Strigolactones as germination stimulants for root parasitic plants

Abstract

Witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.) are the two most devastating root parasitic plants belonging to the family Orobanchaceae and are causing enormous crop losses throughout the world. Seeds of these root parasites will not germinate unless they are exposed to chemical stimuli, 'germination stimulants' produced by and released from plant roots. Most of the germination stimulants identified so far are strigolactones (SLs), which also function as host recognition signals for arbuscular mycorrhizal fungi and a novel class of plant hormones inhibiting shoot branching. In this review, we focus on SLs as germination stimulants for root parasitic plants. In addition, we discuss how quantitative and qualitative differences in SL exudation among sorghum cultivars influence their susceptibility to Striga.

Fig. 1

Life cycle of a root parasitic plant, Orobanche minor. (I) Seed becomes responsive after a pre-incubation period of moist and suitable temperatures. (II) Seed germination is induced by host-derived stimulants including strigolactones. (III) The growing radicle attaches to the host root and develops a haustorium. (IV) The parasite penetrates the host epidermis and cortex tissues and connects with the vascular system. (V) The parasite sucks water, minerals and photosynthates from the host. (VI) The parasite tubercles grow underground for several weeks or several months before emergence of the flowering shoots. (VII) The parasite produces a large number of seeds which remain viable for many years in the soil. I–III (in blue) and IV–VII (in red) are pre-parasitic and parasitic phases, respectively.

Fig. 2

Structures of natural strigolactones and the synthetic analog GR24.Reproduced / Kim, I.L., Xie, X., Kim, H.S., Chun, J.C., Yoneyama, K., Nomura, T., Takeuchi, Y. and Yoneyama. K. (2010) Structure-activity relationship of naturally occurring strigolactones in Orobanche minor seed germination stimulation. J. Pestic. Si. DOI: 10.1584/jpestics.G10-17, with permission from Pesticide Science Society of Japan.

Fig. 3

Orobanche minor and Striga hermonthica seed germination induced by root exudates from Striga-resistant (Nagade, Tetron and SRN39) and Striga-susceptible (Tabat, Wad El Mubark and Korakollow) sorghum cultivars. The root exudates were collected from three seedlings for 20 h. The concentrations of root exudates were equalized for differences in root fresh weight. GR24 at 1 μM induced 78 and 72% germination in O. minor and S. hermonthica, respectively, and no germination was observed for the seeds treated with distilled water. Data are presented as means ± SE (n = 3).

Fig. 4

Orobanche minor seed germination induced by root exudates from Striga-resistant SRN39 and Striga-susceptible Tabat sorghum cultivars at two different concentrations. The root exudates were collected daily from approximately 500 seedlings for 7 d. The germination rate of seeds treated with 1 μM GR24 was 75% and distilled water induced no or negligible germination. Data are presented as means ± SE (n = 3).

Fig. 5

LC–MS/MS analyses of strigolactones in root exudates. (A) MRM chromatogram of root exudate from the Striga-susceptible sorghum cultivar Tabat. (B) MRM chromatogram of root exudate from the Striga-resistant sorghum cultivar SRN39. The transitions monitored were m/z 369 > 272 for sorgomol, m/z 339 > 242 for sorgolactone and m/z 353 > 256 for 5-deoxystrigol.