Quantification of Blumenol Derivatives as Leaf Biomarkers for Plant-AMF Association

Independent regulation of strigolactones and blumenols during arbuscular mycorrhizal symbiosis in rice

The apocarotenoid strigolactones (SLs) facilitate pre-symbiotic communication between arbuscular mycorrhizal (AM) fungi and plants. Related blumenol-C-glucosides (blumenols), have also been associated with symbiosis, but the cues that are involved in the regulation of blumenol accumulation during AM symbiosis remain unclear. In rice, our analyses demonstrated a strict correlation between foliar blumenol abundance and intraradical fungal colonisation. More specifically, rice mutants affected at distinct stages of the interaction revealed that fungal cortex invasion was required for foliar blumenol accumulation. Plant phosphate status and D14L hormone signalling had no effect, contrasting their known role in induction of SLs. This a proportion of the SL biosynthetic enzymes, D27 and D17, are equally required for blumenol production. These results importantly clarify that, while there is a partially shared biosynthetic pathway between SL and blumenols, the dedicated induction of the related apocarotenoids occurs in response to cues acting at distinct stages during the root colonisation process. However, we reveal that neither SLs nor blumenols are essential for fungal invasion of rice roots.

Arbuscular mycorrhizal fungi influence the intraspecific competitive ability of plants under field and glasshouse conditions

MAIN CONCLUSION

Nicotiana attenuata's capacity to interact with arbuscular mycorrhizal fungi influences its intraspecific competitive ability under field and glasshouse conditions, but not its overall community productivity. Arbuscular mycorrhizal (AM) fungi can alter the nutrient status and growth of plants, and they can also affect plant-plant, plant-herbivore, and plant-pathogen interactions. These AM effects are rarely studied in populations under natural conditions due to the limitation of non-mycorrhizal controls. Here we used a genetic approach, establishing field and glasshouse communities of AM-harboring Nicotiana attenuata empty vector (EV) plants and isogenic plants silenced in calcium- and calmodulin-dependent protein kinase expression (irCCaMK), and unable to establish AM symbioses. Performance and growth were quantified in communities of the same (monocultures) or different genotypes (mixed cultures) and both field and glasshouse experiments returned similar responses. In mixed cultures, AM-harboring EV plants attained greater stalk lengths, shoot and root biomasses, clearly out-competing the AM fungal-deficient irCCaMK plants, while in monocultures, both genotypes grew similarly. Competitive ability was also reflected in reproductive traits: EV plants in mixed cultures outperformed irCCaMK plants. When grown in monocultures, the two genotypes did not differ in reproductive performance, though total leaf N and P contents were significantly lower independent of the community type. Plant productivity in terms of growth and seed production at the community level did not differ, while leaf nutrient content of phosphorus and nitrogen depended on the community type. We infer that AM symbioses drastically increase N. attenuata's competitive ability in mixed communities resulting in increased fitness for the individuals harboring AM without a net gain for the community.

microRNA390 modulates Nicotiana attenuata's tolerance response to Manduca sexta herbivory

miR390 is a highly conserved miRNA in plant lineages known to function in growth and development processes, such as lateral root development, and in responses to salt and metal stress. In the ecological model species, Nicotiana attenuata, miR390's biological function remains unknown, which we explore here with a gain-of-function analysis with plants over-expressing (OE-) N. attenuata miR390 (Na-miR390) in glasshouse and natural environments. OEmiR390 plants showed normal developmental processes, including lateral root formation or reproductive output, in plants grown under standard conditions in the glasshouse. OEmiR390 plants did not have dramatically altered interactions with arbuscular mycorrhizal fungi (AMF), Fusarium pathogens, or herbivores. However, Na-miR390 regulated the plant's tolerance of herbivory. Caterpillar feeding elicits the accumulation of a suite of phytohormones, including auxin and jasmonates, which further regulate host-tolerance. The increase in Na-miR390 abundance reduces the accumulation of auxin but does not influence levels of other phytohormones including jasmonates (JA, JA-Ile), salicylic acid (SA), and abscisic acid (ABA). Na-miR390 overexpression reduces reproductive output, quantified as capsule production, when plants are attacked by herbivores. Exogenous auxin treatments of herbivore-attacked plants restored capsule production to wild-type levels. During herbivory, Na-miR390 transcript abundances are increased; its overexpression reduces the abundances of auxin biosynthesizing YUCCA and ARF (mainly ARF4) transcripts during herbivory. Furthermore, the accumulation of auxin-regulated phenolamide secondary metabolites (caffeoylputrescine, dicaffeoylspermidine) is also reduced. In N. attenuata, miR390 functions in modulating tolerance responses of herbivore-attacked plants.

Priming and filtering of antiherbivore defences among Nicotiana attenuata plants connected by mycorrhizal networks

Arbuscular mycorrhizal fungi (AMF) establish symbiotic associations with a majority of terrestrial plants to form underground common mycorrhizal networks (CMNs) that connect neighbouring plants. Because Nicotiana attenuata plants do not respond to herbivory-elicited volatiles from neighbours, we used this ecological model system to evaluate if CMNs function in interplant transmission of herbivory-elicited responses. A mesocosm system was designed to establish and remove CMNs linking N. attenuata plants to examine the herbivory-elicited metabolic and hormone responses in CMNs-connected "receiver" plants after the elicitation of "donor" plants by wounding (W) treated with Manduca sexta larval oral secretions (OS). AMF colonization increased constitutive jasmonate (JA and JA-Ile) levels in N. attenuata roots but did not affect well-characterized JAs-regulated defensive metabolites in systemic leaves. Interestingly, larger JAs bursts, and higher levels of several amino acids and particular sectors of hydroxygeranyllinalool diterpene glycoside metabolism were elevated in the leaves of W + OS-elicited "receivers" with CMN connections with "donors" that had been W + OS-elicited 6 hr previously. Our results demonstrate that AMF colonization alone does not enhance systemic defence responses but that sectors of systemic responses in leaves can be primed by CMNs, suggesting that CMNs can transmit and even filter defence signalling among connected plants.