Plants interact with root microbes via chemical signaling, which modulates competence or symbiosis. Although several volatile organic compounds (VOCs) from fungi may affect plant growth and ...development, the signal transduction pathways mediating VOC sensing are not fully understood. 6‐pentyl‐2H‐pyran‐2‐one (6‐PP) is a major VOC biosynthesized by Trichoderma spp. which is probably involved in plant–fungus cross‐kingdom signaling. Using microscopy and confocal imaging, the effects of 6‐PP on root morphogenesis were found to be correlated with DR5:GFP, DR5:VENUS, H2B::GFP, PIN1::PIN1::GFP, PIN2::PIN2::GFP, PIN3::PIN3::GFP and PIN7::PIN7::GFP gene expression. A genetic screen for primary root growth resistance to 6‐PP in wild‐type seedlings and auxin‐ and ethylene‐related mutants allowed identification of genes controlling root architectural responses to this metabolite. Trichoderma atroviride produced 6‐PP, which promoted plant growth and regulated root architecture, inhibiting primary root growth and inducing lateral root formation. 6‐PP modulated expression of PIN auxin‐transport proteins in a specific and dose‐dependent manner in primary roots. TIR1, AFB2 and AFB3 auxin receptors and ARF7 and ARF19 transcription factors influenced the lateral root response to 6‐PP, whereas EIN2 modulated 6‐PP sensing in primary roots. These results indicate that root responses to 6‐PP involve components of auxin transport and signaling and the ethylene‐response modulator EIN2.
Trichoderma species belong to a class of free-living fungi beneficial to plants that are common in the rhizosphere. We investigated the role of auxin in regulating the growth and development of ...Arabidopsis (Arabidopsis thaliana) seedlings in response to inoculation with Trichoderma virens and Trichoderma atroviride by developing a plant-fungus interaction system. Wild-type Arabidopsis seedlings inoculated with either T. virens or T. atroviride showed characteristic auxin-related phenotypes, including increased biomass production and stimulated lateral root development. Mutations in genes involved in auxin transport or signaling, AUX1, BIG, EIR1, and AXR1, were found to reduce the growth-promoting and root developmental effects of T. virens inoculation. When grown under axenic conditions, T. virens produced the auxin-related compounds indole-3-acetic acid, indole-3-acetaldehyde, and indole-3-ethanol. A comparative analysis of all three indolic compounds provided detailed information about the structure-activity relationship based on their efficacy at modulating root system architecture, activation of auxin-regulated gene expression, and rescue of the root hair-defective phenotype of the rhd6 auxin response Arabidopsis mutant. Our results highlight the important role of auxin signaling for plant growth promotion by T. virens.
Summary
The Mediator (MED) complex plays a key role in the recruitment and assembly of the transcription machinery for the control of gene expression. Here, we report on the role of MEDIATOR18 ...(MED18) subunit in root development, auxin signaling and meristem cell viability in Arabidopsis thaliana seedlings. Loss‐of‐function mutations in MED18 reduce primary root growth, but increase lateral root formation and root hair development. This phenotype correlates with alterations in cell division and elongation likely caused by an increased auxin response and transport at the root tip, as evidenced by DR5:GFP, pPIN1::PIN1‐GFP, pPIN2::PIN2‐GFP and pPIN3::PIN3‐GFP auxin‐related gene expression. Noteworthy, med18 seedlings manifest cell death in the root meristem, which exacerbates with age and/or exposition to DNA‐damaging agents, and display high expression of the cell regeneration factor ERF115. Cell death in the root tip was reduced in med18 seedlings grown in darkness, but remained when only the shoot was exposed to light, suggesting that MED18 acts to protect root meristem cells from local cell death, and/or in response to root‐acting signal(s) emitted by the shoot in response to light stimuli. These data point to MED18 as an important component for auxin‐regulated root development, cell death and cell regeneration in root meristems.
Significance statement
Plant growth and adaptation rely on the activity of meristems. Here, we show that the MEDIATOR subunit 18 (MED18) is critical for viability of root proliferative cells controlling the balance between cell death and differentiation, and demonstrate that light perception in the shoot influences the stability and DNA repair mechanisms in root meristems.
Phosphorus is an essential nutrient that is required for all major developmental processes and reproduction in plants. It is also a major constituent of the fertilizers required to sustain high-yield ...agriculture. Levels of phosphate--the only form of phosphorus that can be assimilated by plants--are suboptimal in most natural and agricultural ecosystems, and when phosphate is applied as fertilizer in soils, it is rapidly immobilized owing to fixation and microbial activity. Thus, cultivated plants use only approximately 20-30% of the applied phosphate, and the rest is lost, eventually causing water eutrophication. Recent advances in the understanding of mechanisms by which wild and cultivated species adapt to low-phosphate stress and the implementation of alternative bacterial pathways for phosphorus metabolism have started to allow the design of more effective breeding and genetic engineering strategies to produce highly phosphate-efficient crops, optimize fertilizer use, and reach agricultural sustainability with a lower environmental cost. In this review, we outline the current advances in research on the complex network of plant responses to low-phosphorus stress and discuss some strategies used to manipulate genes involved in phosphate uptake, remobilization, and metabolism to develop low-phosphate-tolerant crops, which could help in designing more efficient crops.
Research over the last three decades showed that chromium, particularly the oxyanion chromate Cr(VI) behaves as a toxic environmental pollutant that strongly damages plants due to oxidative stress, ...disruption of nutrient uptake, photosynthesis and metabolism, and ultimately, represses growth and development. However, mild Cr(VI) concentrations promote growth, induce adventitious root formation, reinforce the root cap, and produce twin roots from single root meristems under conditions that compromise cell viability, indicating its important role as a driver for root organogenesis. In recent years, considerable advance has been made towards deciphering the molecular mechanisms for root sensing of chromate, including the identification of regulatory proteins such as SOLITARY ROOT and MEDIATOR 18 that orchestrate the multilevel dynamics of the oxyanion. Cr(VI) decreases the expression of several glutamate receptors, whereas amino acids such as glutamate, cysteine and proline confer protection to plants from hexavalent chromium stress. The crosstalk between plant hormones, including auxin, ethylene, and jasmonic acid enables tissues to balance growth and defense under Cr(VI)-induced oxidative damage, which may be useful to better adapt crops to biotic and abiotic challenges. The highly contrasting responses of plants manifested at the transcriptional and translational levels depend on the concentration of chromate in the media, and fit well with the concept of hormesis, an adaptive mechanism that primes plants for resistance to environmental challenges, toxins or pollutants. Here, we review the contrasting facets of Cr(VI) in plants including the cellular, hormonal and molecular aspects that mechanistically separate its toxic effects from biostimulant outputs.
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•Trichoderma biostimulants boost crop nutrition, growth and stress response.•Formulations combining Trichoderma with other beneficial microbes increase yield.•Biostimulants with ...greater stability are critical to successful application.•Trichoderma improves root growth and nutrient uptake via auxin production.•Trichoderma-induced changes in gene expression are integral to phytostimulation.
Plant biostimulants are formulated with diverse microorganisms and/or substances that are applied to crops with the aim of enhancing growth, development and adaptation to abiotic stress. Trichoderma-based products have been particularly successful because of their capacity to control phytopathogenic fungi. Some Trichoderma strains have a predominant biostimulant action that makes them unique for their extended use in horticulture. They are safe for humans, livestock and crop plants and in their natural environment colonize plant roots without apparent adverse reactions. Both solid and liquid formulations containing conidia can be used to produce suitable quantities of active and viable inocula during product formulation and field use. The mechanism of phytostimulation by Trichoderma involves multilevel communication with root and shoot systems, as it releases into the rhizosphere auxins, small peptides, volatiles and other active metabolites, which promote root branching and nutrient uptake capacity, thereby boosting plant growth and yield. Recent proteomic and genetic data suggest that Trichoderma activates the mitogen activated protein kinase 6, transcription factors and DNA processing proteins, which represent promising targets toward formulation of more efficient products.
•Bacteria communicate with plants by means of chemical signaling.•Quorum-sensing enables bacteria to synchronize expression of genes and behavior, which are critical for ...phytostimulation.•N-acyl-L-homoserine lactones and cyclodipeptides are perceived by plants to modulate growth and defense.•N-acyl-L-homoserine lactones may act as alarm signals, pathogen and/or microbe-associated molecular patterns.•Cyclodipeptides may function as hormonal mimics for plants.
Bacteria rely on chemical communication to sense the environment and to retrieve information on their population densities. Accordingly, a vast repertoire of molecules is released, which synchronizes expression of genes, coordinates behavior through a process termed quorum-sensing (QS), and determines the relationships with eukaryotic species. Already identified QS molecules from Gram negative bacteria can be grouped into two main classes, N-acyl-L-homoserine lactones (AHLs) and cyclodipeptides (CDPs), with roles in biofilm formation, bacterial virulence or symbiotic interactions. Noteworthy, plants detect each of these molecules, change their own gene expression programs, re-configurate root architecture, and activate defense responses, improving in this manner their adaptation to natural and agricultural ecosystems. AHLs may act as alarm signals, pathogen and/or microbe-associated molecular patterns, whereas CDPs function as hormonal mimics for plants via their putative interactions with the auxin receptor Transport Inhibitor Response1 (TIR1). A major challenge is to identify the molecular pathways of QS-mediated crosstalk and the plant receptors and interacting proteins for AHLs, CDPs and related signals.
Plant growth-promoting rhizobacteria modulate root development through different mechanisms. This work was conducted to evaluate the effects of root colonization by
Pseudomonas putida
and
Pseudomonas ...fluorescens
in biomass production, lateral root formation, and activation of auxin signaling in
Arabidopsis thaliana.
Selected strains of
P. putida
and
P. fluorescens
were tested for modification of
DR5::uidA
,
BA3::uidA
and
HS::AXR3NT
-
GUS
auxin-related gene expression, and to promote root hair and lateral root formation in WT and
tir1
-
1, tir1
-
1afb2
-
1afb3
-
1, arf7
-
1, arf19
-
1, arf7
-
1arf19
-
1
, and
rhd6
mutants. Production of cyclodipeptides with possible roles in auxin signaling was also determined in
P. putida
and
P. fluorescens
culture supernatants by gas chromatography–mass spectrometry.
P. putida
and
P. fluorescens
stimulated lateral root and root hair formation and increased plant biomass, which correlated with an induction of the auxin response. Genetic analyses suggested that growth promotion involves auxin signaling as
tir1
-
1, tir1
-
1afb2
-
1afb3
-
1, arf7
-
1, arf19
-
1, and arf7
-
1arf19
-
1
mutants showed decreased lateral root response to inoculation and because
P. putida
and
P. fluorescens
restored root hair development in the
rhd6
mutant. It was also found that these bacteria produce the cyclodipeptides cyclo(L-Pro-L-Val), cyclo(L-Pro-L-Phe), and cyclo(L-Pro-L-Tyr), which modulates auxin-responsive gene expression in roots. Our results suggest a role of cyclodipeptides for bacterial phytostimulation.
The plant microbiota can affect host fitness via the emission of microbial volatile organic compounds (mVOCs) that influence growth and development. However, evidence of these molecules and their ...effects in plants from arid ecosystems is limited.
We screened the mVOCs produced by 40 core and representative members of the microbiome of agaves and cacti in their interaction with Arabidopsis thaliana and Nicotiana benthamiana. We used SPME‐GC‐MS to characterize the chemical diversity of mVOCs and tested the effects of selected compounds on growth and development of model and host plants.
Our study revealed that approximately 90% of the bacterial strains promoted plant growth both in A. thaliana and N. benthamiana. Bacterial VOCs were mainly composed of esters, alcohols, and S‐containing compounds with 25% of them not previously characterized. Remarkably, ethyl isovalerate, isoamyl acetate, 3‐methyl‐1‐butanol, benzyl alcohol, 2‐phenylethyl alcohol, and 3‐(methylthio)‐1‐propanol, and some of their mixtures, displayed beneficial effects in A. thaliana and also improved growth and development of Agave tequilana and Agave salmiana in just 60 days.
Volatiles produced by bacteria isolated from agaves and cacti are promising molecules for the sustainable production of crops in arid and semi‐arid regions.
Organic volatile compounds produced by bacteria associated with desert plants, such as agaves and cacti, promote growth and development of model and host plants. The identified molecules and their mixtures, among them ethyl isovalerate, isoamyl acetate, 3‐methyl‐1‐butanol, benzyl alcohol, 2‐phenylethyl alcohol, and 3‐(methylthio)‐1‐propanol, can be used to enhance plant productivity, as well as to reveal the mechanisms underpinning plant‐microbe communication and the ecological relevance of microbial volatiles in arid environments.
: Melatonin (N‐acetyl‐5‐methoxytryptamine) is a tryptophan‐derived signal with important physiological roles in mammals. Although the presence of melatonin in plants may be universal, its endogenous ...function in plant tissues is unknown. On the basis of its structural similarity to indole‐3‐acetic acid, recent studies mainly focusing on root growth in several plant species have suggested a potential auxin‐like activity of melatonin. However, direct evidence about the mechanisms of action of this regulator is lacking. In this work, we used Arabidopsis thaliana seedlings as a model system to evaluate the effects of melatonin on plant growth and development. Melatonin modulated root system architecture by stimulating lateral and adventitious root formation but minimally affected primary root growth or root hair development. The auxin activity of melatonin in roots was investigated using the auxin‐responsive marker constructs DR5:uidA, BA3:uidA, and HS::AXR3NT‐GUS. Our results show that melatonin neither activates auxin‐inducible gene expression nor induces the degradation of HS::AXR3NT‐GUS, indicating that root developmental changes elicited by melatonin were independent of auxin signaling. Taken together, our results suggest that melatonin is beneficial to plants by increasing root branching and that root development processes elicited by this novel plant signal are likely independent of auxin responses.
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