps, prehaustorium formation and haustorium maturation, the processes occurring before and after host attachment, respectively. Prehaustorium formation is provoked by host-derived signal molecules, collectively called haustorium-inducing factors (HIFs). Cell wall-related quinones and phenolics have been known for a long time to induce haustoria in many Orobanchaceae species. Although such phenolics are widely produced in plants, structural specificities exist among these molecules that modulate their competency to induce haustoria in different parasitic plant species. In addition, the plant hormone cytokinins, structurally distinct from phenolic compounds, also trigger prehaustorium formation in Orobanchaceae. Recent findings demonstrate their involvement as rhizopsheric HIFs for Orobanche and Phelipanche species and thus address new activities for cytokinins in haustorium formation in Orobanchaceae, as well as in rhizospheric signaling. This review highlights haustorium-inducing signals in the Orobanchaceae family in the context of their host origin, action mechanisms, and species specificity.

Keywords: Orobanchaceae; Striga; cytokinin; haustorium; haustorium-inducing factor; lignin; parasitic plants; quinone.

Figure 1

Haustorial cell structures formed upon infection of various plants or induced by DMBQ. The upper photos show the longitudinal sections of P. japonicum haustoria after infection of susceptible plants (Arabidopsis thaliana (A) and Oryza sativa (B)) and a resistant plant (Lotus japonicus (C)) or induction by DMBQ treatment (D). The lower drawings show the cell outlines of the upper sections. The colors denote cell types. Red, blue, yellow, purple, and gray indicate intrusive cells, small (procambium-like) cells, large cells, lignin accumulation, and host root cells, respectively. Stripes mark the location of xylem cells. Bars = 100 µm.

Figure 2

Response of Orobanchaceae parasitic plants treated with various chemicals for haustorium induction. Chemical categories previously reported as HIFs are shown on the left. Blue boxes with circles: at least one chemical was reported in the corresponding species. Red boxes with crosses: tested chemical(s) did not induce haustoria. Green boxes with triangles: haustoria-like structures were reported. Gray boxes: not reported. Numbers indicate references: 1, Chang and Lynn (1986); 2, Albrecht et al. (1999); 3, Bandaranayake et al. (2010); 4, Bandaranayake et al. (2012); 5, Cui et al. (2016); 6, Ishida et al. (2016); 7, Cui et al. (2018); 8, Goyet et al. (2017); 9, Salcedo-Morales et al. (2013); 10, Lynn and Chang (1990); 11, Lynn et al. (1981); 12, Steffens et al. (1982); 13, Wada et al. (2019); 14, Fernández-Aparicio et al. (2016b); 15, Wrobel and Yoder (2001); 16, Keyes et al. (2000).