Strigolactones (SLs) are a plant hormone inhibiting shoot branching/tillering and a rhizospheric, chemical signal that triggers seed germination of the noxious root parasitic plant Striga and mediates symbiosis with beneficial arbuscular mycorrhizal fungi. Identifying specific roles of canonical and noncanonical SLs, the two SL subfamilies, is important for developing Striga -resistant cereals and for engineering plant architecture. Here, we report that rice mutants lacking canonical SLs do not show the shoot phenotypes known for SL-deficient plants, exhibiting only a delay in establishing arbuscular mycorrhizal symbiosis, but release exudates with a significantly decreased Striga seed–germinating activity. Blocking the biosynthesis of canonical SLs by TIS108, a specific enzyme inhibitor, significantly lowered Striga infestation without affecting rice growth. These results indicate that canonical SLs are not the determinant of shoot architecture and pave the way for increasing crop resistance by gene editing or chemical treatment.

The plant hormones strigolactones (SLs) regulate shoot branching and mediate the communication with symbiotic mycorrhizal fungi, but also with noxious root parasitic weeds, such as Striga spp. SLs derive from carlactone (CL) and are divided structurally into canonical and non-canonical SLs. However, the questions about particular biological functions of the two groups and the identification of the SL that inhibits shoot branching are still unanswered, hampering targeted modification of SL pattern towards improving plant architecture and resistance against Striga. Here, we reported that 4-deoxyorobanchol (4DO) and orobanchol, the two canonical SLs in rice, do not have major role in determining rice shoot architecture. CRISPR/Cas9 mediated Osmax1-900 mutants, lacking these two SLs, do not show the high tillering and dwarf phenotype typical for SL-deficient plants. However, the absence of 4DO and orobanchol in root exudates significantly decreased their capability in inducing Striga seed germination, while caused only a delay in root colonization by mycorrhizal fungi. To confirm the genetic evidence, we used the SL-biosynthesis inhibitor TIS108. Our results showed that TIS108 is a MAX1-specific inhibitor that lowers 4DO and orobanchol synthesis, conferring a resistance to Striga without a severe impact on rice architecture. Hence, our work uncovers the specific function of canonical SLs as rhizospheric signals and paves the way for establishing chemical and genetic based approaches for combating the root parasitic weeds, by targeted depletion of their release.

Summary The establishment of arbuscular mycorrhiza (AM) between plants and Glomeromycotina fungi is preceded by the exchange of chemical signals: fungal released Myc‐factors, including chitooligosaccharides (CO) and lipo‐chitooligosaccharides (LCO), activate plant symbiotic responses, while root‐exuded strigolactones stimulate hyphal branching and boost CO release. Furthermore, fungal signaling reinforcement through CO application was shown to promote AM development in Medicago truncatula, but the cellular and molecular bases of this effect remained unclear. Here, we focused on long‐term M. truncatula responses to CO treatment, demonstrating its impact on the transcriptome of both mycorrhizal and nonmycorrhizal roots over several weeks and providing an insight into the mechanistic bases of the CO‐dependent promotion of AM colonization. CO treatment caused the long‐lasting regulation of strigolactone biosynthesis and fungal accommodation‐related genes. This was mirrored by an increase in root didehydro‐orobanchol content, and the promotion of accommodation responses to AM fungi in root epidermal cells. Lastly, an advanced downregulation of AM symbiosis marker genes was observed at the latest time point in CO‐treated plants, in line with an increased number of senescent arbuscules. Overall, CO treatment triggered molecular, metabolic, and cellular responses underpinning a protracted acceleration of AM development.

Durum wheat is one of the most important agricultural crops, currently providing 18% of the daily intake of calories and 20% of daily protein intake for humans. However, being wheat that is cultivated in arid and semiarid areas, its productivity is threatened by drought stress, which is being exacerbated by climate change. Therefore, the identification of drought tolerant wheat genotypes is critical for increasing grain yield and also improving the capability of crops to uptake and assimilate nutrients, which are seriously affected by drought. This work aimed to determine the effect of arbuscular mycorrhizal fungi (AMF) on plant growth under normal and limited water availability in two durum wheat genotypes (Svevo and Etrusco). Furthermore, we investigated how the plant nutritional status responds to drought stress. We found that the response of Svevo and Etrusco to drought stress was differentially affected by AMF. Interestingly, we revealed that AMF positively affected sulfur homeostasis under drought conditions, mainly in the Svevo cultivar. The results provide a valuable indication that the identification of drought tolerant plants cannot ignore their nutrient use efficiency or the impact of other biotic soil components (i.e., AMF).

The establishment of arbuscular mycorrhiza (AM) between plants and Glomeromycotina fungi is preceded by the exchange of chemical signals: fungal released Myc-factors, including chitoligosaccharides (CO) and lipo-chitooligosaccharides (LCO), activate plant symbiotic responses, while root exuded strigolactones stimulate hyphal branching and boost CO release. In this frame, the reinforcement of fungal signaling through CO application was shown to promote AM development in Medicago truncatula, but the cellular and molecular bases of this effect remained unclear. Here we demonstrate that CO treatment impacted the transcriptome of both mycorrhizal and non-mycorrhizal M. truncatula roots over several weeks, providing a novel insight into the mechanistic bases of the CO-dependent promotion of AM colonization. CO treatment caused the long-lasting regulation of strigolactone biosynthesis and fungal accommodation related genes. This was further validated by mass spectrometry analyses showing an increase in root didehydro-orobanchol content, and live cell imaging demonstrating the promotion of accommodation responses to AM fungi in root epidermal cells. Lastly, an advanced down-regulation of AM symbiosis marker genes was observed at our latest time point in CO-treated plants, in line with an increased number of senescent arbuscules. Overall, CO treatment triggered molecular, metabolic and cellular responses underpinning a protracted acceleration of AM development.