Characterization of drr1, an alkamide-resistant mutant of Arabidopsis, reveals an important role for small lipid amides in lateral root development and plant senescence

Cellular messengers involved in the inhibition of the Arabidopsis primary root growth by bacterial quorum-sensing signal N-decanoyl-L-homoserine lactone

BACKGROUND

N-acyl-homoserine lactones (AHLs) are used as quorum-sensing signals by Gram-negative bacteria, but they can also affect plant growth and disease resistance. N-decanoyl-L-homoserine lactone (C10-HSL) is an AHL that has been shown to inhibit primary root growth in Arabidopsis, but the mechanisms underlying its effects on root architecture are unclear. Here, we investigated the signaling components involved in C10-HSL-mediated inhibition of primary root growth in Arabidopsis, and their interplay, using pharmacological, physiological, and genetic approaches.

RESULTS

Treatment with C10-HSL triggered a transient and immediate increase in the concentrations of cytosolic free Ca²⁺ and reactive oxygen species (ROS), increased the activity of mitogen-activated protein kinase 6 (MPK6), and induced nitric oxide (NO) production in Arabidopsis roots. Inhibitors of Ca²⁺ channels significantly alleviated the inhibitory effect of C10-HSL on primary root growth and reduced the amounts of ROS and NO generated in response to C10-HSL. Inhibition or scavenging of ROS and NO neutralized the inhibitory effect of C10-HSL on primary root growth. In terms of primary root growth, the respiratory burst oxidase homolog mutants and a NO synthase mutant were less sensitive to C10-HSL than wild type. Activation of MPKs, especially MPK6, was required for C10-HSL to inhibit primary root growth. The mpk6 mutant showed reduced sensitivity of primary root growth to C10-HSL, suggesting that MPK6 plays a key role in the inhibition of primary root growth by C10-HSL.

CONCLUSION

Our results indicate that MPK6 acts downstream of ROS and upstream of NO in the response to C10-HSL. Our data also suggest that Ca²⁺, ROS, MPK6, and NO are all involved in the response to C10-HSL, and may participate in the cascade leading to C10-HSL-inhibited primary root growth in Arabidopsis.

Tracking the time-dependent and tissue-specific processes of arsenic accumulation and stress responses in rice (Oryza sativa L.)

The present study analysed time (0.5 h to 24 h) and tissue [roots, old leaves (OL) and young leaves (YL)] dependent nature of arsenic (As) accumulation and ensuing responses in two contrasting varieties of rice (Oryza sativa L.); Pooja (tolerant) and CO-50 (moderately sensitive). Arsenic accumulation was 5.4-, 4.7- and 7.3-fold higher at 24 h in roots, OL and YL, respectively of var. CO-50 than that in var. Pooja. Arsenic accumulation in YL depicted a delayed accumulation; at 2 h onwards in var. Pooja (0.23 µg g^-1 dw) while at 1 h onwards in var. CO50 (0.26 µg g^-1 dw). The responses of oxidative stress parameters, antioxidant enzymes, metabolites and ions were also found to be tissue- and time-dependent and depicted differential pattern in the two varieties. Among hormone, salicylic acid and abscisic acid showed variable response in var. Pooja and var. CO-50. Metabolite analysis depicted an involvement of various metabolites in As stress responses of two varieties. In conclusion, an early sensing of the As stress, proper coordination of hormones, biochemical responses, ionic and metabolic profiles allowed var. Pooja to resist As stress and reduce As accumulation more effectively as compared to that of var. CO-50.

Review: Phytostimulation and root architectural responses to quorum-sensing signals and related molecules from rhizobacteria

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.

REF6 promotes lateral root formation through de-repression of PIN1/3/7 genes

The H3K27 methyltransferase CLF inhibits lateral root (LR) formation through depositing the repressive H3K27me3 mark to the chromatin of PIN1, a key polar auxin transporter gene. Here, we show that the H3K27me3 demethylase REF6 promotes lateral root primordium initiation and LR emergence. REF6 directly binds to the chromatin of PIN1/3/7. Dysfunction in REF6 results in increased levels of H3K27me3 on PIN1/3/7 and suppressed expression of PIN genes. Genetic analysis of the clf ref6 double mutant revealed an antagonistic action between CLF and REF6, in terms of LR formation. Our findings indicate that H3K27 methylation and demethylation activities are likely coordinated to ensure proper LR organogenesis.

Induction of systemic resistance in tomato against Botrytis cinerea by N-decanoyl-homoserine lactone via jasmonic acid signaling

N-decanoyl-homoserine lactone activates plant systemic resistance against Botrytis cinerea in tomato plants, which is largely dependent on jasmonic acid biosynthesis and signal transduction pathways. Rhizosphere bacteria secrete N-acylated-homoserine lactones (AHLs), a type of specialized quorum-sensing signal molecule, to coordinate their population density during communication with their eukaryotic hosts. AHLs behave as low molecular weight ligands that are sensed by plants and promote the host's resistance against foliar pathogens. In this study, we report on N-decanoyl-homoserine lactone (DHL), which is a type of AHL that induces systemic immunity in tomato plants and protects the host organism against the necrotrophic fungus Botrytis cinerea. Upon DHL treatment, tomato endogenous jasmonic acid (JA) biosynthesis (rather than salicylic acid biosynthesis) and signal transduction were significantly activated. Strikingly, the DHL-induced systemic resistance against B. cinerea was blocked in the tomato JA biosynthesis mutant spr2 and JA signaling gene-silenced plants. Our findings highlight the role of DHL in systemic resistance against economically important necrotrophic pathogens and suggest that DHL-induced immunity against B. cinerea is largely dependent on the JA signaling pathway. Manipulation of DHL-induced resistance is an attractive disease management strategy that could potentially be used to enhance disease resistance in diverse plant species.

N,N-dimethyl hexadecylamine and related amines regulate root morphogenesis via jasmonic acid signaling in Arabidopsis thaliana

Plant growth-promoting rhizobacteria are natural inhabitants of roots, colonize diverse monocot and dicot species, and affect several functional traits such as root architecture, adaptation to adverse environments, and protect plants from pathogens. N,N-dimethyl-hexadecylamine (C16-DMA) is a rhizobacterial amino lipid that modulates the postembryonic development of several plants, likely as part of volatile blends. In this work, we evaluated the bioactivity of C16-DMA and other related N,N-dimethyl-amines with varied length and found that inhibition of primary root growth was related to the length of the acyl chain. C16-DMA inhibited primary root growth affecting cell division and elongation, while promoting lateral root formation and root hair growth and density in Arabidopsis thaliana (Arabidopsis) wild-type (WT) seedlings. Interestingly, C16-DMA induced the expression of the jasmonic acid (JA)-responsive gene marker pLOX2:uidA, while JA-related mutants jar1, coi1-1, and myc2 affected on JA biosynthesis and perception, respectively, are compromised in C16-DMA responses. Comparison of auxin-regulated gene expression, root architectural changes in WT, and auxin-related mutants aux1-7, tir1/afb2/afb3, and arf7-1/arf19-1 to C16-DMA shows that the C16-DMA effects occur independently of auxin signaling. Together, these results reveal a novel class of aminolipids modulating root organogenesis via crosstalk with the JA signaling pathway.

Microbial homoserine lactones (AHLs) are effectors of root morphological changes in barley

While colonizing the rhizosphere, bacterial intra- and inter-specific communication is accomplished by N-Acyl-homoserine-lactones (AHLs) in a density-dependent manner. Moreover, plants are naturally exposed to AHLs and respond with tissue-specificity. In the present study, we investigated the influence of N-hexanoyl- (C6-HSL), N-octanoyl- (C8-HSL) and N-dodecanoyl-d/l-homoserine lactone (C12-HSL) on growth and root development in barley (Hordeum vulgare L.), and identified initial reactions in root cells after AHL exposures using physiological, staining, and electrophysiological methods. Treatment with short- and long-chain AHLs modulated plant growth and branched root architecture and induced nitric oxide (NO) accumulation in the calyptra and root elongation zone of excised roots in an AHL derivative-independent way. Additionally, C6- and C8-HSL treatments stimulated K⁺ uptake in root cells only at certain concentrations, whereas all tested concentrations of C12-HSL induced K⁺ uptake. In further experiments, C8-HSL promoted membrane hyperpolarization in epidermal root cells. Thus, we conclude AHLs promote plant growth and lateral root formation, and cause NO accumulation as an early response to AHLs. Furthermore, the AHL-mediated membrane hyperpolarization is leading to increased K⁺ uptake of the root tissue.

Lipid signalling in plant responses to abiotic stress

Lipids are one of the major components of biological membranes including the plasma membrane, which is the interface between the cell and the environment. It has become clear that membrane lipids also serve as substrates for the generation of numerous signalling lipids such as phosphatidic acid, phosphoinositides, sphingolipids, lysophospholipids, oxylipins, N-acylethanolamines, free fatty acids and others. The enzymatic production and metabolism of these signalling molecules are tightly regulated and can rapidly be activated upon abiotic stress signals. Abiotic stress like water deficit and temperature stress triggers lipid-dependent signalling cascades, which control the expression of gene clusters and activate plant adaptation processes. Signalling lipids are able to recruit protein targets transiently to the membrane and thus affect conformation and activity of intracellular proteins and metabolites. In plants, knowledge is still scarce of lipid signalling targets and their physiological consequences. This review focuses on the generation of signalling lipids and their involvement in response to abiotic stress. We describe lipid-binding proteins in the context of changing environmental conditions and compare different approaches to determine lipid-protein interactions, crucial for deciphering the signalling cascades.

Activated expression of AtEDT1/HDG11 promotes lateral root formation in Arabidopsis mutant edt1 by upregulating jasmonate biosynthesis

Root architecture is crucial for plants to absorb water and nutrients. We previously reported edt1 (edt1D) mutant with altered root architecture that contributes significantly to drought resistance. However, the underlying molecular mechanisms are not well understood. Here we report one of the mechanisms underlying EDT1/HDG11-conferred altered root architecture. Root transcriptome comparison between the wild type and edt1D revealed that the upregulated genes involved in jasmonate biosynthesis and signaling pathway were enriched in edt1D root, which were confirmed by quantitative RT-PCR. Further analysis showed that EDT1/HDG11, as a transcription factor, bound directly to the HD binding sites in the promoters of AOS, AOC3, OPR3, and OPCL1, which encode four key enzymes in JA biosynthesis. We found that the jasmonic acid level was significantly elevated in edt1D root compared with that in the wild type subsequently. In addition, more auxin accumulation was observed in the lateral root primordium of edt1D compared with that of wild type. Genetic analysis of edt1D opcl1 double mutant also showed that HDG11 was partially dependent on JA in regulating LR formation. Taken together, overexpression of EDT1/HDG11 increases JA level in the root of edt1D by directly upregulating the expressions of several genes encoding JA biosynthesis enzymes to activate auxin signaling and promote lateral root formation.

Effects of synthetic alkamides on Arabidopsis fatty acid amide hydrolase activity and plant development

Alkamides and N-acylethanolamines (NAEs) are bioactive, amide-linked lipids that influence plant development. Alkamides are restricted to several families of higher plants and some fungi, whereas NAEs are widespread signaling molecules in both plants and animals. Fatty acid amide hydrolase (FAAH) has been described as a key contributor to NAE hydrolysis; however, no enzyme has been associated with alkamide degradation in plants. Herein reported is synthesis of 12 compounds structurally similar to a naturally occurring alkamide (N-isobutyl-(2E,6Z,8E)decatrienamide or affinin) with different acyl compositions more similar to plant NAEs and various amino alkyl head groups. These "hybrid" synthetic alkamides were tested for activity toward recombinant Arabidopsis FAAH and for their effects on plant development (i.e., cotyledon expansion and primary root length). A substantial increase in FAAH activity was discovered toward NAEs in vitro in the presence of some of these synthetic alkamides, such as N-ethyllauroylamide (4). This "enhancement" effect was found to be due, at least in part, to relief from product inhibition of FAAH by ethanolamine, and not due to an alteration in the oligomerization state of the FAAH enzyme. For several of these alkamides, an inhibition of seedling growth was observed with greater results in FAAH knockouts and less in FAAH over-expressing plants, suggesting that these alkamides could be hydrolyzed by FAAH in planta. The tight regulation of NAE levels in vivo appears to be important for proper seedling establishment, and as such, some of these synthetic alkamides may be useful pharmacological tools to manipulate the effects of NAEs in situ.

Involvement of calmodulin in regulation of primary root elongation by N-3-oxo-hexanoyl homoserine lactone in Arabidopsis thaliana

Many bacteria use signal molecules of low molecular weight to monitor their local population density and to coordinate their collective behavior in a process called "quorum sensing" (QS). N-acyl-homoserine lactones (AHLs) are the primary QS signals among Gram-negative bacteria. AHL-mediated QS plays an essential role in diverse bacterial physiological processes. Recent evidence shows that plants are able to sense bacterial AHLs and respond to them appropriately. However, little is known about the mechanism by which plants perceive and transduce the bacterial AHLs within cells. In this study, we found that the stimulatory effect of N-3-oxo-hexanoyl homoserine lactone (3OC6-HSL) on primary root elongation of Arabidopsis was abolished by the calmodulin (CaM) antagonists N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7) and trifluoperazine (TFP). Western-blot and ELISA analysis revealed that the concentration of CaM protein in Arabidopsis roots increased after treatment with 1 μM 3OC6-HSL. Results from quantitative RT-PCR demonstrated that the transcription of all nine CaM genes in Arabidopsis genome was up-regulated in the plants treated with 3OC6-HSL. The loss-of-function mutants of each AtCaM gene (AtCaM1-9) were insensitive to 3OC6-HSL-stimulation of primary root elongation. On the other hand, the genetic evidence showed that CaM may not participates the inhibition of primary root length caused by application of long-chained AHLs such as C10-HSL and C12-HSL. Nevertheless, our results suggest that CaM is involved in the bacterial 3OC6-HSL signaling in plant cells. These data offer new insight into the mechanism of plant response to bacterial QS signals.

N-Acylethanolamines: lipid metabolites with functions in plant growth and development

Twenty years ago, N-acylethanolamines (NAEs) were considered by many lipid chemists to be biological 'artifacts' of tissue damage, and were, at best, thought to be minor lipohilic constituents of various organisms. However, that changed dramatically in 1993, when anandamide, an NAE of arachidonic acid (N-arachidonylethanolamine), was shown to bind to the human cannabinoid receptor (CB1) and activate intracellular signal cascades in mammalian neurons. Now NAEs of various types have been identified in diverse multicellular organisms, in which they display profound biological effects. Although targets of NAEs are still being uncovered, and probably vary among eukaryotic species, there appears to be remarkable conservation of the machinery that metabolizes these bioactive fatty acid conjugates of ethanolamine. This review focuses on the metabolism and functions of NAEs in higher plants, with specific reference to the formation, hydrolysis and oxidation of these potent lipid mediators. The discussion centers mostly on early seedling growth and development, for which NAE metabolism has received the most attention, but also considers other areas of plant development in which NAE metabolism has been implicated. Where appropriate, we indicate cross-kingdom conservation in NAE metabolic pathways and metabolites, and suggest areas where opportunities for further investigation appear most pressing.

N-acyl-homoserine lactone uptake and systemic transport in barley rest upon active parts of the plant

Bacteria communicate with each other in a population density-dependent process known as quorum sensing. N-acyl-homoserine lactones (HSLs) are the autoinducers of Gram-negative bacteria and the best-studied quorum sensing signals so far. HSLs induce various responses in plants, including systemic resistance and root development. Here, we used different methods, including tritium labelling, sensor strain assays and monoclonal antibodies (mAbs), to analyse the uptake and translocation of C8- and C10- homoserine lactones into barley (Hordeum vulgare cv Barke). Both HSLs were already systemically transported into the shoot at 2 h after application. HSL uptake could be inhibited by orthovanadate, demonstrating that ABC transporters are involved in the uptake. Root transport occurs predominantly via the central cylinder, which was shown by transport inhibition via KCl application and autoradiography of root cross-sections. Furthermore, a newly established detection method with mAbs allowed the first detection of a systemic transport of long-chain HSLs in plants. The coupled use of different HSL detection methods demonstrated that the uptake and transport of HSLs into barley does not occur passively, but relies, at least partially, on active processes in the plant.

Characterization and fine mapping of the rice premature senescence mutant ospse1

Premature senescence can limit crop productivity by limiting the growth phase. In the present study, a spontaneous premature senescence mutant was identified in rice (Oryza sativa L.). Genetic analysis revealed that the premature senescence phenotype was controlled by a recessive mutation, which we named Oryza sativa premature senescence1 (ospse1). The ospse1 mutants showed premature leaf senescence from the booting stage and exhibited more severe symptoms during reproductive and ripening stages. Key yield-related agronomic traits such as 1,000-grain weight and seed-setting rate, but not panicle grain number, were significantly reduced in ospse1 plants. Chlorophyll content, net photosynthetic rate, and transpiration rate of ospse1 flag leaves were similar to the wild-type plants in vegetative stages, but these parameters decreased steeply in the mutant after the heading stage. Consistent with this, the senescence-associated genes OsNYC1 and OsSgr were up-regulated in ospse1 mutant during premature leaf senescence. The ospse1 locus was mapped to a 38-kb region on chromosome 1 and sequence analysis of this region identified a single-nucleotide deletion in the 3' region of an open reading frame (ORF) encoding a putative pectate lyase, leading to a frame shift and a longer ORF. Our results suggested that the premature senescence of the ospse1 may be regulated by a novel mechanism mediated by pectate lyase.

dhm1, an Arabidopsis mutant with increased sensitivity to alkamides shows tumorous shoot development and enhanced lateral root formation

The control of cell division by growth regulators is critical to proper shoot and root development. Alkamides belong to a class of small lipid amides involved in plant morphogenetic processes, from which N-isobutyl decanamide is one of the most active compounds identified. This work describes the isolation and characterization of an N-isobutyl decanamide-hypersensitive (dhm1) mutant of Arabidopsis (Arabidopsis thaliana). dhm1 seedlings grown in vitro develop disorganized tumorous tissue in petioles, leaves and stems. N-isobutyl decanamide treatment exacerbates the dhm1 phenotype resulting in widespread production of callus-like structures in the mutant. Together with these morphological alterations in shoot, dhm1 seedlings sustained increased lateral root formation and greater sensitivity to alkamides in the inhibition of primary root growth. The mutants also show reduced etiolation when grown in darkness. When grown in soil, adult dhm1 plants were characterized by reduced plant size, and decreased fertility. Genetic analysis indicated that the mutant phenotype segregates as a single recessive Mendelian trait. Developmental alterations in dhm1 were related to an enhanced expression of the cell division marker CycB1-uidA both in the shoot and root system, which correlated with altered expression of auxin and cytokinin responsive gene markers. Pharmacological inhibition of auxin transport decreased LR formation in WT and dhm1 seedlings in a similar manner, indicating that auxin transport is involved in the dhm1 root phenotype. These data show an important role of alkamide signaling in cell proliferation and plant architecture remodeling likely acting through the DHM1 protein.

Methyl jasmonate-induced lateral root formation in rice: the role of heme oxygenase and calcium

Lateral roots (LRs) play important roles in increasing the absorptive capacity of roots as well as to anchor the plant in the soil. Therefore, understanding the regulation of LR development is of agronomic importance. In this study, we examined the effect of methyl jasmonate (MJ) on LR formation in rice. Treatment with MJ induced LR formation and heme oxygenase (HO) activity. As well, MJ could induce OsHO1 mRNA expression. Zinc protoporphyrin IX (the specific inhibitor of HO) and hemoglobin [the carbon monoxide/nitric oxide (NO) scavenger] reduced LR formation, HO activity and OsHO1 expression. LR formation and HO activity induced by MJ was reduced by the specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-oxide. The effects of Ca(2+) chelators, Ca(2+)-channel inhibitors, and calmodulin (CaM) antagonists on LR formation induced by MJ were also examined. All these inhibitors were effective in reducing the action of MJ. However, Ca(2+) chelators and Ca(2+) channel inhibitors induced HO activity when combining with MJ further. It is concluded that Ca(2+) may regulate MJ action mainly through CaM-dependent mechanism.

The submergence tolerance gene SUB1A delays leaf senescence under prolonged darkness through hormonal regulation in rice

Leaf senescence is a natural age-dependent process that is induced prematurely by various environmental stresses. Typical alterations during leaf senescence include breakdown of chlorophyll, a shift to catabolism of energy reserves, and induction of senescence-associated genes, all of which can occur during submergence, drought, and constant darkness. Here, we evaluated the influence of the submergence tolerance regulator, SUBMERGENCE1A (SUB1A), in the acclimation responses during leaf senescence caused by prolonged darkness in rice (Oryza sativa). SUB1A messenger RNA was highly induced by prolonged darkness in a near-isogenic line containing SUB1A. Genotypes with conditional and ectopic overexpression of SUB1A significantly delayed loss of leaf color and enhanced recovery from dark stress. Physiological analysis revealed that SUB1A postpones dark-induced senescence through the maintenance of chlorophyll and carbohydrate reserves in photosynthetic tissue. This delay allowed leaves of SUB1A genotypes to recover photosynthetic activity more quickly upon reexposure to light. SUB1A also restricted the transcript accumulation of representative senescence-associated genes. Jasmonate and salicylic acid are positive regulators of leaf senescence, but ectopic overexpression of SUB1A dampened responsiveness to both hormones in the context of senescence. We found that ethylene accelerated senescence stimulated by darkness and jasmonate, although SUB1A significantly restrained dark-induced ethylene accumulation. Overall, SUB1A genotypes displayed altered responses to prolonged darkness by limiting ethylene production and responsiveness to jasmonate and salicylic acid, thereby dampening the breakdown of chlorophyll, carbohydrates, and the accumulation of senescence-associated messenger RNAs. A delay of leaf senescence conferred by SUB1A can contribute to the enhancement of tolerance to submergence, drought, and oxidative stress.

The jasmonate receptor COI1 plays a role in jasmonate-induced lateral root formation and lateral root positioning in Arabidopsis thaliana

Jasmonic acid (JA) regulates a broad range of plant defense and developmental responses. COI1 has been recently found to act as JA receptor. In this report, we show that low micromolar concentrations of JA inhibited primary root (PR) growth and promoted lateral root (LR) formation in Arabidopsis wild-type (WT) seedlings. It was observed that the coi1-1 mutant was less sensitive to JA on pericycle cell activation to induce lateral root primordia (LRP) formation and presented alterations in lateral root positioning and lateral root emergence on bends. To investigate JA-auxin interactions important for remodeling of root system (RS) architecture, we tested the expression of auxin-inducible markers DR5:uidA and BA3:uidA in WT and coi1-1 seedlings in response to indole-3-acetic acid (IAA) and JA and analyzed the RS architecture of a suite of auxin-related mutants under JA treatments. We found that JA did not affect DR5:uidA and BA3:uidA expression in WT and coi1-1 seedlings. Our data also showed that PR growth inhibition in response to JA was likely independent of auxin signaling and that the induction of LRP required ARF7, ARF19, SLR, TIR1, AFB2, AFB3 and AXR1 loci. We conclude that JA regulation of postembryonic root development involves both auxin-dependent and independent mechanisms.

N-Acylethanolamines and related compounds: aspects of metabolism and functions

N-Acylethanolamines (NAE) are fatty acid derivates that are linked with an ethanolamine group via an amide bond. NAE can be characterized as lipid mediators in the plant and animal kingdoms owing to the diverse functions throughout the eukaryotic domain. The functions of NAE have been widely investigated in animal tissues in part due to their abilities to interact with the cannabinoid receptors, vanilloid receptors or peroxisome proliferator activated receptors. However, the interest of studying the functions of these lipids in plants is progressively becoming more apparent. The number of publications about the functions related to NAE and to structural analogs (homoserine lactone and alkamides) is greatly increasing, showing the importance of these lipids in various plant physiological processes. This review sheds light on their role in different processes such as seedling development, plant pathogen interaction, phospholipase D alpha inhibition and senescence of cut flowers, and underlines the interaction between NAE and NAE-related molecules with plant hormone signaling. The different metabolic pathways promoting the synthesis and degradation of NAE are also discussed, in particular the oxygenation of polyunsaturated N-acylethanolamines, which leads to NAE-oxylipins, a new family of bioactive lipids.

N-3-oxo-decanoyl-L-homoserine-lactone activates auxin-induced adventitious root formation via hydrogen peroxide- and nitric oxide-dependent cyclic GMP signaling in mung bean

N-Acyl-homoserine-lactones (AHLs) are bacterial quorum-sensing signaling molecules that regulate population density. Recent evidence demonstrates their roles in plant defense responses and root development. Hydrogen peroxide (H(2)O(2)), nitric oxide (NO), and cyclic GMP (cGMP) are essential messengers that participate in various plant physiological processes, but how these messengers modulate the plant response to N-acyl-homoserine-lactone signals remains poorly understood. Here, we show that the N-3-oxo-decanoyl-homoserine-lactone (3-O-C10-HL), in contrast to its analog with an unsubstituted branch chain at the C3 position, efficiently stimulated the formation of adventitious roots and the expression of auxin-response genes in explants of mung bean (Vigna radiata) seedlings. This response was mimicked by the exogenous application of auxin, H(2)O(2), NO, or cGMP homologs but suppressed by treatment with scavengers or inhibitors of H(2)O(2), NO, or cGMP metabolism. The 3-O-C10-HL treatment enhanced auxin basipetal transport; this effect could be reversed by treatment with H(2)O(2) or NO scavengers but not by inhibitors of cGMP synthesis. Inhibiting 3-O-C10-HL-induced H(2)O(2) or NO accumulation impaired auxin- or 3-O-C10-HL-induced cGMP synthesis; however, blocking cGMP synthesis did not affect auxin- or 3-O-C10-HL-induced H(2)O(2) or NO generation. Additionally, cGMP partially rescued the inhibitory effect of H(2)O(2) or NO scavengers on 3-O-C10-HL-induced adventitious root development and auxin-response gene expression. These results suggest that 3-O-C10-HL, unlike its analog with an unmodified branch chain at the C3 position, can accelerate auxin-dependent adventitious root formation, possibly via H(2)O(2)- and NO-dependent cGMP signaling in mung bean seedlings.

Quantitative analysis of lateral root development: pitfalls and how to avoid them

The advent of the postgenomics era has led to increased interest in exploring the role of gene networks and signaling pathways in controlling plant development. The last two decades have seen a particular increase in the number of studies focusing on the development of the Arabidopsis thaliana root system. However, the investigation of such a seemingly simple system as an Arabidopsis root can lead to problems in quantification and errors in interpretation if knowledge of root organization is lacking. In this article, we identify a number of these problems and give examples of potentially erroneous and correct determinations of lateral root parameters. Our aim is to bring this important issue to the attention of the plant science community and to suggest ways in which the problems inherent in quantifying the process of lateral root development can be avoided.

Alkamides activate jasmonic acid biosynthesis and signaling pathways and confer resistance to Botrytis cinerea in Arabidopsis thaliana

Alkamides are fatty acid amides of wide distribution in plants, structurally related to N-acyl-L-homoserine lactones (AHLs) from Gram-negative bacteria and to N- acylethanolamines (NAEs) from plants and mammals. Global analysis of gene expression changes in Arabidopsis thaliana in response to N-isobutyl decanamide, the most highly active alkamide identified to date, revealed an overrepresentation of defense-responsive transcriptional networks. In particular, genes encoding enzymes for jasmonic acid (JA) biosynthesis increased their expression, which occurred in parallel with JA, nitric oxide (NO) and H₂O₂ accumulation. The activity of the alkamide to confer resistance against the necrotizing fungus Botrytis cinerea was tested by inoculating Arabidopsis detached leaves with conidiospores and evaluating disease symptoms and fungal proliferation. N-isobutyl decanamide application significantly reduced necrosis caused by the pathogen and inhibited fungal proliferation. Arabidopsis mutants jar1 and coi1 altered in JA signaling and a MAP kinase mutant (mpk6), unlike salicylic acid- (SA) related mutant eds16/sid2-1, were unable to defend from fungal attack even when N-isobutyl decanamide was supplied, indicating that alkamides could modulate some necrotrophic-associated defense responses through JA-dependent and MPK6-regulated signaling pathways. Our results suggest a role of alkamides in plant immunity induction.

Nitric oxide is involved in alkamide-induced lateral root development in Arabidopsis

Alkamides are small bioactive lipid signals with a wide distribution in plants. In this report, the role of nitric oxide (NO) in the alterations induced by N-isobutyl decanamide on the Arabidopsis (Arabidopsis thaliana) root system architecture (RSA) was investigated. We first compared the effects of N-isobutyl decanamide and NO donors sodium nitropruside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) on root morphogenetic processes. Both N-isobutyl decanamide and NO donors modulated RSA in a similar way and in a dose-dependent manner, inhibiting primary root growth and promoting lateral root primordia (LRP) formation. RSA alterations induced by N-isobutyl decanamide correlated with NO accumulation in the primary root tip and in developing lateral roots. Morphogenetic effects of N-isobutyl decanamide decreased when NO scavengers were supplied to alkamide-treated seedlings. N-Isobutyl decanamide-regulated root architectural changes were also investigated in mutants defective in NO biosynthesis, nia1 nia2, and NO signalling, Atnoa1, and in the alkamide-resistant mutant drr1. The nia1 nia2 and Atnoa1 mutants were indistinguishable in primary root growth inhibition by the alkamide when compared with wild-type (WT) seedlings, but showed reduced lateral root responses. The drr1 mutant was less sensitive in both primary root growth inhibition and LRP induction by NO donors than WT seedlings. Detailed DR5:uidA and BA3:uidA marker analysis showed that N-isobutyl decanamide and its interacting signals jasmonic acid and NO act downstream or independently of auxin-responsive gene expression to promote LRP formation. Our results provide compelling evidence that NO is an intermediate in alkamide signaling mediating RSA adjustment in Arabidopsis.