Costunolide Influences Germ Tube Orientation in Sunflower Broomrape - A First Step Toward Understanding Chemotropism.

Anna Krupp,Otmar Spring,Barbara Bertsch

doi:10.3389/fpls.2021.699068

Abstract

Orobanche cumana WALLR. is a host-specific root parasite of cultivated sunflowers with increasing economic importance in Europe, North Africa, and parts of Asia. While sesquiterpene lactones (STLs) released from sunflower roots were identified as natural germination stimulants of O. cumana seeds in the soil, the chemical nature of the signals guiding the emerging germ tube toward the host root has remained unknown hitherto. Thus, we designed a bioassay that allowed the observation of broomrape germination and subsequent germ tube development in the presence of substances with putative chemotropic activity. Root exudates and sunflower oil extracts, both containing STLs in micromolar concentrations, caused the positive chemotropic orientation of germ tubes. A similar positive chemotropic effect was achieved with costunolide, one of the four STLs of sunflower present in the exudate and oil extracts. In contrast, GR24, a synthetic strigolactone (SL) with germination-inducing activity on O. cumana seeds, showed no effect on the germ tube orientation. The effect of costunolide was concentration-dependent and within the range of its natural micromolar occurrence in roots. We assume that an STL gradient is responsible for the stronger inhibition of elongation growth on the host-facing flank of the germ tube compared with the far side flank. This would confer a double role of STLs from sunflower root exudates in the sunflower–broomrape interaction, namely, as germination stimulants and as chemotropic signals.

Highlights

Chemical signals are decisive factors for the survival of parasitic plants
Overlap, and seed displacement, clear germ tube orientation could be recorded for only about half of the germinated seeds. In these cases (n = 193 seeds in seven independent experiments), 75 ± 5% showed curving toward the host roots, whereas 25% grew in the opposite direction (Supplementary Table 1)
To test the chemotropic influence of exudates, extracts, or candidate compounds, a bioassay was designed, which allowed the observation of the O. cumana seeds and germ tube development (Figure 2)

Summary

Introduction

Chemical signals are decisive factors for the survival of parasitic plants. particular terpenoids such as strigolactones (SLs) and sesquiterpene lactones (STLs) released from the hosts have been found to stimulate germination of root parasites from the Orobanchaceae family and secure host specificity (Yoneyama et al, 2013), but germination is only the first step in the life cycle, waking up the parasite by breaking its physiological dormancy. The parasite has a very short period of time and limited energy to reach the host root surface by actively growing in the right direction This process, named chemotropism, is apparently guided by host-derived metabolic signals and is not well understood up to date. Studies by Pearson (1913) and Saunders (1933) suggested that a chemotropic signal is involved in the growth of Striga germ tubes. This was supported by Williams (1961) and illustrated by Yoshida and Shirasu (2009). For other genera of Orobanchaceae, a putative chemotropic reaction was observed in Alectra vogelii Benth. (Botha, 1948; Visser et al, 1977) and Orobanche crenata Forssk. (Whitney and Carsten, 1981; Aber and Sallé, 1982), but the chemical nature of the signaling compounds was not unraveled

Results

Discussion

Conclusion