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Phenylpropanoids, particularly flavonoids have been recently suggested as playing primary antioxidant functions in the responses of plants to a wide range of abiotic stresses. Furthermore, flavonoids are effective endogenous regulators of auxin movement, thus behaving as developmental regulators. Flavonoids are capable of controlling the development of individual organs and the whole-plant; and, hence, to contribute to stress-induced morphogenic responses of plants. The significance of flavonoids as scavengers of reactive oxygen species (ROS) in humans has been recently questioned, based on the observation that the flavonoid concentration in plasma and most tissues is too low to effectively reduce ROS. Instead, flavonoids may play key roles as signaling molecules in mammals, through their ability to interact with a wide range of protein kinases, including mitogen-activated protein kinases (MAPK), that supersede key steps of cell growth and differentiation. Here we discuss about the relative significance of flavonoids as reducing agents and signaling molecules in plants and humans. We show that structural features conferring ROS-scavenger ability to flavonoids are also required to effectively control developmental processes in eukaryotic cells.

A growing population coupled with a higher demand for food is putting pressure on agriculture. The use of synthetic pesticides and chemical fertilizers allowed us to boost agricultural productions, but at a great environmental cost. Exploitation of beneficial microorganism (BM)-plant interactions has been proposed as an eco-friendly solution to improve plant resistance to stresses and to increase productivity sustainably. We provide an overview of scientific evidence that this positive interaction is often mediated also by the release of microbial Volatile Organic Compounds (mVOCs). A few mVOCs are reported to have a double, not mutually exclusive, positive effect on plants, as plant growth promoters, and/or inducers of resistance against biotic and abiotic stress factors. They may also alter plant VOCs indirectly improving plant performances. However, mechanisms and functions of mVOCs need deeper investigation. By understanding mVOC modes of action on plants, further tools for sustainably improving plant productivity in agro-ecosystems may become soon available.

Isoprene (C5H8) is a small lipophilic, volatile organic compound (VOC), synthesized in chloroplasts of plants through the photosynthesis-dependent 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. Isoprene-emitting plants are better protected against thermal and oxidative stresses but only about 20% of the terrestrial plants are able to synthesize isoprene. Many studies have been performed to understand the still elusive isoprene protective mechanism. Isoprene reacts with, and quenches, many harmful reactive oxygen species (ROS) like singlet oxygen (1O2). A role for isoprene as antioxidant, made possible by its reduced state and conjugated double bonds, has been often suggested, and sometimes demonstrated. However, as isoprene is present at very low concentrations compared to other molecules, its antioxidant role is still controversial. Here we review updated evidences on the function(s) of isoprene, and outline contrasting indications on whether isoprene is an antioxidant directly scavenging ROS, or a membrane strengthener, or a modulator of genomic, proteomic and metabolomic profiles (perhaps as a secondary effect of ROS removal) eventually leading to priming of antioxidant plant defenses, or a signal of stress for neighbor plants alike other VOCs, or a hormone-like molecule, controlling the metabolic flux of other hormones made by the MEP pathway, or acting itself as a growth and development hormone.

Isoprene is a small lipophilic molecule synthesized in plastids and abundantly released into the atmosphere. Isoprene-emitting plants are better protected against abiotic stresses, but the mechanism of action of isoprene is still under debate. In this study, we compared the physiological responses and proteomic profiles of Arabidopsis which express the isoprene synthase (ISPS) gene and emit isoprene with those of non-emitting plants under both drought-stress (DS) and well-watered (WW) conditions. We aimed to investigate whether isoprene-emitting plants displayed a different proteomic profile that is consistent with the metabolic changes already reported. Only ISPS DS plants were able to maintain the same photosynthesis and fresh weight of WW plants. LC–MS/MS-based proteomic analysis revealed changes in protein abundance that were dependent on the capacity for emitting isoprene in addition to those caused by the DS. The majority of the proteins changed in response to the interaction between DS and isoprene emission. These include proteins that are associated with the activation of secondary metabolisms leading to ABA, trehalose, and proline accumulations. Overall, our proteomic data suggest that isoprene exerts its protective mechanism at different levels: under drought stress, isoprene affects the abundance of chloroplast proteins, confirming a strong direct or indirect antioxidant action and also modulates signaling and hormone pathways, especially those controlling ABA synthesis. Unexpectedly, isoprene also alters membrane trafficking.

Wild roses store and emit a large array of fragrant monoterpenes from their petals. Maximisation of fragrance coincides with floral maturation in many angiosperms, which enhances pollination efficiency, reduces floral predation, and improves plant fitness. We hypothesized that petal monoterpenes serve additional lifelong functions such as limiting metabolic damage from reactive oxygen species (ROS), and altering isoprenoid hormonal abundance to increase floral lifespan. Petal monoterpenes were quantified at three floral life-stages (unopened bud, open mature, and senescent) in 57 rose species and 16 subspecies originating from Asia, America, and Europe, and relationships among monoterpene richness, petal colour, ROS, hormones, and floral lifespan were analysed within a phylogenetic context. Three distinct types of petal monoterpene profiles, revealing significant developmental and functional differences, were identified: Type A, species where monoterpene abundance peaked in open mature flowers depleting thereafter; Type B, where monoterpenes peaked in senescing flowers increasing from bud stage, and a rare Type C (8 species) where monoterpenes depleted from bud stage to senescence. Cyclic monoterpenes peaked during early floral development, whereas acyclic monoterpenes (dominated by geraniol and its derivatives, often 100-fold more abundant than other monoterpenes) peaked during floral maturation in Type A and B roses. Early-diverging roses were geraniol-poor (often Type C) and white-petalled. Lifetime changes in hydrogen peroxide (H₂O₂) revealed a significant negative regression with the levels of petal geraniol at all floral life-stages. Geraniol-poor Type C roses also showed higher cytokinins (in buds) and abscisic acid (in mature petals), and significantly shorter floral lifespan compared with geraniolrich Type A and B roses. We conclude that geraniol enrichment, intensification of petal colour, and lower potential for H₂O₂-related oxidative damage characterise and likely contribute to longer floral lifespan in monoterpene-rich wild roses.

• Isoprene, a major biogenic volatile hydrocarbon of climate-relevance, indisputably mitigates abiotic stresses in emitting plants. However functional relevance of constitutive isoprene emission in unstressed plants remains contested. Isoprene and cytokinins (CKs) are synthesized from a common substrate and pathway in chloroplasts. It was postulated that isoprene emission may affect CK-metabolism. • Using transgenic isoprene-emitting (IE) Arabidopsis and isoprene nonemitting (NE) RNA-interference grey poplars (paired with respective NE and IE genotypes), the life of individual IE and NE leaves from emergence to abscission was followed under stress-free conditions. We monitored plant growth rate, aboveground developmental phenotype, modelled leaf photosynthetic energy status, quantified the abundance of leaf CKs, analysed Arabidopsis and poplar leaf transcriptomes by RNA-sequencing in presence and absence of isoprene during leaf senescence. • Isoprene emission by unstressed leaves enhanced the abundance of CKs (isopentenyl adenine and its precursor) by > 200%, significantly upregulated genes coding for CK-synthesis, CK-signalling and CK-degradation, hastened plant development, increased chloroplast metabolic rate, altered photosynthetic energy status, induced early leaf senescence in both Arabidopsis and poplar. IE leaves senesced sooner even in decapitated poplars where source–sink relationships and hormone homeostasis were perturbed. • Constitutive isoprene emission significantly accelerates CK-led leaf and organismal development and induces early senescence independent of growth constraints. Isoprene emission provides an early-riser evolutionary advantage and shortens lifecycle duration to assist rapid diversification in unstressed emitters.

Plants are central to complex networks of multitrophic interactions. Increasing evidence suggests that beneficial microorganisms (BMs) may be used as plant biostimulants and pest biocontrol agents. We investigated whether tomato ( Solanum lycopersicum ) plants are thoroughly colonized by the endophytic and entomopathogenic fungus Beauveria bassiana , and how such colonization affects physiological parameters and the phenotype of plants grown under unstressed conditions or exposed to the pathogenic fungus Botrytis cinerea . As a positive control, a strain of the well-known biocontrol agent and growth inducer Trichoderma afroharzianum was used. As multitrophic interactions are often driven by (or have consequences on) volatile organic compounds (VOCs) released by plants constitutively or after induction by abiotic or biotic stresses, VOC emissions were also studied. Both B. bassiana and T. afroharzianum induced a significant but transient (one to two-day-long) reduction of stomatal conductance, which may indicate rapid activation of defensive (rejection) responses, but also limited photosynthesis. At later stages, our results demonstrated a successful and complete plant colonization by B. bassiana , which induced higher photosynthesis and lower respiration rates, improved growth of roots, stems, leaves, earlier flowering, higher number of fruits and yield in tomato plants. Beauveria bassiana also helped tomato plants fight B. cinerea , whose symptoms in leaves were almost entirely relieved with respect to control plants. Less VOCs were emitted when plants were colonized by B. bassiana or infected by B. cinerea , alone or in combination, suggesting no activation of VOC-dependent defensive mechanisms in response to both fungi.

Synthesized small molecules are useful as tools to investigate hormonal signaling involved in plant growth and development. They are also important as agrochemicals to promote beneficial properties of crops in the field. We describe here the synthesis and mode of action of a novel growth‐promoting chemical, A1. A1 stimulates enhanced growth in both shoot and root tissues of plants, acting by increasing both dry and fresh weight. This suggests that A1 not only promotes uptake of water but also increases production of cellular material. A1 treatment of Arabidopsisleads to the degradation of DELLA growth‐inhibitory proteins suggesting that A1‐mediated growth promotion is dependent upon this mechanism. We performed genetic analysis to confirm this and further dissect the mechanism of A1 action upon growth in Arabidopsis. A quintuple dellamutant was insensitive to A1, confirming that the mode of action was indeed via a DELLA‐dependent mechanism. The ga1‐5gibberellin synthesis mutant was similarly insensitive, suggesting that to promote growth in ArabidopsisA1 requires the presence of endogenous gibberellins. This was further suggested by the observation that double mutants of GID1 gibberellin receptor genes were insensitive to A1. Taken together, our data suggest that A1 acts to enhance sensitivity to endogenous gibberellins thus leading to observed enhanced growth via DELLA degradation. A1 and related compounds will be useful to identify novel signaling components involved in plant growth and development, and as agrochemicals suitable for a wide range of crop species.

• Background New roles for flavonoids, as developmental regulators and/or signalling molecules, have recently been proposed in eukaryotic cells exposed to a wide range of environmental stimuli. In plants, these functions are actually restricted to flavonols, the ancient and widespread class of flavonoids. In mosses and liverworts, the whole set of genes for flavonol biosynthesis - CHS, CHI, F3H, FLS and F3'H - has been detected. The flavonol branch pathway has remained intact for millions of years, and is almost exclusively involved in the responses of plants to a wide array of stressful agents, despite the fact that evolution of flavonoid metabolism has produced > 10 000 structures. • Scope Here the emerging functional roles of flavonoids in the responses of present-day plants to different stresses are discussed based on early, authoritative views of their primary functions during the colonization of land by plants. Flavonols are not as efficient as other secondary metabolites in absorbing wavelengths in the 290-320 nm spectral region, but display the greatest potential to keep stress-induced changes in cellular reactive oxygen species homeostasis under control, and to regulate the development of individual organs and the whole plant. Very low flavonol concentrations, as probably occurred in early terrestrial plants, may fully accomplish these regulatory functions. • Conclusions During the last two decades the routine use of genomic, chromatography/mass spectrometry and fluorescence microimaging techniques has provided new insights into the regulation of flavonol metabolism as well as on the inter- and intracellular distribution of stress-responsive flavonols. These findings offer new evidence on how flavonols may have performed a wide array of functional roles during the colonization of land by plants. In our opinion this ancient flavonoid class is still playing the same old and robust roles in presentday plants.

Isoprene-emitting plants are better protected against thermal and oxidative stresses, which is a desirable trait in a climate-changing (drier and warmer) world. Here we compared the ecophysiological performances of transgenic isoprene-emitting and wild-type non-emitting tobacco plants during water stress and after re-watering in actual environmental conditions (400 ppm of CO2 and 28 °C of average daily temperature) and in a future climate scenario (600 ppm of CO2 and 32 °C of average daily temperature). Furthermore, we intended to complement the present knowledge on the mechanisms involved in isoprene-induced resistance to water deficit stress by examining the proteome of transgenic isoprene-emitting and wild-type non-emitting tobacco plants during water stress and after re-watering in actual climate. Isoprene emitters maintained higher photosynthesis and electron transport rates under moderate stress in future climate conditions. However, physiological resistance to water stress in the isoprene-emitting plants was not as marked as expected in actual climate conditions, perhaps because the stress developed rapidly. In actual climate, isoprene emission capacity affected the tobacco proteomic profile, in particular by upregulating proteins associated with stress protection. Our results strengthen the hypothesis that isoprene biosynthesis is related to metabolic changes at the gene and protein levels involved in the activation of general stress defensive mechanisms of plants.