Phenanthrene-Induced Cytochrome P450 Genes and Phenanthrene Tolerance Associated with Arabidopsis thaliana CYP75B1 Gene.

Polycyclic aromatic hydrocarbons (PAHs) form an important group of organic pollutants due to their distribution in the environment and their carcinogenic and/or mutagenic effects. In order to identify at the molecular level some of the players in the biodegradation and tolerance response to PAHs in plants, we have phenotyped 32 Arabidopsis thaliana T-DNA mutant lines corresponding to 16 cytochrome P450 (CYP) genes that showed to be differentially expressed under contrasted stress conditions induced by phenanthrene, a 3-ring PAH. This screening has allowed us to identify CYP75B1 (At5g07990) T-DNA mutants as the only ones being sensitive to phenanthrene-induced stress, supporting that CYP75B1 protein is necessary for PAH tolerance. CYP75B1 codes for a 3'flavonol hydroxylase. CYP75B1 gene was heterologously expressed on yeast in order to investigate whether it affects the A. thaliana response to phenanthrene by participating in its metabolization. Heterologously-produced CYP75B1 enzyme shows to be catalytically efficient against its physiological substrates (e.g., naringenin) but unable to metabolize phenanthrene or 9-phenanthrenol. In contrast, CYP75B1 seems rather involved in phenanthrene tolerance as a crucial element by regulating concentration of antioxidants through the production of 3'-hydroxylated flavonoids such as quercetin and cyanidin. In particular, we report a highly increased generation of reactive oxygen species (H2O2 and singlet oxygen) in cyp75b1 mutants compared to control plants in response to phenanthrene treatment. Overall, CYP75B1 shows to play an important role in the response to the deleterious effects of phenanthrene exposure and this is related to oxidative stress sensitivity rather than metabolization.

Are nanoplastics potentially toxic for plants and rhizobiota? Current knowledge and recommendations.

Soil is now becoming a reservoir of plastics in response to global production, use/disposal patterns and low recovery rates. Their degradation is caused by numerous processes, and this degradation leads to the formation and release of plastic nanoparticles, i.e., nanoplastics. The occurrence of nanoplastics in the soil is expected to both directly and indirectly impact its properties and functioning. Nanoplastics may directly impact the physiology and development of living organisms, especially plants, e.g., by modifying their production yield. Nanoplastics can also indirectly modify the physicochemical properties of the soil and, as a result, favour the release of related contaminants (organic or inorganic) and have an impact on soil biota, and therefore have a negative effect on the functioning of rhizospheres. However all these results have to be taken carefully since performed with polymer nano-bead not representative of the nanoplastics observed in the environment. This review highlight thus the current knowledge on the interactions between plants, rhizosphere and nanoplastics, their consequences on plant physiology and development in order to identify gaps and propose scientific recommendations.

Analysis of Small Non-coding RNAs as Signaling Intermediates of Environmentally Integrated Responses to Abiotic Stress.

Research to date on abiotic stress responses in plants has been largely focused on the plant itself, but current knowledge indicates that microorganisms can interact with and help plants during periods of abiotic stress. In our research, we aim to investigate the interkingdom communication between the plant root and the rhizo-microbiota. Our investigation showed that miRNA plays a pivotal role in this interkingdom communication. Here, we describe a protocol for the analysis of miRNA secreted by the plant root, which includes all of the steps from the isolation of the miRNA to the bioinformatics analysis. Because of their short nucleotide length, Next Generation Sequencing (NGS) library preparation from miRNAs can be challenging due to the presence of dimer adapter contaminants. Therefore, we highlight some strategies we adopt to inhibit the generation of dimer adapters during library preparation. Current screens of miRNA targets mostly focus on the identification of targets present in the same organism expressing the miRNA. Our bioinformatics analysis challenges the barrier of evolutionary divergent organisms to identify candidate sequences of the microbiota targeted by the miRNA of plant roots. This protocol should be of interest to researchers investigating interkingdom RNA-based communication between plants and their associated microorganisms, particularly in the context of holobiont responses to abiotic stresses.

Recent allopolyploidy alters Spartina microRNA expression in response to xenobiotic-induced stress.

Environmental contamination by xenobiotics represents a major threat for natural ecosystems and public health. In response, xenobiotic detoxification is a fundamental trait of organisms for developmental plasticity and stress tolerance, but the underlying molecular mechanisms remain poorly understood in plants. To decipher this process, we explored the consequences of allopolyploidy on xenobiotic tolerance in the genus Spartina Schreb. Specifically, we focused on microRNAs (miRNAs) owing to their central function in the regulation of gene expression patterns, including responses to stress. Small RNA-Seq was conducted on the parents S. alterniflora and S. maritima, their F1 hybrid S. x townsendii and the allopolyploid S. anglica under phenanthrene-induced stress (phe), a model Polycyclic Aromatic Hydrocarbon (PAH) compound. Differentially expressed miRNAs in response to phe were specifically identified within species. In complement, the respective impacts of hybridization and genome doubling were detected, through changes in miRNA expression patterns between S. x townsendii, S. anglica and the parents. The results support the impact of allopolyploidy in miRNA-guided regulation of plant response to phe. In total, we identified 17 phe-responsive miRNAs in Spartina among up-regulated MIR156 and down-regulated MIR159. We also describe novel phe-responsive miRNAs as putative Spartina-specific gene expression regulators in response to stress. Functional validation using Arabidopsis (L.) Heynh. T-DNA lines inserted in homologous MIR genes was performed, and the divergence of phe-responsive miRNA regulatory networks between Arabidopsis and Spartina was discussed.

Rhizospheric Plant-Microbe Interactions: miRNAs as a Key Mediator.

The importance of microorganisms in plant development, nutrition, and stress resistance is unquestioned and has led to a more holistic approach of plant-microbe interactions, under the holobiont concept. The structure of the plant microbiota is often described as host driven, especially in the rhizosphere, where microbial communities are shaped by diverse rhizodeposits. Gradually, this anthropogenic vision is fading and being replaced by the idea that plants and microorganisms co-shape the plant microbiota. Through coevolution, plants and microbes have developed cross-kingdom communication channels. Here, we propose that miRNAs are crucial mediators of plant-microbe interactions and microbiota shaping in the rhizosphere. Moreover, we suggest, as an alternative to generally unsuccessful strategies based on microbial inoculants, miRNAs as a promising tool for novel holobiont engineering.

Phenanthrene contamination and ploidy level affect the rhizosphere bacterial communities of Spartina spp.

Spartina spp. are widely distributed salt marsh plants that have a recent history of hybridization and polyploidization. These events have resulted in a heightened tolerance to hydrocarbon contaminants, but the effects of this phenomenon on the rhizosphere microbial communities are unknown. Here, we grew two parental Spartina species, their hybrid and the resulting allopolyploid in salt marsh sediments that were contaminated or not with phenanthrene. The DNA from the rhizosphere soil was extracted and the bacterial 16S rRNA gene was amplified and sequenced, whereas the abundances of the genes encoding for the PAH (polycyclic aromatic hydrocarbon) ring-hydroxylating dioxygenase (RHD) of Gram-negative and Gram-positive bacteria were quantified by real-time PCR. Both the contamination and the plant genotype significantly affected the bacterial communities. In particular, the allopolyploid S. anglica harbored a more diverse bacterial community in its rhizosphere. The interspecific hybrid and the allopolyploid also harbored significantly more copies of the PAH-RHD gene of Gram-negative bacteria in their rhizosphere than the parental species, irrespective of the contamination treatments. Overall, our results are showing that the recent polyploidization events in the Spartina affected its rhizosphere bacterial communities, both under normal and contaminated conditions, possibly increasing its phytoremediation potential.

Do Specialized Cells Play a Major Role in Organic Xenobiotic Detoxification in Higher Plants?

In the present work, we used a double cell screening approach based on phenanthrene (phe) epifluorescence histochemical localization and oxygen radical detection to generate new data about how some specialized cells are involved in tolerance to organic xenobiotics. Thereby, we bring new insights about phe [a common Polycyclic Aromatic Hydrocarbon (PAH)] cell specific detoxification, in two contrasting plant lineages thriving in different ecosystems. Our data suggest that in higher plants, detoxification may occur in specialized cells such as trichomes and pavement cells in Arabidopsis, and in the basal cells of salt glands in Spartina species. Such features were supported by a survey from the literature, and complementary data correlating the size of basal salt gland cells and tolerance abilities to PAHs previously reported between Spartina species. Furthermore, we conducted functional validation in two independent Arabidopsis trichomeless glabrous T-DNA mutant lines (GLABRA1 mutants). These mutants showed a sensitive phenotype under phe-induced stress in comparison with their background ecotypes without the mutation, indicating that trichomes are key structures involved in the detoxification of organic xenobiotics. Interestingly, trichomes and pavement cells are known to endoreduplicate, and we discussed the putative advantages given by endopolyploidy in xenobiotic detoxification abilities. The same feature concerning basal salt gland cells in Spartina has been raised. This similarity with detoxification in the endopolyploid liver cells of the animal system is included.

Evolution of small RNA expression following hybridization and allopolyploidization: insights from Spartina species (Poaceae, Chloridoideae).

KEY MESSAGE: Differential expression of mi-RNAs targeting developmental processes and progressive downregulation of repeat-associated siRNAs following genome merger and genome duplication in the context of allopolyploid speciation in Spartina. The role of small RNAs on gene expression regulation and genome stability is arousing increased interest and is being explored in various plant systems. In spite of prominence of reticulate evolution and polyploidy that affects the evolutionary history of all plant lineages, very few studies analysed RNAi mechanisms with this respect. Here, we explored small RNAs diversity and expression in the context of recent allopolyploid speciation, using the Spartina system, which offers a unique opportunity to explore the immediate changes following hybridization and genome duplication. Small RNA-Seq analyses were conducted on hexaploid parental species (S. alterniflora and S. maritima), their F1 hybrid S. x townsendii, and the neoallododecaploid S. anglica. We identified 594 miRNAs, 2197 miRNA-target genes, and 3730 repeat-associated siRNAs (mostly targeting Class I/Copia-Ivana- Copia-SIRE and LINEs elements). For both mi- and ra-siRNAs, we detected differential expression patterns following genome merger and genome duplication. These misregulations include non-additive expression of miRNAs in the F1 hybrid and additional changes in the allopolyploid targeting developmental processes. Expression of repeat-associated siRNAs indicates a strengthen of transposable element repression during the allopolyploidization process. Altogether, these results confirm the central role small RNAs play in shaping regulatory changes in naturally formed recent allopolyploids.

Involvement of polyamines in sucrose-induced tolerance to atrazine-mediated chemical stress in Arabidopsis thaliana.

Treatment of Arabidopsis thaliana seedlings with the PSII-inhibiting herbicide atrazine results in xenobiotic and oxidative stress, developmental arrest, induction of senescence and cell death processes. In contrast, exogenous sucrose supply confers a high level of atrazine stress tolerance, in relation with genome-wide modifications of transcript levels and regulation of genes involved in detoxification, defense and repair. However, the regulation mechanisms related to exogenous sucrose, involved in this sucrose-induced tolerance, are largely unknown. Characterization of these mechanisms was carried out through a combination of transcriptomic, metabolic, functional and mutant analysis under different conditions of atrazine exposure. Exogenous sucrose was found to differentially regulate genes involved in polyamine synthesis. ARGININE DECARBOXYLASE ADC1 and ADC2 paralogues, which encode the rate-limiting enzyme (EC 4.1.1.19) of the first step of polyamine biosynthesis, were strongly upregulated by sucrose treatment in the presence of atrazine. Such regulation occurred concomitantly with significant changes of major polyamines (putrescine, spermidine, spermine). Physiological characterization of a mutant affected in ADC activity and exogenous treatments with sucrose, putrescine, spermidine and spermine further showed that modification of polyamine synthesis and of polyamine levels could play adaptive roles in response to atrazine stress, and that putrescine and spermine had antagonistic effects, especially in the presence of sucrose. This interplay between sucrose, putrescine and spermine is discussed in relation with survival and anti-death mechanisms in the context of chemical stress exposure.

Increased tolerance to organic xenobiotics following recent allopolyploidy in Spartina (Poaceae).

Genome doubling or polyploidy is a widespread phenomenon in plants where it has important evolutionary consequences affecting the species distribution and ecology. PAHs are ubiquitous organic pollutants, which represent a major environmental concern. Recent data showed that tolerance to organic xenobiotics involve specific signaling pathways, and detoxifying gene sets referred as 'the xenome'. However, no data are available about how polyploidy impacts tolerance to organic xenobiotics. In the present paper, we investigated PAH tolerance following allopolyploidization in Spartina alterniflora, S. maritima and their derived allopolyploid species S. anglica. We performed comparative analyses of cellular compartmentalization, photosynthetic indices, and oxidative stress markers under phenanthrene-induced stress, and found that S. anglica exhibit increased tolerance compared to its parents. Based on 52 genes potentially involved in phenanthrene detoxification previously identified in A. thaliana, we investigated the Spartina xenome using genomic and transcriptomic available resources. Subsequently, we focused on GSTs, a ubiquitous enzymes class involved in organic xenobiotic detoxification. We examined expression profiles of selected genes by RT-qPCR, and revealed various patterns of parental expression alteration in the allopolyploid. The impacts of allopolyploidization on phenanthrene-induced stress and their potential ecological implications are discussed. The neo-allopolyploid S. anglica appears as a potential candidate for phytoremediation in PAH-polluted marshes.

Transcriptome response of the foundation plant Spartina alterniflora to the Deepwater Horizon oil spill.

Despite the severe impacts of the Deepwater Horizon oil spill, the foundation plant species Spartina alterniflora proved resilient to heavy oiling, providing an opportunity to identify mechanisms of response to the anthropogenic stress of crude oil exposure. We assessed plants from oil-affected and unaffected populations using a custom DNA microarray to identify genomewide transcription patterns and gene expression networks that respond to crude oil exposure. In addition, we used T-DNA insertion lines of the model grass Brachypodium distachyon to assess the contribution of four novel candidate genes to crude oil response. Responses in S. alterniflora to hydrocarbon exposure across the transcriptome as well as xenobiotic specific response pathways had little overlap with those previously identified in the model plant Arabidopsis thaliana. Among T-DNA insertion lines of B. distachyon, we found additional support for two candidate genes, one (ATTPS21) involved in volatile production, and the other (SUVH5) involved in epigenetic regulation of gene expression, that may be important in the response to crude oil. The architecture of crude oil response in S. alterniflora is unique from that of the model species A. thaliana, suggesting that xenobiotic response may be highly variable across plant species. In addition, further investigations of regulatory networks may benefit from more information about epigenetic response pathways.

Long-term investigation of constructed wetland wastewater treatment and reuse: Selection of adapted plant species for metaremediation.

A highly diverse plant community in a constructed wetland was used to investigate an ecological treatment system for human wastewater in an arid climate. The eight-year operation of the system has allowed the identification of a highly adapted and effective plant consortium that is convenient for plant-assisted metaremediation of wastewater. This constructed wetland pilot station demonstrated effective performance over this extended period. Originally, there were twenty-five plant species. However, because of environmental constraints and pressure from interspecific competition, only seven species persisted. Interestingly, the molecular phylogenetic analyses and an investigation of the photosynthetic physiology showed that the naturally selected plants are predominately monocot species with C4 or C4-like photosynthetic pathways. Despite the loss of 72% of initially used species in the constructed wetland, the removal efficiencies of BOD, COD, TSS, total phosphorus, ammonia and nitrate were maintained at high levels, approximately 90%, 80%, 94%, 60% and 50%, respectively. Concomitantly, the microbiological water tests showed an extremely high reduction of total coliform bacteria and streptococci, about 99%, even without a specific disinfection step. Hence, the constructed wetland system produced water of high quality that can be used for agricultural purposes. In the present investigation, we provide a comprehensive set of plant species that might be used for long-term and large-scale wastewater treatment.

Unraveling the early molecular and physiological mechanisms involved in response to phenanthrene exposure.

BACKGROUND: Higher plants have to cope with increasing concentrations of pollutants of both natural and anthropogenic origin. Given their capacity to concentrate and metabolize various compounds including pollutants, plants can be used to treat environmental problems - a process called phytoremediation. However, the molecular mechanisms underlying the stabilization, the extraction, the accumulation and partial or complete degradation of pollutants by plants remain poorly understood. RESULTS: Here, we determined the molecular events involved in the early plant response to phenanthrene, used as a model of polycyclic aromatic hydrocarbons. A transcriptomic and a metabolic analysis strongly suggest that energy availability is the crucial limiting factor leading to high and rapid transcriptional reprogramming that can ultimately lead to death. We show that the accumulation of phenanthrene in leaves inhibits electron transfer and photosynthesis within a few minutes, probably disrupting energy transformation. CONCLUSION: This kinetic analysis improved the resolution of the transcriptome in the initial plant response to phenanthrene, identifying genes that are involved in primary processes set up to sense and detoxify this pollutant but also in molecular mechanisms used by the plant to cope with such harmful stress. The identification of first events involved in plant response to phenanthrene is a key step in the selection of candidates for further functional characterization, with the prospect of engineering efficient ecological detoxification systems for polycyclic aromatic hydrocarbons.

Moderate salinity reduced phenanthrene-induced stress in the halophyte plant model Thellungiella salsuginea compared to its glycophyte relative Arabidopsis thaliana: Cross talk and metabolite profiling.

It was shown that halophytes experience higher cross-tolerance to stresses than glycophytes, which was often associated with their more powerful antioxidant systems. Moreover, salinity was reported to enhance halophyte tolerance to several stresses. The aim of the present work was to investigate whether a moderate salinity enhances phenanthrene stress tolerance in the halophyte Thellungiella salsuginea. The model plant Arabidopsis thaliana, considered as its glycophyte relative, was used as reference. Our study was based on morpho-physiological, antioxidant, and metabolomic parameters. Results showed that T. salsuginea was more tolerant to phenanthrene stress as compared to A. thaliana. An improvement of phenanthrene-induced responses was recorded in the two plants in the presence of 25 mM NaCl, but the effect was significantly more obvious in the halophyte. This observation was particularly related to the higher antioxidant activities and the induction of more adapted metabolism in the halophyte. Gas Chromatography coupled with Mass Spectrometry (GC-MS) was used to quantify alcohols, ammonium, sugars, and organic acids. It showed the accumulation of several metabolites, many of them are known to be involved in signaling and abiotic stress tolerance. Moderate salinity and phenanthrene cross-tolerance involved in these two stresses was discussed.

"Omics" Insights into PAH Degradation toward Improved Green Remediation Biotechnologies.

This review summarizes recent knowledge of polycyclic aromatic hydrocarbons (PAHs) biotransformation by microorganisms and plants. Whereas most research has focused on PAH degradation either by plants or microorganisms separately, this review specifically addresses the interactions of plants with their rhizosphere microbial communities. Indeed, plant roots release exudates that contain various nutritional and signaling molecules that influence bacterial and fungal populations. The complex interactions of these populations play a pivotal role in the biodegradation of high-molecular-weight PAHs and other complex molecules. Emerging integrative approaches, such as (meta-) genomics, (meta-) transcriptomics, (meta-) metabolomics, and (meta-) proteomics studies are discussed, emphasizing how "omics" approaches bring new insight into decipher molecular mechanisms of PAH degradation both at the single species and community levels. Such knowledge address new pictures on how organic molecules are cometabolically degraded in a complex ecosystem and should help in setting up novel decontamination strategies based on the rhizosphere interactions between plants and their microbial associates.

Abiotic stressors and stress responses: What commonalities appear between species across biological organization levels?

Organisms are regularly subjected to abiotic stressors related to increasing anthropogenic activities, including chemicals and climatic changes that induce major stresses. Based on various key taxa involved in ecosystem functioning (photosynthetic microorganisms, plants, invertebrates), we review how organisms respond and adapt to chemical- and temperature-induced stresses from molecular to population level. Using field-realistic studies, our integrative analysis aims to compare i) how molecular and physiological mechanisms related to protection, repair and energy allocation can impact life history traits of stressed organisms, and ii) to what extent trait responses influence individual and population responses. Common response mechanisms are evident at molecular and cellular scales but become rather difficult to define at higher levels due to evolutionary distance and environmental complexity. We provide new insights into the understanding of the impact of molecular and cellular responses on individual and population dynamics and assess the potential related effects on communities and ecosystem functioning.

The Halophyte Cakile maritima Reduces Phenanthrene Phytotoxicity.

In a previous study, we showed that the halophyte plant model Thellungiella salsuginea was more tolerant to phenanthrene (Polycyclic Aromatic Hydrocarbon: PAH) than its relative glycophyte Arabidopsis thaliana. In the present work, we investigated the potential of another halophyte with higher biomass production, Cakile maritma, to reduce phenanthrene phytotoxicity. Sand was used instead of arable soil with the aim to avoid pollutant degradation by microorganisms or their interaction with the plant. After 6 weeks of treatment by 500 ppm phenanthrene (Phe), stressed plants showed a severe reduction (-73%) in their whole biomass, roots being more affected than leaves and stems. In parallel, Guaiacol peroxidase (GPX) activity was increased by 185 and 62% in leaves and roots, respectively. Non-enzymatic antioxidant capacity (assayed by ABTS test) was maintained unchanged in all plant organs. The model halophytic plant Thellungiella salsuginea was used as a biomarker of phenanthrene stress severity and was grown at 0 (control), 125, 250, and 375 ppm. T. salsuginea plants grown on the sand previously contaminated by 500 ppm Phe then treated by C. maritma culture (phytoremediation culture) showed similar biomass production as plants subjected to 125 ppm Phe. This suggests that the phytotoxic effects of phenanthrene were reduced by 75% by the 6-week treatment by C. maritima. Our findings indicate that C. maritima can constitute a potentially good candidate for PAH phytoremediation.

γ-Aminobutyric acid transaminase deficiency impairs central carbon metabolism and leads to cell wall defects during salt stress in Arabidopsis roots.

Environmental constraints challenge cell homeostasis and thus require a tight regulation of metabolic activity. We have previously reported that the γ-aminobutyric acid (GABA) metabolism is crucial for Arabidopsis salt tolerance as revealed by the NaCl hypersensitivity of the GABA transaminase (GABA-T, At3g22200) gaba-t/pop2-1 mutant. In this study, we demonstrate that GABA-T deficiency during salt stress causes root and hypocotyl developmental defects and alterations of cell wall composition. A comparative genome-wide transcriptional analysis revealed that expression levels of genes involved in carbon metabolism, particularly sucrose and starch catabolism, were found to increase upon the loss of GABA-T function under salt stress conditions. Consistent with the altered mutant cell wall composition, a number of cell wall-related genes were also found differentially expressed. A targeted quantitative analysis of primary metabolites revealed that glutamate (GABA precursor) accumulated while succinate (the final product of GABA metabolism) significantly decreased in mutant roots after 1 d of NaCl treatment. Furthermore, sugar concentration was twofold reduced in gaba-t/pop2-1 mutant roots compared with wild type. Together, our results provide strong evidence that GABA metabolism is a major route for succinate production in roots and identify GABA as a major player of central carbon adjustment during salt stress.

Carbon dynamics, development and stress responses in Arabidopsis: involvement of the APL4 subunit of ADP-glucose pyrophosphorylase (starch synthesis).

An Arabidopsis thaliana T-DNA insertional mutant was identified and characterized for enhanced tolerance to the singlet-oxygen-generating herbicide atrazine in comparison to wild-type. This enhanced atrazine tolerance mutant was shown to be affected in the promoter structure and in the regulation of expression of the APL4 isoform of ADP-glucose pyrophosphorylase, a key enzyme of the starch biosynthesis pathway, thus resulting in decrease of APL4 mRNA levels. The impact of this regulatory mutation was confirmed by the analysis of an independent T-DNA insertional mutant also affected in the promoter of the APL4 gene. The resulting tissue-specific modifications of carbon partitioning in plantlets and the effects on plantlet growth and stress tolerance point out to specific and non-redundant roles of APL4 in root carbon dynamics, shoot-root relationships and sink regulations of photosynthesis. Given the effects of exogenous sugar treatments and of endogenous sugar levels on atrazine tolerance in wild-type Arabidopsis plantlets, atrazine tolerance of this apl4 mutant is discussed in terms of perception of carbon status and of investment of sugar allocation in xenobiotic and oxidative stress responses.

GABA accumulation causes cell elongation defects and a decrease in expression of genes encoding secreted and cell wall-related proteins in Arabidopsis thaliana.

GABA (γ-aminobutyric acid), a non-protein amino acid, is a signaling factor in many organisms. In plants, GABA is known to accumulate under a variety of stresses. However, the consequence of GABA accumulation, especially in vegetative tissues, remains poorly understood. Moreover, gene expression changes as a consequence of GABA accumulation in plants are largely unknown. The pop2 mutant, which is defective in GABA catabolism and accumulates GABA, is a good model to examine the effects of GABA accumulation on plant development. Here, we show that the pop2 mutants have pollen tube elongation defects in the transmitting tract of pistils. Additionally, we observed growth inhibition of primary root and dark-grown hypocotyl, at least in part due to cell elongation defects, upon exposure to exogenous GABA. Microarray analysis of pop2-1 seedlings grown in GABA-supplemented medium revealed that 60% of genes whose expression decreased encode secreted proteins. Besides, functional classification of genes with decreased expression in the pop2-1 mutant showed that cell wall-related genes were significantly enriched in the microarray data set, consistent with the cell elongation defects observed in pop2 mutants. Our study identifies cell elongation defects caused by GABA accumulation in both reproductive and vegetative tissues. Additionally, our results show that genes that encode secreted and cell wall-related proteins may mediate some of the effects of GABA accumulation. The potential function of GABA as a growth control factor under stressful conditions is discussed.