Differential activity of Striga hermonthica seed germination stimulants and Gigaspora rosea hyphal branching factors in rice and their contribution to underground communication

The lactonase BxdA mediates metabolic specialisation of maize root bacteria to benzoxazinoids

Root exudates contain specialised metabolites that shape the plant's root microbiome. How host-specific microbes cope with these bioactive compounds, and how this ability affects root microbiomes, remains largely unknown. We investigated how maize root bacteria metabolise benzoxazinoids, the main specialised metabolites of maize. Diverse and abundant bacteria metabolised the major compound in the maize rhizosphere MBOA (6-methoxybenzoxazolin-2(3H)-one) and formed AMPO (2-amino-7-methoxy-phenoxazin-3-one). AMPO forming bacteria were enriched in the rhizosphere of benzoxazinoid-producing maize and could use MBOA as carbon source. We identified a gene cluster associated with AMPO formation in microbacteria. The first gene in this cluster, bxdA encodes a lactonase that converts MBOA to AMPO in vitro. A deletion mutant of the homologous bxdA genes in the genus Sphingobium, did not form AMPO nor was it able to use MBOA as a carbon source. BxdA was identified in different genera of maize root bacteria. Here we show that plant-specialised metabolites select for metabolisation-competent root bacteria. BxdA represents a benzoxazinoid metabolisation gene whose carriers successfully colonize the maize rhizosphere and thereby shape the plant's chemical environmental footprint.

Unveiling the complexity of strigolactones: exploring structural diversity, biosynthesis pathways, and signaling mechanisms

Strigolactone is the collective name for compounds containing a butenolide as a part of their structure, first discovered as compounds that induce seed germination of root parasitic plants. They were later found to be rhizosphere signaling molecules that induce hyphal branching of arbuscular mycorrhizal fungi, and, finally, they emerged as a class of plant hormones. Strigolactones are found in root exudates, where they display a great variability in their chemical structure. Their structure varies among plant species, and multiple strigolactones can exist in one species. Over 30 strigolactones have been identified, yet the chemical structure of the strigolactone that functions as an endogenous hormone and is found in the above-ground parts of plants remains unknown. We discuss our current knowledge of the synthetic pathways of diverse strigolactones and their regulation, as well as recent progress in identifying strigolactones as plant hormones. Strigolactone is perceived by the DWARF14 (D14), receptor, an α/β hydrolase which originated by gene duplication of KARRIKIN INSENSITIVE 2 (KAI2). D14 and KAI2 signaling pathways are partially overlapping paralogous pathways. Progress in understanding the signaling mechanisms mediated by two α/β hydrolase receptors as well as remaining challenges in the field of strigolactone research are reviewed.

Vitislactone, a non-canonical strigolactone exudated by grapevine rootstocks in response to nitrogen starvation

Strigolactones are compounds produced by plant roots in response to nutrient deficiency, acting both as local and systemic signals to control development and nutrition. Strigolactones are exuded in the rhizosphere to positively influence interactions with beneficial microbes. LC-MS/MS analysis shows that two genetically distinct grapevine rootstocks exudate one or two non-canonical strigolactones when subjected to low nitrogen conditions. Gene expression profiles and orobanche seed germination assays confirm that the biosynthesis and exudation of non-canonical compounds is the preferred pathway. The first compound, corresponding to heliolactone or 6-epi-heliolactone, is only exuded by the rootstock showing lower shoot branching and a higher level of mycorrhization with arbuscular mycorrhizal fungi. The structure of the second compound exuded by both rootstocks was identified by NMR and LC-MS/MS analysis. It is a non-canonical strigolactone, which has never been identified in another species. This first identification of a natural compound with the potential to stimulate beneficial root-microbe interactions in grapevines opens new perspectives in viticulture.

Hormonomic Changes Driving the Negative Impact of Broomrape on Plant Host Interactions with Arbuscular Mycorrhizal Fungi

Belowground interactions of plants with other organisms in the rhizosphere rely on extensive small-molecule communication. Chemical signals released from host plant roots ensure the development of beneficial arbuscular mycorrhizal (AM) fungi which in turn modulate host plant growth and stress tolerance. However, parasitic plants have adopted the capacity to sense the same signaling molecules and to trigger their own seed germination in the immediate vicinity of host roots. The contribution of AM fungi and parasitic plants to the regulation of phytohormone levels in host plant roots and root exudates remains largely obscure. Here, we studied the hormonome in the model system comprising tobacco as a host plant, Phelipanche spp. as a holoparasitic plant, and the AM fungus Rhizophagus irregularis. Co-cultivation of tobacco with broomrape and AM fungi alone or in combination led to characteristic changes in the levels of endogenous and exuded abscisic acid, indole-3-acetic acid, cytokinins, salicylic acid, and orobanchol-type strigolactones. The hormonal content in exudates of broomrape-infested mycorrhizal roots resembled that in exudates of infested non-mycorrhizal roots and differed from that observed in exudates of non-infested mycorrhizal roots. Moreover, we observed a significant reduction in AM colonization of infested tobacco plants, pointing to a dominant role of the holoparasite within the tripartite system.

The Many Models of Strigolactone Signaling

Strigolactones (SLs) are a class of plant hormones involved in several biological processes that are of great agricultural concern. While initiating plant-fungal symbiosis, SLs also trigger germination of parasitic plants that pose a major threat to farming. In vascular plants, SLs control shoot branching, which is linked to crop yield. SL research has been a fascinating field that has produced a variety of different signaling models, reflecting a complex picture of hormone perception. Here, we review recent developments in the SL field and the crystal structures that gave rise to various models of receptor activation. We also highlight the increasing number of discovered SL molecules, reflecting the existence of cross-kingdom SL communication.

Genetic variation in Sorghum bicolor strigolactones and their role in resistance against Striga hermonthica

Sorghum is an important food, feed, and industrial crop worldwide. Parasitic weeds of the genus Striga constitute a major constraint to sorghum production, particularly in the drier parts of the world. In this study we analysed the Striga germination stimulants, strigolactones, in the root exudates of 36 sorghum genotypes and assessed Striga germination and infection. Low germination-stimulating activity and low Striga infection correlated with the exudation of low amounts of 5-deoxystrigol and high amounts of orobanchol, whereas susceptibility to Striga and high germination-stimulating activity correlated with high concentrations of 5-deoxystrigol and low concentrations of orobanchol. Marker analysis suggested that similar genetics to those previously described for the resistant sorghum variety SRN39 and the susceptible variety Shanqui Red underlie these differences. This study shows that the strigolactone profile in the root exudate of sorghum has a large impact on the level of Striga infection. High concentrations of 5-deoxystrigol result in high infection, while high concentrations of orobanchol result in low infection. This knowledge should help to optimize the use of low germination stimulant-based resistance to Striga by the selection of sorghum genotypes with strigolactone profiles that favour normal growth and development, but reduce the risk of Striga infection.

Target-based selectivity of strigolactone agonists and antagonists in plants and their potential use in agriculture

Strigolactones (SLs) are small carotenoid-derived molecules that possess a wide spectrum of functions, including plant hormonal activities and chemical mediation of rhizosphere communication with both root parasitic plants and symbiotic arbuscular mycorrhizal fungi. Chemicals that regulate the functions of SLs may therefore have the potential to become widely used in agricultural applications. For example, various SL analogs and mimics have been developed to reduce the seed banks of root parasites in the field. Other analogs and mimics act selectively to suppress branching, with weak, or no stimulation, of germination in root parasites. In addition, some antagonists for SL receptors have been developed based on the mechanisms of SL perception. A better understanding of the modes of action of SL perception by various receptors will help to support the design of SL analogs, mimics, and antagonists with high activity and selectivity. Here, we review the compounds reported so far from the viewpoint of their selectivity to their targets, and the possibilities for their use in agriculture.

Strigolactone Signaling and Evolution

Strigolactones are a structurally diverse class of plant hormones that control many aspects of shoot and root growth. Strigolactones are also exuded by plants into the rhizosphere, where they promote symbiotic interactions with arbuscular mycorrhizal fungi and germination of root parasitic plants in the Orobanchaceae family. Therefore, understanding how strigolactones are made, transported, and perceived may lead to agricultural innovations as well as a deeper knowledge of how plants function. Substantial progress has been made in these areas over the past decade. In this review, we focus on the molecular mechanisms, core developmental roles, and evolutionary history of strigolactone signaling. We also propose potential translational applications of strigolactone research to agriculture.

Strigolactones, super hormones in the fight against Striga

Strigolactones are plant hormones that control diverse aspects of plant growth, but are also exuded into soil as recruitment signals for arbuscular mycorrhizal fungi interactions. Highly damaging parasitic weeds in the Orobanchaceae family have coopted strigolactones as germination cues that indicate the presence of a host. Recent studies have established how strigolactones are actively transported within and out of plants. Key components of the strigolactone signaling system have been identified, including strigolactone receptors in angiosperms and parasites, as well as downstream targets that are polyubiquitinated and proteolyzed following strigolactone perception. The basis for protein-protein interactions among these signaling components has also been explored. We propose several strategies to translate current knowledge of strigolactone transport and signaling into parasite control methods.

Functional redundancy in the control of seedling growth by the karrikin signaling pathway

SMAX1 and SMXL2 control seedling growth, demonstrating functional redundancy within a gene family that mediates karrikin and strigolactone responses. Strigolactones (SLs) are plant hormones with butenolide moieties that control diverse aspects of plant growth, including shoot branching. Karrikins (KARs) are butenolide molecules found in smoke that enhance seed germination and seedling photomorphogenesis. In Arabidopsis thaliana, SLs and KARs signal through the α/β hydrolases D14 and KAI2, respectively. The F-box protein MAX2 is essential for both signaling pathways. SUPPRESSOR OF MAX2 1 (SMAX1) plays a prominent role in KAR-regulated growth downstream of MAX2, and SMAX1-LIKE genes SMXL6, SMXL7, and SMXL8 mediate SL responses. We previously found that smax1 loss-of-function mutants display constitutive KAR response phenotypes, including reduced seed dormancy and hypersensitive growth responses to light in seedlings. However, smax1 seedlings remain slightly responsive to KARs, suggesting that there is functional redundancy in karrikin signaling. SMXL2 is a strong candidate for this redundancy because it is the closest paralog of SMAX1, and because its expression is regulated by KAR signaling. Here, we present evidence that SMXL2 controls hypocotyl growth and expression of the KAR/SL transcriptional markers KUF1, IAA1, and DLK2 redundantly with SMAX1. Hypocotyl growth in the smax1 smxl2 double mutant is insensitive to KAR and SL, and etiolated smax1 smxl2 seedlings have reduced hypocotyl elongation. However, smxl2 has little or no effect on seed germination, leaf shape, or petiole orientation, which appear to be predominantly controlled by SMAX1. Neither SMAX1 nor SMXL2 affect axillary branching or inflorescence height, traits that are under SL control. These data support the model that karrikin and strigolactone responses are mediated by distinct subclades of the SMXL family, and further the case for parallel butenolide signaling pathways that evolved through ancient KAI2 and SMXL duplications.

Engineering the plant rhizosphere

Plant natural products are low molecular weight compounds playing important roles in plant survival under biotic and abiotic stresses. In the rhizosphere, several groups of plant natural products function as semiochemicals that mediate the interactions of plants with other plants, animals and microorganisms. The knowledge on the biosynthesis and transport of these signaling molecules is increasing fast. This enables us to consider to optimize plant performance by changing the production of these signaling molecules or their exudation into the rhizosphere. Here we discuss recent advances in the understanding and metabolic engineering of these rhizosphere semiochemicals.