The phytohormone jasmonoyl-isoleucine (JA-Ile) regulates defense, growth and developmental responses in vascular plants. Bryophytes have conserved sequences for all JA-Ile signaling pathway components but lack JA-Ile. We show that, in spite of 450 million years of independent evolution, the JA-Ile receptor COI1 is functionally conserved between the bryophyte Marchantia polymorpha and the eudicot Arabidopsis thaliana but COI1 responds to different ligands in each species. We identified the ligand of Marchantia MpCOI1 as two isomeric forms of the JA-Ile precursor dinor-OPDA (dinor-cis-OPDA and dinor-iso-OPDA). We demonstrate that AtCOI1 functionally complements Mpcoi1 mutation and confers JA-Ile responsiveness and that a single-residue substitution in MpCOI1 is responsible for the evolutionary switch in ligand specificity. Our results identify the ancestral bioactive jasmonate and clarify its biosynthetic pathway, demonstrate the functional conservation of its signaling pathway, and show that JA-Ile and COI1 emergence in vascular plants required co-evolution of hormone biosynthetic complexity and receptor specificity.

Biosynthesis of the phytohormone jasmonoyl-isoleucine (JA-Ile) requires reduction of the JA precursor 12-oxo-phytodienoic acid (OPDA) by OPDA reductase 3 (OPR3). Previous analyses of the opr3-1 Arabidopsis mutant suggested an OPDA signaling role independent of JA-Ile and its receptor COI1; however, this hypothesis has been challenged because opr3-1 is a conditional allele not completely impaired in JA-Ile biosynthesis. To clarify the role of OPR3 and OPDA in JA-independent defenses, we isolated and characterized a loss-of-function opr3-3 allele. Strikingly, opr3-3 plants remained resistant to necrotrophic pathogens and insect feeding, and activated COI1-dependent JA-mediated gene expression. Analysis of OPDA derivatives identified 4,5-didehydro-JA in wounded wild-type and opr3-3 plants. OPR2 was found to reduce 4,5-didehydro-JA to JA, explaining the accumulation of JA-Ile and activation of JA-Ile-responses in opr3-3 mutants. Our results demonstrate that in the absence of OPR3, OPDA enters the β-oxidation pathway to produce 4,5-ddh-JA as a direct precursor of JA and JA-Ile, thus identifying an OPR3-independent pathway for JA biosynthesis.

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intimate mechanisms involved, as well as its effect on the production of signalling molecules 4 associated to the host plant-AM fungus interaction remains largely unknown. In the present 5 work, the effects of drought on lettuce and tomato plant performance and hormone levels 6 were investigated in non-AM and AM plants. Three different water regimes were applied and 7 their effects analysed over time. AM plants showed an improved growth rate and efficiency of 8 photosystem II than non-AM plants under drought from very early stages of plant 9 colonization. The levels of the phytohormone abscisic acid, as well as the expression of the 10 corresponding marker genes, were influenced by drought stress in non-AM and AM plants. 11The levels of strigolactones and the expression of corresponding marker genes were affected 12 by both AM symbiosis and drought. The results suggest that AM symbiosis alleviates drought 13 stress by altering the hormonal profiles and affecting plant physiology in the host plant. In 14 addition, a correlation between AM root colonization, strigolactone levels and drought 15 severity is shown, suggesting that under these unfavourable conditions plants might increase 16 strigolactone production in order to promote symbiosis establishment to cope with the stress. 17 18 19