Krupp Anna, Bertsch Barbara, Spring Otmar
Department of Biochemistry of Plant Secondary Metabolism (190b), Institute of Biology, University of Hohenheim, Stuttgart, Germany.
Front Plant Sci. 2021 Aug 13;12:699068. doi: 10.3389/fpls.2021.699068. eCollection 2021.
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 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 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.
列当属是栽培向日葵的专性根寄生植物，在欧洲、北非和亚洲部分地区，其经济重要性日益增加。虽然已确定从向日葵根系释放的倍半萜内酯（STLs）是土壤中种子的天然萌发刺激物，但迄今为止，引导新出现的胚管朝向寄主根的信号的化学性质仍不清楚。因此，我们设计了一种生物测定法，可在存在具有假定向化活性物质的情况下观察列当属植物的萌发及随后胚管的发育。根系分泌物和向日葵油提取物都含有微摩尔浓度的STLs，它们能使胚管产生正向化性定向。分泌物和油提取物中存在的向日葵四种STLs之一的木香烃内酯，也产生了类似的正向化性效应。相比之下，GR24是一种对种子具有萌发诱导活性的合成独脚金内酯（SL），对胚管定向没有影响。木香烃内酯的作用具有浓度依赖性，且在其在根系中天然微摩尔浓度范围内。我们认为，STL梯度是胚管朝向寄主一侧的侧翼伸长生长比远离寄主一侧的侧翼受到更强抑制的原因。这将赋予向日葵根系分泌物中的STLs在向日葵与列当属植物相互作用中的双重作用，即作为萌发刺激物和向化信号。
