Beyond the Canon: Within-Plant and Population-Level Heterogeneity in Jasmonate Signaling Engaged by Plant-Insect Interactions

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.

Argonaute4 Modulates Resistance to Fusarium brachygibbosum Infection by Regulating Jasmonic Acid Signaling

Argonautes (AGOs) associate with noncoding RNAs to regulate gene expression during development and stress adaptation. Their role in plant immunity against hemibiotrophic fungal infection remains poorly understood. Here, we explore the function of AGOs in the interaction of wild tobacco (Nicotiana attenuata) with a naturally occurring hemibiotrophic pathogen, Fusarium brachygibbosum Among all AGOs, only transcripts of AGO4 were elicited after fungal infection. The disease progressed more rapidly in AGO4-silenced (irAGO4) plants than in wild type, and small RNA (smRNA) profiling revealed that 24-nucleotide smRNA accumulation was severely abrogated in irAGO4 plants. Unique microRNAs (miRNAs: 130 conserved and 208 novel, including 11 canonical miRNA sequence variants known as "isomiRs") were identified in infected plants; silencing of AGO4 strongly changed miRNA accumulation dynamics. Time-course studies revealed that infection increased accumulation of abscisic acid, jasmonates, and salicylic acid in wild type; in irAGO4 plants, infection accumulated lower jasmonate levels and lower transcripts of jasmonic acid (JA) biosynthesis genes. Treating irAGO4 plants with JA, methyl jasmonate, or cis-(+)-12-oxo-phytodienoic acid restored wild-type levels of resistance. Silencing expression of RNA-directed RNA polymerases RdR1 and RdR2 (but not RdR3) and Dicer-like3 (DCL3, but not DCL2 or DCL4) increased susceptibility to F brachygibbosum The relevance of AGO4, RdR1, RdR2, and DCL3 in a natural setting was revealed when plants individually silenced in their expression (and their binary combinations) were planted in a diseased field plot in the Great Basin Desert of Utah. These plants were more susceptible to infection and accumulated lower JA levels than wild type. We infer that AGO4-dependent smRNAs play a central role in modulating JA biogenesis and signaling during hemibiotrophic fungal infections.

Using natural variation to achieve a whole-plant functional understanding of the responses mediated by jasmonate signaling

The dramatic advances in our understanding of the molecular biology and biochemistry of jasmonate (JA) signaling have been the subject of several excellent recent reviews that have highlighted the phytohormonal function of this signaling pathway. Here, we focus on the responses mediated by JA signaling which have consequences for a plant's Darwinian fitness, i.e. the organism-level function of JA signaling. The most diverse module in the signaling cascade, the JAZ proteins, and their interactions with other proteins and transcription factors, allow this canonical signaling cascade to mediate a bewildering array of traits in different tissues at different times; the functional coherence of these diverse responses are best appreciated in an organismal/ecological context. From published work, it appears that jasmonates can function as the 'Swiss Army knife' of plant signaling, mediating many different biotic and abiotic stress and developmental responses that allow plants to contextualize their responses to their frequently changing local environments and optimize their fitness. We propose that a deeper analysis of the natural variation in both within-plant and within-population JA signaling components is a profitable means of attaining a coherent whole-plant functional perspective of this signaling cascade, and provide examples of this approach from the Nicotiana attenuata system.

Endogenous phytohormones of frankincense producing Boswellia sacra tree populations

Boswellia sacra, an endemic tree to Oman, is exposed to man-made incisions for commercial level frankincense production, whereas unsustainable harvesting may lead to population decline. In this case, assessment of endogenous phytohormones (gibberellic acid (GA), indole-acetic acid (IAA), salicylic acid (SA) and kinetin) can help to understand population health and growth dynamics. Hence, it was aimed to devise a robust method using Near-Infrared spectroscopy (NIRS) coupled with multivariate methods for phytohormone analysis of thirteen different populations of B. sacra. NIRS data was recorded in absorption mode (10000-4000 cm-1) to build partial least squares regression model (calibration set 70%). Model was externally cross validated (30%) as a test set to check their prediction ability before the application to quantify the unknown amount of phytohormones in thirteen different populations of B. sacra. The results showed that phytohormonal contents varied significantly, showing a trend of SA>GA/IAA>kinetin across different populations. SA and GA contents were significantly higher in Pop13 (Hasik), followed by Pop2 (Dowkah)-an extreme end of B. sacra tree cover in Dhofar region. A similar trend in the concentration of phytohormones was found when the samples from 13 populations were subjected to advance liquid chromatography mass spectrophotometer and gas chromatograph with selected ion monitor analysis. The current analysis provides alternative tool to assess plant health, which could be important to in situ propagation of tree population as well as monitoring tree population growth dynamics.

Field studies reveal functions of chemical mediators in plant interactions

Plants are at the trophic base of most ecosystems, embedded in a rich network of ecological interactions in which they evolved. While their limited range and speed of motion precludes animal-typical behavior, plants are accomplished chemists, producing thousands of specialized metabolites which may function to convey information, or even to manipulate the physiology of other organisms. Plants' complex interactions and their underlying mechanisms are typically dissected within the controlled environments of growth chambers and glasshouses, but doing so introduces conditions alien to plants evolved in natural environments, such as being pot-bound, and receiving artificial light with a spectrum very different from sunlight. The mechanistic understanding gained from a reductionist approach provides the tools required to query and manipulate plant interactions in real-world settings. The few tests conducted in natural ecosystems and agricultural fields have highlighted the limitations of studying plant interactions only in artificial environments. Here, we focus on three examples of known or hypothesized chemical mediators of plants' interactions: the volatile phytohormone ethylene (ET), more complex plant volatile blends, and as-yet-unknown mediators transferred by common mycorrhizal networks (CMNs). We highlight how mechanistic knowledge has advanced research in all three areas, and the critical importance of field work if we are to put our understanding of chemical ecology on rigorous experimental and theoretical footing, and demonstrate function.

Aboveground herbivory induced jasmonates disproportionately reduce plant reproductive potential by facilitating root nematode infestation

Different plant feeders, including insects and parasitic nematodes, can influence each other by triggering systemic changes in their shared host plants. In most cases, however, the underlying mechanisms are unclear, and the consequences for plant fitness are not well understood. We studied the interaction between leaf feeding Manduca sexta caterpillars and root parasitic nematodes in Nicotiana attenuata. Simulated M. sexta attack increased the abundance of root parasitic nematodes in the field and facilitated Meloidogyne incognita reproduction in the glasshouse. Intact jasmonate biosynthesis was found to be required for both effects. Flower counts revealed that the jasmonate-dependent facilitation of nematode infestation following simulated leaf attack reduces the plant's reproductive potential to a greater degree than would be expected from the additive effects of the individual stresses. This work reveals that jasmonates mediate the interaction between a leaf herbivore and root parasitic nematodes and illustrates how plant-mediated interactions can alter plant's reproductive potential. The selection pressure resulting from the demonstrated fitness effects is likely to influence the evolution of plant defense traits in nature.

The Active Jasmonate JA-Ile Regulates a Specific Subset of Plant Jasmonate-Mediated Resistance to Herbivores in Nature

The jasmonate hormones are essential regulators of plant defense against herbivores and include several dozen derivatives of the oxylipin jasmonic acid (JA). Among these, the conjugate jasmonoyl isoleucine (JA-Ile) has been shown to interact directly with the jasmonate co-receptor complex to regulate responses to jasmonate signaling. However, functional studies indicate that some aspects of jasmonate-mediated defense are not regulated by JA-Ile. Thus, it is not clear whether JA-Ile is best characterized as the master jasmonate regulator of defense, or if it regulates more specific aspects. We investigated possible functions of JA-Ile in anti-herbivore resistance of the wild tobacco Nicotiana attenuata, a model system for plant-herbivore interactions. We first analyzed the soluble and volatile secondary metabolomes of irJAR4xirJAR6, asLOX3, and WT plants, as well as an RNAi line targeting the jasmonate co-receptor CORONATINE INSENSITIVE 1 (irCOI1), following a standardized herbivory treatment. irJAR4xirJAR6 were the most similar to WT plants, having a ca. 60% overlap in differentially regulated metabolites with either asLOX3 or irCOI1. In contrast, while at least 25 volatiles differed between irCOI1 or asLOX3 and WT plants, there were few or no differences in herbivore-induced volatile emission between irJAR4xirJAR6 and WT plants, in glasshouse- or field-collected samples. We then measured the susceptibility of jasmonate-deficient vs. JA-Ile-deficient plants in nature, in comparison to wild-type (WT) controls, and found that JA-Ile-deficient plants (irJAR4xirJAR6) are much better defended even than a mildly jasmonate-deficient line (asLOX3). The differences among lines could be attributed to differences in damage from specific herbivores, which appeared to prefer either one or the other jasmonate-deficient phenotype. We further investigated the elicitation of one herbivore-induced volatile known to be jasmonate-regulated and to mediate resistance to herbivores: (E)-α-bergamotene. We found that JA was a more potent elicitor of (E)-α-bergamotene emission than was JA-Ile, and when treated with JA, irJAR4xirJAR6 plants emitted 20- to 40-fold as much (E)-α-bergamotene than WT. We conclude that JA-Ile regulates specific aspects of herbivore resistance in N. attenuata. This specificity may allow plants flexibility in their responses to herbivores and in managing trade-offs between resistance, vs. growth and reproduction, over the course of ontogeny.

Specificity of root microbiomes in native-grown Nicotiana attenuata and plant responses to UVB increase Deinococcus colonization

Plants recruit microbial communities from the soil in which they germinate. Our understanding of the recruitment process and the factors affecting it is still limited for most microbial taxa. We analysed several factors potentially affecting root microbiome structure - the importance of geographic location of natural populations, the microbiome of native seeds as putative source of colonization and the effect of a plant's response to UVB exposure on root colonization of highly abundant species. The microbiome of Nicotiana attenuata seeds was determined by a culture-dependent and culture-independent approach, and the root microbiome of natural N. attenuata populations from five different locations was analysed using 454-pyrosequencing. To specifically address the influence of UVB light on root colonization by Deinococcus, a genus abundant and consistently present in N. attenuata roots, transgenic lines impaired in UVB perception (irUVR8) and response (irCHAL) were investigated in a microcosm experiment with/without UVB supplementation using a synthetic bacterial community. The seed microbiome analysis indicated that N. attenuata seeds are sterile. Alpha and beta diversities of native root bacterial communities differed significantly between soil and root, while location had only a significant effect on the fungal but not the bacterial root communities. With UVB supplementation, root colonization of Deinococcus increased in wild type, but decreased in irUVR8 and irCHAL plants compared to nontreated plants. Our results suggest that N. attenuata recruits a core root microbiome exclusively from soil, with fungal root colonization being less selective than bacterial colonization. Root colonization by Deinococcus depends on the plant's response to UVB.