An ancestral function of strigolactones as symbiotic rhizosphere signals

Ancestral sequence reconstruction of the CYP711 family reveals functional divergence in strigolactone biosynthetic enzymes associated with gene duplication events in monocot grasses

The strigolactone (SL) class of phytohormones shows broad chemical diversity, the functional importance of which remains to be fully elucidated, along with the enzymes responsible for the diversification of the SL structure. Here we explore the functional evolution of the highly conserved CYP711A P450 family, members of which catalyze several key monooxygenation reactions in the strigolactone pathway. Ancestral sequence reconstruction was utilized to infer ancestral CYP711A sequences based on a comprehensive set of extant CYP711 sequences. Eleven ancestral enzymes, corresponding to key points in the CYP711A phylogenetic tree, were resurrected and their activity was characterized towards the native substrate carlactone and the pure enantiomers of the synthetic strigolactone analogue, GR24. The ancestral and extant CYP711As tested accepted GR24 as a substrate and catalyzed several diversifying oxidation reactions on the structure. Evidence was obtained for functional divergence in the CYP711A family. The monocot group 3 ancestor, arising from gene duplication events within monocot grasses, showed both increased catalytic activity towards GR24 and high stereoselectivity towards the GR24 isomer resembling strigol-type SLs. These results are consistent with a role for CYP711As in strigolactone diversification in early land plants, which may have extended to the diversification of strigol-type SLs.

Noncanonical Strigolactone Analogues Highlight Selectivity for Stimulating Germination in Two Phelipanche ramosa Populations

Strigolactones (SLs) are plant hormones exuded in the rhizosphere with a signaling role for the development of arbuscular mycorrhizal (AM) fungi and as stimulants of seed germination of the parasitic weeds Orobanche, Phelipanche, and Striga, the most threatening weeds of major crops worldwide. Phelipanche ramosa is present mainly on rape, hemp, and tobacco in France. P. ramosa 2a preferentially attacks hemp, while P. ramosa 1 attacks rapeseed. The recently isolated cannalactone (14) from hemp root exudates has been characterized as a noncanonical SL that selectively stimulates the germination of P. ramosa 2a seeds in comparison with P. ramosa 1. In the present work, (-)-solanacol (5), a canonical orobanchol-type SL exuded by tobacco and tomato, was established to possess a remarkable selective germination stimulant activity for P. ramosa 2a seeds. Two cannalactone analogues, named (±)-SdL19 and (±)-SdL118, have been synthesized. They have an unsaturated acyclic carbon chain with a tertiary hydroxy group and a methyl or a cyclopropyl group instead of a cyclohexane A-ring, respectively. (±)-SdL analogues are able to selectively stimulate P. ramosa 2a, revealing that these minimal structural elements are key for this selective bioactivity. In addition, (±)-SdL19 is able to inhibit shoot branching in Pisum sativum and Arabidopsis thaliana and induces hyphal branching in the AM fungus Rhizophagus irregularis, like SLs.

Masks Start to Drop: Suppressor of MAX2 1-Like Proteins Reveal Their Many Faces

Although the main players of the strigolactone (SL) signaling pathway have been characterized genetically, how they regulate plant development is still poorly understood. Of central importance are the SUPPRESSOR OF MAX2 1-LIKE (SMXL) proteins that belong to a family of eight members in Arabidopsis thaliana, of which one subclade is involved in SL signaling and another one in the pathway of the chemically related karrikins. Through proteasomal degradation of these SMXLs, triggered by either DWARF14 (D14) or KARRIKIN INSENSITIVE2 (KAI2), several physiological processes are controlled, such as, among others, shoot and root architecture, seed germination, and seedling photomorphogenesis. Yet another clade has been shown to be involved in vascular development, independently of the D14 and KAI2 actions and not relying on proteasomal degradation. Despite their role in several aspects of plant development, the exact molecular mechanisms by which SMXLs regulate them are not completely unraveled. To fill the major knowledge gap in understanding D14 and KAI2 signaling, SMXLs are intensively studied, making it challenging to combine all the insights into a coherent characterization of these important proteins. To this end, this review provides an in-depth exploration of the recent data regarding their physiological function, evolution, structure, and molecular mechanism. In addition, we propose a selection of future perspectives, focusing on the apparent localization of SMXLs in subnuclear speckles, as observed in transient expression assays, which we couple to recent advances in the field of biomolecular condensates and liquid-liquid phase separation.