A Divergent Clade KAI2 Protein in the Root Parasitic Plant Orobanche minor Is a Highly Sensitive Strigolactone Receptor and Is Involved in the Perception of Sesquiterpene Lactones

Abstract

Strigolactones (SLs) were initially discovered as germination inducers for root parasitic plants. In 2015, three groups independently reported the characterization of the SL receptor in the root parasitic plant Striga hermonthica, which causes significant damage to crop production, particularly in sub-Saharan Africa. The characterized receptors belong to HYPOSENSITIVE TO LIGHT/KARRIKIN INSENSITIVE2 (HTL/KAI2), which is a member of the α/β-hydrolase protein superfamily. In non-parasitic plants, HTL/KAI2 perceives the smoke-derived germination inducer karrikin and a yet-unidentified endogenous ligand. However, root parasitic plants evolved a specific clade of HTL/KAI2 that has diverged from the KAI2 clade of non-parasitic plants. The S. hermonthica SL receptors are included in this specific clade, which is called KAI2 divergent (KAI2d). Orobanche minor is an obligate root holoparasitic plant that grows completely dependent on the host for water and nutrients because of a lack of photosynthetic ability. Previous phylogenetic analysis of KAI2 proteins in O. minor has demonstrated the presence of at least five KAI2d clade genes. Here, we report that KAI2d3 and KAI2d4 in O. minor have the ability to act as the SL receptors. They directly interact with SLs in vitro, and when expressed in Arabidopsis, they rescue thermo-inhibited germination in response to the synthetic SL analog GR24. In particular, KAI2d3 showed high sensitivity to GR24 when expressed in Arabidopsis, suggesting that this receptor enables highly sensitive SL recognition in O. minor. Furthermore, we provide evidence that these KAI2d receptors are involved in the perception of sesquiterpene lactones, non-strigolactone-type germination inducers.

Sesquiterpene Lactones Isolated from Centaurea cineraria L. subsp. cineraria Inhibit the Radicle Growth of Broomrape Weeds

“…The family of phytohormones named strigolactones represents the most studied structures, though diverse sesquiterpene lactones produced by plants have been discovered as potent elicitors of germination of some broomrape species, being inactive on the germination of other broomrape species, with the case of costunolide and dehydrocostus lactone produced by sunflower being one of the most representative [ 10 , 23 ]. Thus, the study of sesquiterpene lactones also focused on the interest of parasitic weed research, covering topics like mechanisms, biosynthesis, and the use of materials for the development of bioactive derivatives against parasitic weeds [ 38 , 39 ]. Regarding the inhibition of parasitic weeds, few references are available in the literature, mainly the discovery of inuloxins as inhibitors of the germination of O. crenata and Cuscuta campestris [ 40 ].…”

Chemistry of Strigolactones, Key Players in Plant Communication

“…Contrary to GR24 analogs, CTLs and karrikins are non-hydrolytic ligands for KAI2. In the course of evolution, in parasitic plants, the family of KAI2 genes has grown and evolved in encoding SL receptors for germination (12 HTLs/KAI2s in Striga hermonthica, [127] 4 KAI2s in P. ramosa, [72] 5 KAI2 s in Orobanche minor [128] ). The most widely studied SL receptors in parasitic plants that are particularly sensitive to SL are ShHTL7, [127a] PrKAI2d3, [72] and OmPrKAI2d3 [128] present in S. hermonthica, O. minor, and P. ramosa, respectively.…”

“…[39] Later, Pavan, et al mentioned to have used a 13 C 2 -labeled strigol derivative as an internal standard to characterized and quantify Pisum sativum L. (pea) SLs. [77] 2018 marks the expansion of several labeled SLs with (i) an enantiomerically pure synthesis of two monodeuterated MeCLA (124 and 125) by Dieckmann and colleagues, [78] (ii) the use of 13 C-and deuterium labeled 5DS (3) and 4DO ( 12) derivatives (126)(127)(128)(129)(130), highlighting the conversion of deoxy-SLs into HO-SLs in various plant species during the biosynthetic pathway of HO-SLs, [76b] (iii) the development of an original method of SLs quantification by UHPLC using deuterium labeled orobanchyl acetate (131), fabacyl acetate (132), and GR24 (133D3) as internal standards. [13b] Two tritium labeled GR24s were reported to study their transport in plant: (�)-[7-T 1 ]-GR24 (133T) (29 Ci/mmol) [79] and one labeled GR24 (40 Ci/mmol) [80] with no structure given.…”

“…(�)-[7'-D 3 ]-5DS ( 126), (�)-[7'-D 3 , 7'-13 C]-5DS ( 127), (�)-[7'-13 C]-5DS (128) and their 2'-epimers 222-224, respectively, containing a deuterium or/and a carbone-13 labeled D-ring, were synthesized by Cheng, et al [109] This was facilitated by the synthesis of corresponding labeled D-ring 216-218 on four steps from commercially available 2[5H]-furanone (214) with about 50 % yield overall (Scheme 8C). A two steps reaction of 2[5H]-furanone (214) with successively bromine then triethylamine and triisopropylsilyl trifluoromethanesulfonate and triethylamine gave the O-protected (O-TIPS) 3-bromofuranone 215.…”

Radicle growth regulation of root parasitic plants by auxin-related compounds

“…Toward the practical application of this method, many synthetic analogs of SLs have been synthesized (Tsuchiya 2018). Recently, SL receptors in root parasitic plants, such as Striga hermonthica , Phelipanche ramosa , and Orobanche minor , have been characterized (Conn et al 2015; de Saint Germain et al 2021; Takei et al 2023; Toh et al 2015; Tsuchiya et al 2015), and this has facilitated the identification of novel chemicals that target these receptors (Uraguchi et al 2018; Wang et al 2022). Such synthetic agonists can be used as suicidal germination inducers that promote germination in the absence of the host plants.…”