A hidden mystery: root adaptive responses to environmental factors in Nicotiana attenuata

Abstract

As one of well-established plant model systems, Nicotiana attenuata has been intensively researched in plant resistance/tolerance, plant physiology and interactions with other environmental components in the last two decades. Although root systems, hidden belowground, are much harder to study than other organs, considering their importance in dynamic regulation of plant physiology, morphology, biochemistry, flowering and synthesis of metabolites, and their rich environmental interactions, the exploration of root responses to abiotic and biotic factors in N. attenuata will help to better understand mechanisms of how plants cope with multifaceted environmental factors. Smoke cues are long-live chemical signals in native habitats which play important roles in many aspects for N.attenuata plants, for example by promoting seed germination. Roots are likely exposed to smoke cues in the soil, but how roots respond to such chemical cues was poorly understand. We observed clear root developmental defects in primary root and root hair elongation with liquid smoke incubation. Bioassay-guided fractionation allowed us to obtain the active compound (catechol) responsible for these root defects. Amongst many root growth regulators, comparative transcriptomics narrowed them down to auxin and reactive oxygen species (ROS) signaling pathwaysAuxin was excluded as the main regulator to mediate smokeinduced root morphilogical alteration. Extenal catechol application caused a spatial disruption of H2O2 accumulation, although no bulk quantitative differences. Such misslocation of H2O2 is likely responsible for root defects caused by catechol, which can be partially recovered by H2O2 supplementation that suggested catechol is the main regulator in smoke leads to the smokemediated root defects. Further investigation demonstared the exsistence of catechol in burned areas in the native habitat of Utah, indicating possible ecological functions of smoke cues tuning root growth in nature. As one the 80% of higher plants associated with arbuscular mycorrhizal (AMF) for a mutualistic relationship, N.attenuata has been observed to harbor the characteristic symbiotic fungus structure in roots along with sequence identity confirmation; R. irregularis and F.mosesa are domestic fungus species colonizing N.attenuata roots in a native habitat. We were aiming to seek representative systemic responses in leaves by transcripts or metabolites after AMF colonization. Comparative transcriptomics and metabolomics were performed to compare changes in EV and irCCaMK plants after AMF colonization, and a class of blumenols were targeted for their specificity and high inducibility. By optimizing quantification methods for an improvement of sensitivity, 11-hydroxyblumenol C-9-O-Glc and 11-carboxyblumenol C-9-O-Gl were traced in aerial tissues as foliar markers to mirror root colonization ratio belowground. By application of “real-time” genetic modification using a chemically inducible promoter, we locally blocked biosynthesis of these two compounds in leaves, but their levels in modified leaves were not reduced. Thus, we inferred a root-to-shoot translocation mechanism. Using these more easily applied foliar markers of AMF association, a large screening was performed for a QTL analysis, and we successfully targeted some reported genes regulating symbioses such as NOPE1. We further confirmed that such foliar markers, found in many di- and monocotyledonous crop and model plants (Solanum lycopersicum, Solanum tuberosum, Hordeum vulgare, Triticum aestivum, Medicago truncatula and Brachypodium distachyon), are not restricted to particular mycorrhizal species.