Differential location of alpha-expansin proteins during the accommodation of root cells to an arbuscular mycorrhizal fungus

Enhancing arbuscular mycorrhiza symbiosis effectiveness through the involvement of the tomato GRAS transcription factor SCL3/SlGRAS18

Arbuscular mycorrhizal (AM) fungi improve plant growth, nutrition, fitness and stress tolerance while AM fungi obtain carbohydrates and lipids from the host. This whole process of mutual benefit requires substantial alterations in the structural and functional aspects of the host root cells. These modifications ultimately culminate in the formation of arbuscules, which are specialized intraradical and highly branched fungal structures. Arbuscule-containing cells undergo massive reprogramming to hosting arbuscule and members of the GRAS transcription factor family have been characterized as AM inducible genes which play a pivotal role in these process. Here, we show a functional analysis for the GRAS transcription factor SCL3/SlGRAS18 in tomato. SlGRAS18 interacts with SlDELLA, a central regulator of AM formation. Silencing of SlGRAS18 positively impacts arbuscule development and the improvement in symbiotic status, favouring flowering and therefore progress in the formation and development of fruits in SlGRAS18 silenced plants which parallel to a discernible pattern of mineral nutrient redistribution in leaves. Our results advance the knowledge of GRAS transcription factors involved in the formation and establishment of AM symbiosis and provide experimental evidence for how specific genetic alterations can lead to more effective AM symbiosis.

EXPANSIN15 is involved in flower and fruit development in Arabidopsis

KEY MESSAGE

EXPANSIN15 is involved in petal cell morphology and size, the fusion of the medial tissues in the gynoecium and expansion of fruit valve cells. It genetically interacts with SPATULA and FRUITFULL. Cell expansion is fundamental for the formation of plant tissues and organs, contributing to their final shape and size during development. To better understand this process in flower and fruit development, we have studied the EXPANSIN15 (EXPA15) gene, which showed expression in petals and in the gynoecium. By analyzing expa15 mutant alleles, we found that EXPA15 is involved in petal shape and size determination, by affecting cell morphology and number. EXPA15 also has a function in fruit size, by affecting cell size and number. Furthermore, EXPA15 promotes fusion of the medial tissues in the gynoecium. In addition, we observed genetic interactions with the transcription factors SPATULA (SPT) and FRUITFULL (FUL) in gynoecium medial tissue fusion, style and stigma development and fruit development in Arabidopsis. These findings contribute to the importance of EXPANSINS in floral and fruit development in Arabidopsis.

At the core of the endomycorrhizal symbioses: intracellular fungal structures in orchid and arbuscular mycorrhiza

Arbuscular (AM) and orchid (OrM) mycorrhiza are the most widespread mycorrhizal symbioses among flowering plants, formed by distinct fungal and plant species. They are both endosymbioses because the fungal hyphae can enter inside the plant cell to develop intracellular fungal structures that are surrounded by the plant membrane. The symbiotic plant-fungus interface is considered to be the major site of nutrient transfer to the host plant. We summarize recent data on nutrient transfer in OrM and compare the development and function of the arbuscules formed in AM and the pelotons formed in OrM in order to outline differences and conserved traits. We further describe the unexpected similarities in the form and function of the intracellular mycorrhizal fungal structures observed in orchids and in the roots of mycoheterotrophic plants forming AM. We speculate that these similarities may be the result of convergent evolution of mycorrhizal types in mycoheterotrophic plants and highlight knowledge gaps and new research directions to explore this scenario.

Hormone-regulated expansins: Expression, localization, and cell wall biomechanics in Arabidopsis root growth

Expansins facilitate cell expansion by mediating pH-dependent cell wall (CW) loosening. However, the role of expansins in controlling CW biomechanical properties in specific tissues and organs remains elusive. We monitored hormonal responsiveness and spatial specificity of expression and localization of expansins predicted to be the direct targets of cytokinin signaling in Arabidopsis (Arabidopsis thaliana). We found EXPANSIN1 (EXPA1) homogenously distributed throughout the CW of columella/lateral root cap, while EXPA10 and EXPA14 localized predominantly at 3-cell boundaries in the epidermis/cortex in various root zones. EXPA15 revealed cell-type-specific combination of homogenous vs. 3-cell boundaries localization. By comparing Brillouin frequency shift and AFM-measured Young's modulus, we demonstrated Brillouin light scattering (BLS) as a tool suitable for non-invasive in vivo quantitative assessment of CW viscoelasticity. Using both BLS and AFM, we showed that EXPA1 overexpression upregulated CW stiffness in the root transition zone (TZ). The dexamethasone-controlled EXPA1 overexpression induced fast changes in the transcription of numerous CW-associated genes, including several EXPAs and XYLOGLUCAN:XYLOGLUCOSYL TRANSFERASEs (XTHs), and associated with rapid pectin methylesterification determined by in situ Fourier-transform infrared spectroscopy in the root TZ. The EXPA1-induced CW remodeling is associated with the shortening of the root apical meristem, leading to root growth arrest. Based on our results, we propose that expansins control root growth by a delicate orchestration of CW biomechanical properties, possibly regulating both CW loosening and CW remodeling.

Gibberellic Acid Inhibits Dendrobium nobile-Piriformospora Symbiosis by Regulating the Expression of Cell Wall Metabolism Genes

Orchid seeds lack endosperms and depend on mycorrhizal fungi for germination and nutrition acquisition under natural conditions. Piriformospora indica is a mycorrhizal fungus that promotes seed germination and seedling development in epiphytic orchids, such as Dendrobium nobile. To understand the impact of P. indica on D. nobile seed germination, we examined endogenous hormone levels by using liquid chromatography-mass spectrometry. We performed transcriptomic analysis of D. nobile protocorm at two developmental stages under asymbiotic germination (AG) and symbiotic germination (SG) conditions. The result showed that the level of endogenous IAA in the SG protocorm treatments was significantly higher than that in the AG protocorm treatments. Meanwhile, GA3 was only detected in the SG protocorm stages. IAA and GA synthesis and signaling genes were upregulated in the SG protocorm stages. Exogenous GA3 application inhibited fungal colonization inside the protocorm, and a GA biosynthesis inhibitor (PAC) promoted fungal colonization. Furthermore, we found that PAC prevented fungal hyphae collapse and degeneration in the protocorm, and differentially expressed genes related to cell wall metabolism were identified between the SG and AG protocorm stages. Exogenous GA3 upregulated SRC2 and LRX4 expression, leading to decreased fungal colonization. Meanwhile, GA inhibitors upregulated EXP6, EXB16, and EXP10-2 expression, leading to increased fungal colonization. Our findings suggest that GA regulates the expression of cell wall metabolism genes in D. nobile, thereby inhibiting the establishment of mycorrhizal symbiosis.

Characterization of Arbuscular Mycorrhizal Effector Proteins

Plants are colonized by various fungi with both pathogenic and beneficial lifestyles. One type of colonization strategy is through the secretion of effector proteins that alter the plant's physiology to accommodate the fungus. The oldest plant symbionts, the arbuscular mycorrhizal fungi (AMF), may exploit effectors to their benefit. Genome analysis coupled with transcriptomic studies in different AMFs has intensified research on the effector function, evolution, and diversification of AMF. However, of the current 338 predicted effector proteins from the AM fungus Rhizophagus irregularis, only five have been characterized, of which merely two have been studied in detail to understand which plant proteins they associate with to affect the host physiology. Here, we review the most recent findings in AMF effector research and discuss the techniques used for the functional characterization of effector proteins, from their in silico prediction to their mode of action, with an emphasis on high-throughput approaches for the identification of plant targets of the effectors through which they manipulate their hosts.

Pectin modifications at the symbiotic interface

Plant cells are surrounded by a structured cell wall, which not only defines cell shape but also provides a structural barrier for protection against pathogen infection. However, the presence of this barrier does not impede the establishment of mutualistic symbioses between plants and several microbes (e.g. ectomycorrhizal fungi, arbuscular mycorrhizal fungi, and rhizobia). To establish such beneficial associations, symbiotic microbes need to colonize the plant tissues via intercellular and/or intracellular infection, a process that requires cell wall modifications. Although cell wall composition and changes during this process have interested researchers for years, the functional characterization of the molecular players involved is still limited. In this viewpoint, based on several new studies, I discuss how the PME-PL/PG pathway mediates cell wall pectin modifications at the symbiotic interface and highlight further research directions which can broaden our understanding of how beneficial root symbioses are established.

Long-lasting impact of chitooligosaccharide application on strigolactone biosynthesis and fungal accommodation promotes arbuscular mycorrhiza in Medicago truncatula

The establishment of arbuscular mycorrhiza (AM) between plants and Glomeromycotina fungi is preceded by the exchange of chemical signals: fungal released Myc-factors, including chitooligosaccharides (CO) and lipo-chitooligosaccharides (LCO), activate plant symbiotic responses, while root-exuded strigolactones stimulate hyphal branching and boost CO release. Furthermore, fungal signaling reinforcement through CO application was shown to promote AM development in Medicago truncatula, but the cellular and molecular bases of this effect remained unclear. Here, we focused on long-term M. truncatula responses to CO treatment, demonstrating its impact on the transcriptome of both mycorrhizal and nonmycorrhizal roots over several weeks and providing an insight into the mechanistic bases of the CO-dependent promotion of AM colonization. CO treatment caused the long-lasting regulation of strigolactone biosynthesis and fungal accommodation-related genes. This was mirrored by an increase in root didehydro-orobanchol content, and the promotion of accommodation responses to AM fungi in root epidermal cells. Lastly, an advanced downregulation of AM symbiosis marker genes was observed at the latest time point in CO-treated plants, in line with an increased number of senescent arbuscules. Overall, CO treatment triggered molecular, metabolic, and cellular responses underpinning a protracted acceleration of AM development.

Microtubule cytoskeleton and mycorrhizal roots

For the establishment of the Arbuscular Mycorrhiza (AM) symbiosis it is essential that epidermis and cortical cells from plant roots suffer a strong reorganization to allow the penetration of intracellular fungal hyphae. In the same manner, the new formation of a periarbuscular membrane and a symbiotic interface with specific compositions are required for a functional symbiosis. It is believed that the cytoskeleton of the plant host plays an essential role in these processes, particularly the microtubule (MT) cytoskeleton, as huge modifications have been observed in the MT array of root cells accompanying the establishment of the AM symbiosis. Recent research has established a link between microtubule rearrangements and arbuscule functioning. However, further research is required to elucidate the specific functions of MT cytoskeleton along the different stages of the arbuscule life cycle and to unravel the signals triggering these changes.

Overexpression of DsEXLA2 gene from Dendrocalamus sinicus accelerates the plant growth rate of Arabidopsis

Expansins play crucial roles in cell wall loosening and a range of life activities involving cell wall modification. Nevertheless, the biological functions of expansin genes during fast growth of bamboo remain unclear. In this study, Dendrocalamus sinicus, the largest and fastest growing bamboo species in the world, was used as the research material, and the full length of DsEXLA2 was cloned. Bioinformatics analysis revealed that DsEXLA2 contained expansin family typical domains (DPBB_1 and Pollen_allerg_1, CDRC motif) and amino acid sequence was highly conserved among different species. The expression level of DsEXLA2 increased from top section to basal section in different internodes. Subcellular localization verified that DsEXLA2 protein was located in the cell wall. Further genetic transformation studies in Arabidopsis indicated that compared with the wild type, DsEXLA2 overexpressed transgenic plants exhibited higher plant height, thicker stem, larger leaf, and less epidermal hair number and smaller stomatal aperture in the prophase and metaphase of growth. In addition, the cellulose content in the stem of transgenic plants was increased, and cell wall was thickened significantly. Moreover, a total of 1656 differentially expressed genes (DEGs) were identified by RNA-seq. The upregulated genes were predominantly enriched in the plant-pathogen interaction, MAPK signaling pathway-plant, plant hormone signal transduction, lipid metabolism and amino acid metabolism, while the downregulated genes were mainly enriched in energy metabolism, carbohydrate metabolism, plant hormone signal transduction and ribosome. These data implied that overexpression of DsEXLA2 gene accelerates the plant growth rate of Arabidopsis. This study is helpful to reveal the molecular mechanism of DsEXLA2 in culm growth and development of D. sinicus, and to understand the rapid growth of bamboos.

Dual Roles of OsGH3.2 in Modulating Rice Root Morphology and Affecting Arbuscular Mycorrhizal Symbiosis

Several angiosperm GRETCHEN HAGEN 3 (GH3) genes, including tomato SlGH3.4 and rice OsGH3.2 are induced during arbuscular mycorrhizal (AM) symbiosis, but their functions remain largely unclear. Recently, tomato SlGH3.4 was suggested to negatively regulate arbuscule incidence via decreasing auxin levels in colonized cells. In this study, by acquiring rice OsGH3.2pro:β-glucuronidase (GUS) transgenic plants and generating Osgh3.2 mutants via CRISPR/Cas9 technique, the roles of OsGH3.2 in modulating rice root morphology and affecting AM symbiosis were investigated through time course experiments. Unlike SlGH3.4, OsGH3.2 showed asymbiotic expression in rice young lateral roots, and its mutation resulted in a "shallow" root architecture. Such root morphological change was also observed under symbiotic condition and it likely promoted AM fungal colonization, as the mutants exhibited higher colonization levels and arbuscule incidence than wild-type at early stages. Similar to SlGH3.4, OsGH3.2 showed symbiotic expression in cortical cells that have formed mature arbuscules. At late stages of symbiosis, Osgh3.2 mutants showed elongated cortical cells and larger arbuscules than wild-type, indicating elevated auxin level in the colonized cells. Together, these results revealed both asymbiotic and symbiotic roles of OsGH3.2 in modulating rice root architecture and controlling auxin levels in arbusculated cells, which further affected colonization rate and arbuscule phenotype.

Potential use of arbuscular mycorrhizal fungi for simultaneous mitigation of arsenic and cadmium accumulation in rice

Rice polluted by metal(loid)s, especially arsenic (As) and cadmium (Cd), imposes serious health risks. Numerous studies have demonstrated that the obligate plant symbionts arbuscular mycorrhizal fungi (AMF) can reduce As and Cd concentrations in rice. The behaviours of metal(loid)s in the soil-rice-AMF system are of significant interest for scientists in the fields of plant biology, microbiology, agriculture, and environmental science. We review the mechanisms of As and Cd accumulation in rice with and without the involvement of AMF. In the context of the soil-rice-AMF system, we assess and discuss the role of AMF in affecting soil ion mobility, chemical forms, transport pathways (including the symplast and apoplast), and genotype variation. A potential strategy for AMF application in rice fields is considered, followed by future research directions to improve theoretical understanding and encourage field application.

Root cortex development is fine-tuned by the interplay of MIGs, SCL3 and DELLAs during arbuscular mycorrhizal symbiosis

Root development is a crucial process that determines the ability of plants to acquire nutrients, adapt to the substrate and withstand changing environmental conditions. Root plasticity is controlled by a plethora of transcriptional regulators that allow, in contrast to tissue development in animals, post-embryonic changes that give rise to new tissue and specialized cells. One of these changes is the accommodation in the cortex of hyperbranched hyphae of symbiotic arbuscular mycorrhizal (AM) fungi, called arbuscules. Arbuscule-containing cells undergo massive reprogramming to coordinate developmental changes with transport processes. Here we describe a novel negative regulator of arbuscule development, MIG3. MIG3 induces and interacts with SCL3, both of which modulate the activity of the central regulator DELLA, restraining cortical cell growth. As in a tug-of-war, MIG3-SCL3 antagonizes the function of the complex MIG1-DELLA, which promotes the cell expansion required for arbuscule development, adjusting cell size during the dynamic processes of the arbuscule life cycle. Our results in the legume plant Medicago truncatula advance the knowledge of root development in dicot plants, showing the existence of additional regulatory elements not present in Arabidopsis that fine-tune the activity of conserved central modules.

Cell wall modification induced by an arbuscular mycorrhizal fungus enhanced cadmium fixation in rice root

The chemistry of root cell wall of rice could be changed by inoculation of arbuscular mycorrhizal fungi (AMF). Hydroponic experiments were conducted to investigate the roles of such changes on cadmium (Cd) uptake and distribution in rice. Results showed that inoculation of AM fungus Rhizophagus intraradices (RI) significantly enhanced (p < 0.05) shoot biomass, plant height and root length of rice, and decreased Cd concentration in shoot and root under Cd stress. Moreover, Cd in root was mainly found in pectin and hemicellulose 1 (HC1) components of root cell wall. Inoculation of RI increased the levels of pectin, HC1 and lignin content, accompanied by the increments of L-phenylalanine ammonia-lyase (PAL) and pectin methylesterase (PME) activities. Results of Fourier transform infrared spectroscopy further showed that more hydroxyl and carboxyl groups in root cell wall were observed in mycorrhizal treatment, compared with control. This study demonstrates that cell wall components could be the locations for Cd fixation, which reduced Cd transportation from root to shoot. Inoculation of AMF may remodel root cell wall biosynthesis and affect the characteristics of Cd fixation. The entering and fixing mechanisms should be further studied.

Variations in Growth, Physiology, and Antioxidative Defense Responses of Two Tomato (Solanum lycopersicum L.) Cultivars after Co-Infection of Fusarium oxysporum and Meloidogyne incognita

The soil-borne fungus Fusarium oxysporum (Fo) and the nematode Meloidogyne incognita (Mi) are destructive pathogens that cause substantial yield losses to tomato (Solanum lycopersicum L.) crops worldwide. The present study sought to elucidate the physiological, biochemical, and cytological responses of tomato cultivars (Gailing maofen 802 and Zhongza 09) by root invasion of Fo (1 × 105 CFUmL−1) and Mi (1500 second-stage juveniles (J2) alone and in combination after 14 days. Results revealed that combined inoculation of Fo and Mi significantly increased disease intensity, electrolyte leakage, and hydrogen peroxide and malondialdehyde contents; and decreased photosynthetic capacity and enzyme activity in both cultivars as compared to their solo inoculation. Increasing the disease intensity reduced the maximum morphological traits, such as shoot length, total dry weight, and total chlorophyll contents, in G. maofen 802 (by 32%, 54.2%, and 52.3%, respectively) and Zhongza 09 (by 18%, 32%, and 21%, respectively) as compared to the control. Others factors were also reduced in G. maofen 802 and Zhongza 09, such as photosynthetic capacity (by 70% and 57%, respectively), stomatal conductance (by 86% and 70%, respectively), photochemical quantum yield of photosystem II (YII) (by 36.6% and 29%, respectively), and electron transport rate (by 17.7% and 10%, respectively), after combined inoculation of Fo and Mi. Furthermore, the combined infestation of Fo and Mi resulted in reduced activity of plant-defense-related antioxidants in G. maofen 802 compared with their single application or control. However, these antioxidants were highly up-regulated in Zhongza 09 (by 59%–93%), revealing the induction of tolerance against studied pathogens. The transmission electron microscopy (TEM) results further demonstrated that root cells of Zhongza 09 had unique tetrahedral crystal-like structures in the membrane close to mitochondria under all treatments except control. Therefore, it is concluded that Mi caused severe root damage, suppressed plant growth, depleted antioxidants, and caused high generation of ROS in the presence of Fo as compared to its solo inoculation. Tolerant cultivars adopted different mechanistic strategies at the structural and cellular levels to tolerate the Mi and Fo stresses.

Transcriptomic Responses to Water Deficit and Nematode Infection in Mycorrhizal Tomato Roots

Climate changes include the intensification of drought in many parts of the world, increasing its frequency, severity and duration. However, under natural conditions, environmental stresses do not occur alone, and, in addition, more stressed plants may become more susceptible to attacks by pests and pathogens. Studies on the impact of the arbuscular mycorrhizal (AM) symbiosis on tomato response to water deficit showed that several drought-responsive genes are differentially regulated in AM-colonized tomato plants (roots and leaves) during water deficit. To date, global changes in mycorrhizal tomato root transcripts under water stress conditions have not been yet investigated. Here, changes in root transcriptome in the presence of an AM fungus, with or without water stress (WS) application, have been evaluated in a commercial tomato cultivar already investigated for the water stress response during AM symbiosis. Since root-knot nematodes (RKNs, Meloidogyne incognita) are obligate endoparasites and cause severe yield losses in tomato, the impact of the AM fungal colonization on RKN infection at 7 days post-inoculation was also evaluated. Results offer new information about the response to AM symbiosis, highlighting a functional redundancy for several tomato gene families, as well as on the tomato and fungal genes involved in WS response during symbiosis, underlying the role of the AM fungus. Changes in the expression of tomato genes related to nematode infection during AM symbiosis highlight a role of AM colonization in triggering defense responses against RKN in tomato. Overall, new datasets on the tomato response to an abiotic and biotic stress during AM symbiosis have been obtained, providing useful data for further researches.

Understanding Changes in Tomato Cell Walls in Roots and Fruits: The Contribution of Arbuscular Mycorrhizal Colonization

Modifications in cell wall composition, which can be accompanied by changes in its structure, were already reported during plant interactions with other organisms, such as the mycorrhizal fungi. Arbuscular mycorrhizal (AM) fungi are among the most widespread soil organisms that colonize the roots of land plants, where they facilitate mineral nutrient uptake from the soil in exchange for plant-assimilated carbon. In AM symbiosis, the host plasma membrane invaginates and proliferates around all the developing intracellular fungal structures, and cell wall material is laid down between this membrane and the fungal cell surface. In addition, to improve host nutrition and tolerance/resistance to environmental stresses, AM symbiosis was shown to modulate fruit features. In this study, Comprehensive Microarray Polymer Profiling (CoMMP) technique was used to verify the impact of the AM symbiosis on the tomato cell wall composition both at local (root) and systemic level (fruit). Multivariate data analyses were performed on the obtained datasets looking for the effects of fertilization, inoculation with AM fungi, and the fruit ripening stage. Results allowed for the discernment of cell wall component modifications that were correlated with mycorrhizal colonization, showing a different tomato response to AM colonization and high fertilization, both at the root and the systemic level.

OsEXPA10 mediates the balance between growth and resistance to biotic stress in rice

OsEXPA10 gene coordinates the balance between rice development and biotic resistance. Expansins are proteins that can loosen the cell wall. Previous studies have indicated that expansin-encoding genes were involved in defense against abiotic stress, but little is known about the involvement of expansins in biotic stress. Brown planthopper (BPH) is one of the worst insect pests of rice in the Asia-Pacific planting area, and many efforts have been made to identify and clone BPH-resistance genes for use in breeding resistant cultivars. At the same time, rice blast caused by Magnaporthe grisea is one of the three major diseases that severely affect rice production worldwide. Here, we demonstrated that one rice expansin-encoding gene, OsEXPA10, functions in both rice growth and biotic resistance. Over expression of OsEXPA10 improved rice growth but also increased susceptibility to BPH infestation and blast attack, while knock-down OsEXPA10 gene expression resulted in reduced plant height and grain size, but also increased resistance to BPH and the blast pathogen. These results imply that OsEXPA10 mediates the balance between rice development and biotic resistance.

Manipulation of Bryophyte Hosts by Pathogenic and Symbiotic Microbes

The colonization of plant tissues by pathogenic and symbiotic microbes is associated with a strong and directed effort to reprogram host cells in order to permit, promote and sustain microbial growth. In response to colonization, hosts accommodate or sequester invading microbes by activating a set of complex regulatory programs that initiate symbioses or bolster defenses. Extensive research has elucidated a suite of molecular and physiological responses occurring in plant hosts and their microbial partners; however, this information is mostly limited to model systems representing evolutionarily young plant lineages such as angiosperms. The extent to which these processes are conserved across land plants is therefore poorly understood. In this review, we outline key aspects of host reprogramming that occur during plant-microbe interactions in early diverging land plants belonging to the bryophytes (liverworts, hornworts and mosses). We discuss how further knowledge of bryophyte-microbe interactions will advance our understanding of how plants and microbes co-operated and clashed during the conquest of land.

Transcriptome changes induced by arbuscular mycorrhizal fungi in sunflower (Helianthus annuus L.) roots

Arbuscular mycorrhizal (AM) fungi are essential elements of soil fertility, plant nutrition and productivity, facilitating soil mineral nutrient uptake. Helianthus annuus is a non-model, widely cultivated species. Here we used an RNA-seq approach for evaluating gene expression variation at early and late stages of mycorrhizal establishment in sunflower roots colonized by the arbuscular fungus Rhizoglomus irregulare. mRNA was isolated from roots of plantlets at 4 and 16 days after inoculation with the fungus. cDNA libraries were built and sequenced with Illumina technology. Differential expression analysis was performed between control and inoculated plants. Overall 726 differentially expressed genes (DEGs) between inoculated and control plants were retrieved. The number of up-regulated DEGs greatly exceeded the number of down-regulated DEGs and this difference increased in later stages of colonization. Several DEGs were specifically involved in known mycorrhizal processes, such as membrane transport, cell wall shaping, and other. We also found previously unidentified mycorrhizal-induced transcripts. The most important DEGs were carefully described in order to hypothesize their roles in AM symbiosis. Our data add a valuable contribution for deciphering biological processes related to beneficial fungi and plant symbiosis, adding an Asteraceae, non-model species for future comparative functional genomics studies.

The Novel Secreted Meloidogyne incognita Effector MiISE6 Targets the Host Nucleus and Facilitates Parasitism in Arabidopsis

Meloidogyne incognita is highly specialized parasite that interacts with host plants using a range of strategies. The effectors are synthesized in the esophageal glands and secreted into plant cells through a needle-like stylet during parasitism. In this study, based on RNA-seq and bioinformatics analysis, we predicted 110 putative Meloidogyne incognita effectors that contain nuclear localization signals (NLSs). Combining the Burkholderia glumae-pEDV based screening system with subcellular localization, from 20 randomly selected NLS effector candidates, we identified an effector MiISE6 that can effectively suppress B. glumae-induced cell death in Nicotiana benthamiana, targets to the nuclei of plant cells, and is highly expressed in early parasitic J2 stage. Sequence analysis showed that MiISE6 is a 157-amino acid peptide, with an OGFr_N domain and two NLS motifs. Hybridization in situ verified that MiISE6 is expressed in the subventral esophageal glands. Yeast invertase secretion assay validated the function of the signal peptide harbored in MiISE6. Transgenic Arabidopsis thaliana plants expressing MiISE6 become more susceptible to M. incognita. Inversely, the host-derived RNAi of MiISE6 of the nematode can decrease its parasitism on host. Based on transcriptome analysis of the MiISE6 transgenic Arabidopsis samples and the wild-type samples, we obtained 852 differentially expressed genes (DEGs). Integrating Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses, we found that expression of MiISE6 in Arabidopsis can suppress jasmonate signaling pathway. In addition, the expression of genes related to cell wall modification and the ubiquitination proteasome pathway also have detectable changes in the transgenic plants. Results from the present study suggest that MiISE6 is involved in interaction between nematode-plant, and plays an important role during the early stages of parasitism by interfering multiple signaling pathways of plant. Moreover, we found homologs of MiISE6 in other sedentary nematodes, Meloidogyne hapla and Globodera pallida. Our experimental results provide evidence to decipher the molecular mechanisms underlying the manipulation of host immune defense responses by plant parasitic nematodes, and transcriptome data also provide useful information for further study nematode-plant interactions.

Analysis of tomato plasma membrane H(+)-ATPase gene family suggests a mycorrhiza-mediated regulatory mechanism conserved in diverse plant species

In plants, the plasma membrane H(+)-ATPase (HA) is considered to play a crucial role in regulating plant growth and respoding to environment stresses. Multiple paralogous genes encoding different isozymes of HA have been identified and characterized in several model plants, while limited information of the HA gene family is available to date for tomato. Here, we describe the molecular and expression features of eight HA-encoding genes (SlHA1-8) from tomato. All these genes are interrupted by multiple introns with conserved positions. SlHA1, 2, and 4 were widely expressed in all tissues, while SlHA5, 6, and 7 were almost only expressed in flowers. SlHA8, the transcripts of which were barely detectable under normal or nutrient-/salt-stress growth conditions, was strongly activated in arbuscular mycorrhizal (AM) fungal-colonized roots. Extreme lack of SlHA8 expression in M161, a mutant defective to AM fungal colonization, provided genetic evidence towards the dependence of its expression on AM symbiosis. A 1521-bp SlHA8 promoter could direct the GUS reporter expression specifically in colonized cells of transgenic tobacco, soybean, and rice mycorrhizal roots. Promoter deletion assay revealed a 223-bp promoter fragment of SlHA8 containing a variant of AM-specific cis-element MYCS (vMYCS) sufficient to confer the AM-induced activity. Targeted deletion of this motif in the corresponding promoter region causes complete abolishment of GUS staining in mycorrhizal roots. Together, these results lend cogent evidence towards the evolutionary conservation of a potential regulatory mechanism mediating the activation of AM-responsive HA genes in diverse mycorrhizal plant species.

Genome-wide identification and expression analysis of the expansin gene family in tomato

Plant expansins are capable of inducing pH-dependent cell wall extension and stress relaxation. They may be useful as targets for crop improvement to enhance fruit development and stress resistance. Tomato is a major agricultural crop and a model plant for studying fruit development. Because only some tomato expansins have been studied, a genome-wide analysis of the tomato expansin family is necessary. In this study, we identified 25 SlEXPAs, eight SlEXPBs, one SlEXLA, four SlEXLBs, and five short homologs in the tomato genome. 25 of these genes were identified as being expressed. Bioinformatic analysis showed that although tomato expansins share similarities with those from other plants, they also exhibit specific features regarding genetic structure and amino acid sequences, which indicates a unique evolutionary process. Segmental and tandem duplication events have played important roles in expanding the tomato expansin family. Additionally, the 3-exon/2-intron structure may form the basic organization of expansin genes. We identified new expansin genes preferentially expressed in fruits (SlEXPA8, SlEXPB8, and SlEXLB1), roots (SlEXPA9, SlEXLB2, and SlEXLB4), and floral organs. Among the analyzed genes those that were inducible by hormone or stress treatments, including SlEXPA3, SlEXPA7, SlEXPB1-B2, SlEXPB8, SlEXLB1-LB2, and SlEXLB4. Our findings may further clarify the biological activities of tomato expansins, especially those related to fruit development and stress resistance, and contribute to the genetic modification of tomato plants to improve crop quality and yield.

Plant expansins: diversity and interactions with plant cell walls

Expansins were discovered two decades ago as cell wall proteins that mediate acid-induced growth by catalyzing loosening of plant cell walls without lysis of wall polymers. In the interim our understanding of expansins has gotten more complex through bioinformatic analysis of expansin distribution and evolution, as well as through expression analysis, dissection of the upstream transcription factors regulating expression, and identification of additional classes of expansin by sequence and structural similarities. Molecular analyses of expansins from bacteria have identified residues essential for wall loosening activity and clarified the bifunctional nature of expansin binding to complex cell walls. Transgenic modulation of expansin expression modifies growth and stress physiology of plants, but not always in predictable or even understandable ways.

The role of the cell wall compartment in mutualistic symbioses of plants

Plants engage in mutualistic interactions with microbes that improve their mineral nutrient supply. The most wide-spread symbiotic association is arbuscular mycorrhiza (AM), in which fungi of the order Glomeromycota invade roots and colonize the cellular lumen of cortical cells. The establishment of this interaction requires a dedicated molecular-genetic program and a cellular machinery of the plant host. This program is partially shared with the root nodule symbiosis (RNS), which involves prokaryotic partners collectively referred to as rhizobia. Both, AM and RNS are endosymbioses that involve intracellular accommodation of the microbial partner in the cells of the plant host. Since plant cells are surrounded by sturdy cell walls, root penetration and cell invasion requires mechanisms to overcome this barrier while maintaining the cytoplasm of the two partners separate during development of the symbiotic association. Here, we discuss the diverse functions of the cell wall compartment in establishment and functioning of plant symbioses with the emphasis on AM and RNS, and we describe the stages of the AM association between the model organisms Petunia hybrida and Rhizophagus irregularis.

Characterization of soybean β-expansin genes and their expression responses to symbiosis, nutrient deficiency, and hormone treatment

Expansins are plant cell wall-loosening proteins encoded by a superfamily of genes including α-expansin, β-expansin, expansin-like A, and expansin-like B proteins. They play a variety of biological roles during plant growth and development. Expansin genes have been reported in many plant species, and results primarily from graminaceous members indicate that β-expansins are more abundant in monocots than in dicots. Soybean [Glycine max (L.) Merr] is an important legume crop. This work identified nine β-expansin gene family members in soybean (GmEXPBs) that were divided into two distinct classes based on phylogeny and gene structure, with divergence between the two groups occurring more in introns than in exons. A total of 887 hormone-responsive and environmental stress-related putative cis-elements from 188 families were found in the 2-kb upstream region of GmEXPBs. Variations in number and type of cis-elements associated with each gene indicate that the function of these genes is differentially regulated by these signals. Expression analysis confirmed that the family members were ubiquitously, yet differentially expressed in soybean. Responsiveness to nutrient deficiency stresses and regulation by auxin (indole-3-acetic acid) and cytokinin (6-benzylaminopurine) varied among GmEXPBs. In addition, most β-expansin genes were associated with symbiosis of soybean inoculated with Rhizobium or abuscular mycorrhizal fungi (AMF). Taken together, these results systematically investigate the characteristics of the entire GmEXPB family in soybean and comprise the first report analyzing the relationship of GmEXPBs with rhizobial or AMF symbiosis. This information is a valuable step in the process of understanding the expansin protein functions in soybean and opens avenues for continued researches.

Stipe wall extension of Flammulina velutipes could be induced by an expansin-like protein from Helix aspersa

Expansin proteins extend plant cell walls by a hydrolysis-free process that disrupts hydrogen bonding between cell wall polysaccharides. However, it is unknown if this mechanism is operative in mushrooms. Herein we report that the native wall extension activity was located exclusively in the 10 mm apical region of 30 mm Flammulina velutipes stipes. The elongation growth was restricted also to the 9 mm apical region of the stipes where the elongation growth of the 1st millimetre was 40-fold greater than that of the 5th millimetre. Therefore, the wall extension activity represents elongation growth of the stipe. The low concentration of expansin-like protein in F. velutipes stipes prevented its isolation. However, we purified an expansin-like protein from snail stomach juice which reconstituted heat-inactivated stipe wall extension without hydrolytic activity. So the previous hypotheses that stipe wall extension was resulted from hydrolysis of wall polymers by enzymes or disruption of hydrogen bonding of wall polymers exclusively by turgor pressure are challenged. We suggest that stipe wall extension may be mediated by endogenous expansin-like proteins that facilitate cell wall polymer slippage by disrupting noncovalent bonding between glucan chains or chitin chains.

Different transcriptional response to Xanthomonas citri subsp. citri between kumquat and sweet orange with contrasting canker tolerance

Citrus canker disease caused by Xanthomonas citri subsp. citri (Xcc) is one of the most devastating biotic stresses affecting the citrus industry. Meiwa kumquat (Fortunella crassifolia) is canker-resistant, while Newhall navel orange (Citrus sinensis Osbeck) is canker-sensitive. To understand the molecular mechanisms underlying the differences in responses to Xcc, transcriptomic profiles of these two genotypes following Xcc attack were compared by using the Affymetrix citrus genome GeneChip. A total of 794 and 1324 differentially expressed genes (DEGs) were identified as canker-responsive genes in Meiwa and Newhall, respectively. Of these, 230 genes were expressed in common between both genotypes, while 564 and 1094 genes were only significantly expressed in either Meiwa or Newhall. Gene ontology (GO) annotation and Singular Enrichment Analysis (SEA) of the DEGs showed that genes related to the cell wall and polysaccharide metabolism were induced for basic defense in both Meiwa and Newhall, such as chitinase, glucanase and thaumatin-like protein. Moreover, apart from inducing basic defense, Meiwa showed specially upregulated expression of several genes involved in the response to biotic stimulus, defense response, and cation binding as comparing with Newhall. And in Newhall, abundant photosynthesis-related genes were significantly down-regulated, which may be in order to ensure the basic defense. This study revealed different molecular responses to canker disease in Meiwa and Newhall, affording insight into the response to canker and providing valuable information for the identification of potential genes for engineering canker tolerance in the future.

Rhizobium-legume symbiosis shares an exocytotic pathway required for arbuscule formation

Endosymbiotic interactions are characterized by the formation of specialized membrane compartments, by the host in which the microbes are hosted, in an intracellular manner. Two well-studied examples, which are of major agricultural and ecological importance, are the widespread arbuscular mycorrhizal symbiosis and the Rhizobium-legume symbiosis. In both symbioses, the specialized host membrane that surrounds the microbes forms a symbiotic interface, which facilitates the exchange of, for example, nutrients in a controlled manner and, therefore, forms the heart of endosymbiosis. Despite their key importance, the molecular and cellular mechanisms underlying the formation of these membrane interfaces are largely unknown. Recent studies strongly suggest that the Rhizobium-legume symbiosis coopted a signaling pathway, including receptor, from the more ancient arbuscular mycorrhizal symbiosis to form a symbiotic interface. Here, we show that two highly homologous exocytotic vesicle-associated membrane proteins (VAMPs) are required for formation of the symbiotic membrane interface in both interactions. Silencing of these Medicago VAMP72 genes has a minor effect on nonsymbiotic plant development and nodule formation. However, it blocks symbiosome as well as arbuscule formation, whereas root colonization by the microbes is not affected. Identification of these VAMP72s as common symbiotic regulators in exocytotic vesicle trafficking suggests that the ancient exocytotic pathway forming the periarbuscular membrane compartment has also been coopted in the Rhizobium-legume symbiosis.

Mycorrhizal symbiosis stimulates endoreduplication in angiosperms

Symbiotic and parasitic relationships can alter the degree of endoreduplication in plant cells, and a limited number of studies have documented this occurrence in root cells colonized by arbuscular mycorrhizal (AM) fungi. However, this phenomenon has not been tested in a wide range of plant species, including species that are non-endopolyploid and those that do not associate with AM fungi. We grew 37 species belonging to 16 plant families, with a range of genome sizes and a range in the degree of endopolyploidy. The endoreduplication index (EI) was compared between plants that were inoculated with Glomus irregulare and plants that were not inoculated. Of the species colonized with AM fungi, 22 of the 25 species had a significant increase in endopolyploid root nuclei over non-mycorrhizal plants, including species that do not normally exhibit endopolyploidy. Changes in the EI were strongly correlated (R(2) = 0.619) with the proportion of root length colonized by arbuscules. No change was detected in the EI for the 12 non-mycorrhizal species. This work indicates that colonization by symbiotic fungi involves a mechanism to increase nuclear DNA content in roots across many angiosperm groups and is likely linked to increased metabolism and protein production.

Overexpression of PhEXPA1 increases cell size, modifies cell wall polymer composition and affects the timing of axillary meristem development in Petunia hybrida

• Expansins are cell wall proteins required for cell enlargement and cell wall loosening during many developmental processes. The involvement of the Petunia hybrida expansin A1 (PhEXPA1) gene in cell expansion, the control of organ size and cell wall polysaccharide composition was investigated by overexpressing PhEXPA1 in petunia plants. • PhEXPA1 promoter activity was evaluated using a promoter-GUS assay and the protein's subcellular localization was established by expressing a PhEXPA1-GFP fusion protein. PhEXPA1 was overexpressed in transgenic plants using the cauliflower mosaic virus (CaMV) 35S promoter. Fourier transform infrared (FTIR) and chemical analysis were used for the quantitative analysis of cell wall polymers. • The GUS and GFP assays demonstrated that PhEXPA1 is present in the cell walls of expanding tissues. The constitutive overexpression of PhEXPA1 significantly affected expansin activity and organ size, leading to changes in the architecture of petunia plants by initiating premature axillary meristem outgrowth. Moreover, a significant change in cell wall polymer composition in the petal limbs of transgenic plants was observed. • These results support a role for expansins in the determination of organ shape, in lateral branching, and in the variation of cell wall polymer composition, probably reflecting a complex role in cell wall metabolism.

Integration of molecular biology tools for identifying promoters and genes abundantly expressed in flowers of Oncidium Gower Ramsey

BACKGROUND

Orchids comprise one of the largest families of flowering plants and generate commercially important flowers. However, model plants, such as Arabidopsis thaliana do not contain all plant genes, and agronomic and horticulturally important genera and species must be individually studied.

RESULTS

Several molecular biology tools were used to isolate flower-specific gene promoters from Oncidium 'Gower Ramsey' (Onc. GR). A cDNA library of reproductive tissues was used to construct a microarray in order to compare gene expression in flowers and leaves. Five genes were highly expressed in flower tissues, and the subcellular locations of the corresponding proteins were identified using lip transient transformation with fluorescent protein-fusion constructs. BAC clones of the 5 genes, together with 7 previously published flower- and reproductive growth-specific genes in Onc. GR, were identified for cloning of their promoter regions. Interestingly, 3 of the 5 novel flower-abundant genes were putative trypsin inhibitor (TI) genes (OnTI1, OnTI2 and OnTI3), which were tandemly duplicated in the same BAC clone. Their promoters were identified using transient GUS reporter gene transformation and stable A. thaliana transformation analyses.

CONCLUSIONS

By combining cDNA microarray, BAC library, and bombardment assay techniques, we successfully identified flower-directed orchid genes and promoters.

Synergism between cucumber alpha-expansin, fungal endoglucanase and pectin lyase

Several recombinant fungal enzymes (endoglucanase and pectinase) were studied for their interactions with alpha-expansin in cell wall extension and polysaccharide degradation. Both Cel12A and Cel5A were able to hydrolyze cellulose CMC-Na and mixed-linkage beta-glucan. In contrast to Cel5A, Cel12A could also hydrolyze xyloglucan and induce wall extension of cucumber hypocotyls in an in vitro assay. Combining alpha-expansin, even at high concentrations, with Cel12A did not enhance the maximum/final wall extension rate induced by Cel12A alone. These results strongly suggest that modification/degradation of the xyloglucan molecule/network is the key for cell wall extension, and alpha-expansin and Cel12A may share the same acting site in the substrate. Pectinase (Pel1, a pectin lyase) enhanced alpha-expansin-induced wall extension in a concentration-dependent manner, suggesting that the pectin network may normally regulate accessibility of expansin to the xyloglucan-cellulose complex. alpha-Expansin enhanced Cel12A's hydrolytic activity on cellulose CMC-Na but not on xyloglucan and beta-glucan. Expansin did not affect Cel5A's hydrolytic activity. Interestingly, expansin also enhanced Pel1's activity on degrading high esterified pectin. A potential explanation for why expansin could synergistically interact with only certain enzymes on specific polysaccharides is discussed. Additional results also suggested that cell wall swelling may not be a significant event during the action of expansin and hydrolases.

Microarray analysis and functional tests suggest the involvement of expansins in the early stages of symbiosis of the arbuscular mycorrhizal fungus Glomus intraradices on tomato (Solanum lycopersicum)

Arbuscular mycorrhizal (AM) symbiosis occurs between fungi of the phylum Glomeromycota and most terrestrial plants. However, little is known about the molecular symbiotic signalling between AM fungi (AMFs) and non-leguminous plant species. We sought to further elucidate the molecular events occurring in tomato, a non-leguminous host plant, during the early, pre-symbiotic stage of AM symbiosis, i.e. immediately before and after contact between the AMF (Glomus intraradices) and the host. We adopted a semi-synchronized AMF root infection protocol, followed by genomic-scale, microarray-based, gene expression profiling at several defined time points during pre-symbiotic AM stages. The microarray results suggested differences in the number of differentially expressed genes and in the differential regulation of several functional groups of genes at the different time points examined. The microarray results were validated and one of the genes induced during contact between AMF and tomato, the expansin-like EXLB1, was functionally analysed. Expansins, encoded by a large multigene family, facilitate plant cell expansion. However, no biological or biochemical function has yet been established for plant-originated expansin-like proteins. EXLB1 transcripts were localized early during the association to cells that may perceive the fungal signal, and later during the association in close proximity to sites of AMF hypha-root colonization. Moreover, in transgenic roots, we demonstrated that a reduction in the steady-state level of EXLB1 transcript was correlated with a reduced rate of infection, reduced arbuscule expansion and reduced AMF spore formation.

Activation of the indole-3-acetic acid-amido synthetase GH3-8 suppresses expansin expression and promotes salicylate- and jasmonate-independent basal immunity in rice

New evidence suggests a role for the plant growth hormone auxin in pathogenesis and disease resistance. Bacterial infection induces the accumulation of indole-3-acetic acid (IAA), the major type of auxin, in rice (Oryza sativa). IAA induces the expression of expansins, proteins that loosen the cell wall. Loosening the cell wall is key for plant growth but may also make the plant vulnerable to biotic intruders. Here, we report that rice GH3-8, an auxin-responsive gene functioning in auxin-dependent development, activates disease resistance in a salicylic acid signaling- and jasmonic acid signaling-independent pathway. GH3-8 encodes an IAA-amino synthetase that prevents free IAA accumulation. Overexpression of GH3-8 results in enhanced disease resistance to the rice pathogen Xanthomonas oryzae pv oryzae. This resistance is independent of jasmonic acid and salicylic acid signaling. Overexpression of GH3-8 also causes abnormal plant morphology and retarded growth and development. Both enhanced resistance and abnormal development may be caused by inhibition of the expression of expansins via suppressed auxin signaling.

Check-in procedures for plant cell entry by biotrophic microbes

Significant advances in the cell biology of plant-microbe interactions have been achieved recently, to a large extent based on new technical approaches such as the use of fluorescent protein tags in model plants exploited in conjunction with available genetic resources. They have highlighted the pivotal role played by epidermal cells as the first site at which direct cell-to-cell contact takes place between the plant and microbes it may host. Here, we compare the cellular aspects of early biotrophic interactions with symbiotic and pathogenic microbes and evaluate the hypothesis that their hosting by plant cells share common traits related to the necessity of preserving host-cell integrity. The cellular events that accompany cell entry by the different biotrophs are divided into three categories, depending on whether the cellular changes are triggered by diffusible molecules, direct contact, or cell lumen penetration. Similarities and differences mirror the nutritional and developmental strategies of each plant-interacting organism, underlining the fact that plant cell entry represents a key aspect in the establishment of biotrophy.

Transcriptome analysis of arbuscular mycorrhizal roots during development of the prepenetration apparatus

Information on changes in the plant transcriptome during early interaction with arbuscular mycorrhizal (AM) fungi is still limited since infections are usually not synchronized and plant markers for early stages of colonization are not yet available. A prepenetration apparatus (PPA), organized in epidermal cells during appressorium development, has been reported to be responsible for assembling a trans-cellular tunnel to accommodate the invading fungus. Here, we used PPAs as markers for cell responsiveness to fungal contact to investigate gene expression at this early stage of infection with minimal transcript dilution. PPAs were identified by confocal microscopy in transformed roots of Medicago truncatula expressing green fluorescent protein-HDEL, colonized by the AM fungus Gigaspora margarita. A PPA-targeted suppressive-subtractive cDNA library was built, the cDNAs were cloned and sequenced, and, consequently, 107 putative interaction-specific genes were identified. The expression of a subset of 15 genes, selected by reverse northern dot blot screening, and five additional genes, potentially involved in PPA formation, was analyzed by real-time reverse transcription-polymerase chain reaction and compared with an infection stage, 48 h after the onset of the PPA. Comparison of the expression profile of G. margarita-inoculated wild type and the mycorrhiza-defective dmi3-1 mutant of M. truncatula revealed that an expansin-like gene, expressed in wild-type epidermis during PPA development, can be regarded as an early host marker for successful mycorrhization. A putative Avr9/Cf-9 rapidly elicited gene, found to be up-regulated in the mutant, suggests novel regulatory roles for the DMI3 protein in the early mycorrhization process.

Fungal and plant gene expression in arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizas (AMs) are a unique example of symbiosis between two eukaryotes, soil fungi and plants. This association induces important physiological changes in each partner that lead to reciprocal benefits, mainly in nutrient supply. The symbiosis results from modifications in plant and fungal cell organization caused by specific changes in gene expression. Recently, much effort has gone into studying these gene expression patterns to identify a wider spectrum of genes involved. We aim in this review to describe AM symbiosis in terms of current knowledge on plant and fungal gene expression profiles.

Expansins are involved in the formation of nematode-induced syncytia in roots of Arabidopsis thaliana

Parasitism of the cyst nematode Heterodera schachtii is characterized by the formation of syncytial feeding structures in the host root. Syncytia are formed by the fusion of root cells, accompanied by local cell wall degradation, fusion of protoplasts and hypertrophy. Expansins are cell wall-loosening proteins involved in growth and cell wall disassembly. In this study, we analysed whether members of the expansin gene family are specifically and developmentally regulated during syncytium formation in the roots of Arabidopsis thaliana. We used PCR to screen a cDNA library of 5-7-day-old syncytia for expansin transcripts with primers differentiating between 26 alpha- and three beta-expansin cDNAs. AtEXPA1, AtEXPA3, AtEXPA4, AtEXPA6, AtEXPA8, AtEXPA10, AtEXPA15, AtEXPA16, AtEXPA20 and AtEXPB3 could be amplified from the library. In a semi-quantitative RT-PCR and a Genechip analysis AtEXPA3, AtEXPA6, AtEXPA8, AtEXPA10 and AtEXPA16 were found to be upregulated specifically in syncytia, but not to be transcribed in surrounding root tissue. Histological analyses were performed with the aid of promoter::GUS lines and in situ RT-PCR. Results from both approaches supported the specific expression pattern. Among the specifically expressed genes, AtEXPA3 and AtEXPA16 turned out to be of special interest as they are shoot-specific in uninfected plants. We conclude that syncytium formation involves the specific regulation of expansin genes, indicating that the encoded expansins take part in cell growth and cell wall disassembly during syncytium formation.

The expansin superfamily

The expansin superfamily of plant proteins is made up of four families, designated alpha-expansin, beta-expansin, expansin-like A and expansin-like B. alpha-Expansin and beta-expansin proteins are known to have cell-wall loosening activity and to be involved in cell expansion and other developmental events during which cell-wall modification occurs. Proteins in these two families bind tightly to the cell wall and their activity is typically assayed by their stimulation of cell-wall extension and stress relaxation; no bona fide enzymatic activity has been detected for these proteins. Alpha-expansin proteins and some, but not all, beta-expansin proteins are implicated as catalysts of 'acid growth', the enlargement of plant cells stimulated by low extracellular pH. A divergent group of beta-expansin genes are expressed at high levels in the pollen of grasses but not of other plant groups. They probably function to loosen maternal cell walls during growth of the pollen tube towards the ovary. All expansins consist of two domains; domain 1 is homologous to the catalytic domain of proteins in the glycoside hydrolase family 45 (GH45); expansin domain 2 is homologous to group-2 grass pollen allergens, which are of unknown biological function. Experimental evidence suggests that expansins loosen cell walls via a nonenzymatic mechanism that induces slippage of cellulose microfibrils in the plant cell wall.

Expression profiles of a phosphate transporter gene (GmosPT) from the endomycorrhizal fungus Glomus mosseae

Arbuscular mycorrhizal (AM) fungi have long been shown to successfully contribute to phosphate uptake by plant roots. The first step of the fungus-mediated uptake is carried out by fungal membrane Pi transporters (PT) that transfer Pi from the soil into the extraradical hyphae. In the present work we report the identification and characterisation of a PT gene from Glomus mosseae, an AM fungus important for natural and agricultural ecosystems. Degenerate primers and rapid amplification of cDNA ends-polymerase chain reaction (PCR) allowed us to obtain a sequence (GmosPT) showing a highly significant similarity with GiPT and GvPT, the only two other PT genes already isolated from AM fungi. Reverse transcriptase-PCR experiments were carried out to study GmosPT expression profiles in structures corresponding to different fungal life stages (quiescent and germinated sporocarps, intraradical and extraradical hyphae) and in extra- and intraradical hyphae exposed to high and low Pi concentrations. GmosPT showed an expression pattern similar to GiPT, the Glomus intraradices PT gene, since its transcript was more abundant in the extraradical mycelium treated with micromolar Pi levels. In addition, the intraradical mycelium also showed a significant GmosPT expression level that was independent from external Pi concentrations. This finding opens new questions about the role and functioning of high-affinity PT in AM fungi.