Fast, efficient and reproducible genetic transformation of Phaseolus spp. by Agrobacterium rhizogenes

A viral small interfering RNA-host plant mRNA pathway modulates virus-induced drought tolerance by enhancing autophagy

Virus-induced drought tolerance presents a fascinating facet of biotic-abiotic interaction in plants, yet its molecular intricacies remain unclear. Our study shows that cowpea mild mottle virus (CPMMV) infection enhances drought tolerance in common bean (Phaseolus vulgaris) plants through a virus-derived small interfering RNA (vsiRNA)-activated autophagy pathway. Specifically, a 21 nt vsiRNA originating from the CPMMV Triple Gene Block1 (TGB1) gene targeted the 5' untranslated region (UTR) of the host Teosinte branched 1, Cycloidea, Proliferating Cell Factor (TCP) transcription factor gene PvTCP2, independent of the known role of TGB1 as an RNA silencing suppressor. This targeting attenuated the expression of PvTCP2, which encodes a transcriptional repressor, and in turn upregulated the core autophagy-related gene (ATG) PvATG8c, leading to activated autophagy activity surpassing the level induced by drought or CPMMV infection alone. The downstream EARLY RESPONSIVE TO DEHYDRATION (ERD) effector PvERD15 is a homologue of Arabidopsis thaliana AtERD15, which positively regulates stomatal aperture. PvERD15 was degraded in PvATG8c-mediated autophagy. Therefore, we establish a TGB1-PvTCP2-PvATG8c-PvERD15 module as a trans-kingdom fine-tuning mechanism that contributes to virus-induced drought tolerance in plant-drought-virus interactions.

A comprehensive, improved protocol for generating common bean (Phaseolus vulgaris L.) transgenic hairy roots and their use in reverse-genetics studies

Generating transgenic hairy roots has been the preferred strategy for molecular studies in common bean (Phaseolus vulgaris L.), since generating stable knockout lines in this species is challenging. However, the number of plants producing hairy roots following the original protocol published in 2007 is usually low, which has impeded progress. Since its initial publication, the original protocol has been extensively modified, but these modifications have not been adequately or systematically reported, making it difficult to assess the reproducibility of the method. The protocol presented here is an update and expansion of the original method. Importantly, it includes new, critical steps for generating transgenic hairy roots and using them in molecular analyses based on reverse-genetics approaches. Using this protocol, the expression of two different genes, used as an example, was significantly increased or decreased in approximately 30% of the transformed plants. In addition, the promoter activity of a given gene was observed, and the infection process of rhizobia in transgenic hairy roots was monitored successfully. Thus, this improved protocol can be used to upregulate, downregulate, and perform promoter activity analysis of various genes in common bean transgenic hairy roots as well as to track rhizobia infection.

An ex vitro hairy root system from petioles of detached soybean leaves for in planta screening of target genes and CRISPR strategies associated with nematode bioassays

MAIN CONCLUSION

The ex vitro hairy root system from petioles of detached soybean leaves allows the functional validation of genes using classical transgenesis and CRISPR strategies (e.g., sgRNA validation, gene activation) associated with nematode bioassays. Agrobacterium rhizogenes-mediated root transformation has been widely used in soybean for the functional validation of target genes in classical transgenesis and single-guide RNA (sgRNA) in CRISPR-based technologies. Initial data showed that in vitro hairy root induction from soybean cotyledons and hypocotyls were not the most suitable strategies for simultaneous performing genetic studies and nematode bioassays. Therefore, an ex vitro hairy root system was developed for in planta screening of target molecules during soybean parasitism by root-knot nematodes (RKNs). Applying this method, hairy roots were successfully induced by A. rhizogenes from petioles of detached soybean leaves. The soybean GmPR10 and GmGST genes were then constitutively overexpressed in both soybean hairy roots and tobacco plants, showing a reduction in the number of Meloidogyne incognita-induced galls of up to 41% and 39%, respectively. In addition, this system was evaluated for upregulation of the endogenous GmExpA and GmExpLB genes by CRISPR/dCas9, showing high levels of gene activation and reductions in gall number of up to 58.7% and 67.4%, respectively. Furthermore, morphological and histological analyses of the galls were successfully performed. These collective data validate the ex vitro hairy root system for screening target genes, using classical overexpression and CRISPR approaches, directly in soybean in a simple manner and associated with nematode bioassays. This system can also be used in other root pathosystems for analyses of gene function and studies of parasite interactions with plants, as well as for other purposes such as studies of root biology and promoter characterization.

Role of a LORELEI- like gene from Phaseolus vulgaris during a mutualistic interaction with Rhizobium tropici

Reactive oxygen species (ROS), produced by NADPH oxidases known as RBOHs in plants, play a key role in plant development, biotic and abiotic stress responses, hormone signaling, and reproduction. Among the subfamily of receptor-like kinases referred to as CrRLK, there is FERONIA (FER), a regulator of RBOHs, and FER requires a GPI-modified membrane protein produced by LORELEI (LRE) or LORELEI-like proteins (LLG) to reach the plasma membrane and generate ROS. In Arabidopsis, AtLLG1 is involved in interactions with microbes as AtLLG1 interacts with the flagellin receptor (FLS2) to trigger the innate immune response, but the role of LLGs in mutualistic interactions has not been examined. In this study, two Phaseolus vulgaris LLG genes were identified, PvLLG2 that was expressed in floral tissue and PvLLG1 that was expressed in vegetative tissue. Transcripts of PvLLG1 increased during rhizobial nodule formation peaking during the early period of well-developed nodules. Also, P. vulgaris roots expressing pPvLLG1:GFP-GUS showed that this promoter was highly active during rhizobium infections, and very similar to the subcellular localization using a construct pLLG1::PvLLG1-Neon. Compared to control plants, PvLLG1 silenced plants had less superoxide (O2-) at the root tip and elongation zone, spotty hydrogen peroxide (H2O2) in the elongation root zone, and significantly reduced root hair length, nodule number and nitrogen fixation. Unlike control plants, PvLLG1 overexpressing plants showed superoxide beyond the nodule meristem, and significantly increased nodule number and nodule diameter. PvLLG1 appears to play a key role during this mutualistic interaction, possibly due to the regulation of the production and distribution of ROS in roots.

Highly efficient Agrobacterium rhizogenes-mediated hairy root transformation in citrus seeds and its application in gene functional analysis

Highly efficient genetic transformation technology is beneficial for plant gene functional research and molecular improvement breeding. However, the most commonly used Agrobacterium tumefaciens-mediated genetic transformation technology is time-consuming and recalcitrant for some woody plants such as citrus, hampering the high-throughput functional analysis of citrus genes. Thus, we dedicated to develop a rapid, simple, and highly efficient hairy root transformation system induced by Agrobacterium rhizogenes to analyze citrus gene function. In this report, a rapid, universal, and highly efficient hairy root transformation system in citrus seeds was described. Only 15 days were required for the entire workflow and the system was applicable for various citrus genotypes, with a maximum transformation frequency of 96.1%. After optimization, the transformation frequency of Citrus sinensis, which shows the lowest transformation frequency of 52.3% among four citrus genotypes initially, was increased to 71.4% successfully. To test the applicability of the hairy roots transformation system for gene functional analysis of citrus genes, we evaluated the subcellular localization, gene overexpression and gene editing in transformed hairy roots. Compared with the traditional transient transformation system performed in tobacco leaves, the transgenic citrus hairy roots displayed a more clear and specific subcellular fluorescence localization. Transcript levels of genes were significantly increased in overexpressing transgenic citrus hairy roots as compared with wild-type (WT). Additionally, hairy root transformation system in citrus seeds was successful in obtaining transformants with knocked out targets, indicating that the Agrobacterium rhizogenes-mediated transformation enables the CRISPR/Cas9-mediated gene editing. In summary, we established a highly efficient genetic transformation technology with non-tissue-culture in citrus that can be used for functional analysis such as protein subcellular localization, gene overexpression and gene editing. Since the material used for genetic transformation are roots protruding out of citrus seeds, the process of planting seedlings prior to transformation of conventional tissue culture or non-tissue-culture was eliminated, and the experimental time was greatly reduced. We anticipate that this genetic transformation technology will be a valuable tool for routine research of citrus genes in the future.

TETRASPANIN 8-1 from Phaseolus vulgaris plays a key role during mutualistic interactions

Arbuscular mycorrhizal (AM) fungi and rhizobia form two of the most important plant-microbe associations for the assimilation of phosphorus (P) and nitrogen (N). Symbiont-derived signals are able to coordinate the infection process by triggering multiple responses in the plant root, such as calcium influxes and oscillations, increased reactive oxygen species (ROS), cytoskeletal rearrangements and altered gene expression. An examination was made of the role of tetraspanins, which are transmembrane proteins that self-organize into tetraspanin web regions, where they recruit specific proteins into platforms required for signal transduction, membrane fusion, cell trafficking, and ROS generation. In plant cells, tetraspanins are scaffolding proteins associated with root radial patterning, biotic and abiotic stress responses, cell fate determination, plasmodesmata and hormonal regulation. Some plant tetraspanins, such as Arabidopsis thaliana TETRASPANIN 8 and TETRASPANIN 9 (AtTET8 and AtTET9) are associated with exosomes during inter-kingdom communication. In this study, a homolog of AtTET8, PvTET8-1, in common bean (Phaseolus vulgaris L. var. Negro Jamapa) was examined in roots during interactions with Rhizobium tropici and Rhizophagus irregularis. The promoter of PvTET8-1 contained several cis-acting regulatory DNA elements potentially related to mutualistic interactions, and PvTET8-1 was transcriptionally activated during AM fungal and rhizobial associations. Silencing it decreased the size and number of nodules, nitrogen fixation, and mycorrhizal arbuscule formation, whereas overexpressing it increased the size and number of nodules, and mycorrhizal arbuscule formation but decreased nitrogen fixation. PvTET8-1 appears to be an important element in both of these mutualistic interactions, perhaps through its interaction with NADPH oxidase and the generation of ROS during the infection processes.

Development of transgenic composite Stylosanthes plants to study root growth regulated by a β-expansin gene, SgEXPB1, under phosphorus deficiency

A highly efficient transformation procedure to generate transgenic Stylosanthes roots was established. SgEXPB1 is involved in Stylosanthes root growth under phosphorus deficiency. Stylo (Stylosanthes spp.) is an important forage legume widely applied in agricultural systems in the tropics. Due to the recalcitrance of stylo genetic transformation, functional characterization of candidate genes involved in stylo root growth is limited. This study established an efficient procedure for Agrobacterium rhizogenes-mediated transformation for generating transgenic composite plants of S. guianensis cultivar 'Reyan No. 5'. Results showed that composite stylo plants with transgenic hairy roots were efficiently generated by A. rhizogenes strains K599 and Arqual, infecting the residual hypocotyl at 1.0 cm of length below the cotyledon leaves of 9-d-old seedlings, leading to a high transformation efficiency of > 95% based on histochemical β-glucuronidase (GUS) staining. Notably, 100% of GUS staining-positive hairy roots can be achieved per composite stylo plant. Subsequently, SgEXPB1, a β-expansin gene up-regulated by phosphorus (P) deficiency in stylo roots, was successfully overexpressed in hairy roots. Analysis of hairy roots showed that root growth and P concentration in the transgenic composite plants were increased by SgEXPB1 overexpression under low-P treatment. Taken together, a highly efficient A. rhizogenes-mediated transformation procedure for generating composite stylo plants was established to study the function of SgEXPB1, revealing that this gene is involved in stylo root growth during P deficiency.

Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein and hairy roots: a perfect match for gene functional analysis and crop improvement

Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas) gene editing has become a powerful tool in genome manipulation for crop improvement. Advances in omics technologies, including genomics, transcriptomics, and metabolomics, allow the identification of causal genes that can be used to improve crops. However, the functional validation of these genetic components remains a challenge due to the lack of efficient protocols for crop engineering. Hairy roots gene editing using CRISPR/Cas, coupled with omics analyses, provide a platform for rapid, precise, and cost-effective functional analysis of genes. Here, we describe common requirements for efficient crop genome editing, focused on the transformation of recalcitrant legumes, and highlight the great opportunities that gene editing in hairy roots offers for future crop improvement.

Argonaute5 and its associated small RNAs modulate the transcriptional response during the rhizobia-Phaseolus vulgaris symbiosis

Both plant- and rhizobia-derived small RNAs play an essential role in regulating the root nodule symbiosis in legumes. Small RNAs, in association with Argonaute proteins, tune the expression of genes participating in nodule development and rhizobial infection. However, the role of Argonaute proteins in this symbiosis has been overlooked. In this study, we provide transcriptional evidence showing that Argonaute5 (AGO5) is a determinant genetic component in the root nodule symbiosis in Phaseolus vulgaris. A spatio-temporal transcriptional analysis revealed that the promoter of PvAGO5 is active in lateral root primordia, root hairs from rhizobia-inoculated roots, nodule primordia, and mature nodules. Transcriptional analysis by RNA sequencing revealed that gene silencing of PvAGO5 affected the expression of genes involved in the biosynthesis of the cell wall and phytohormones participating in the rhizobial infection process and nodule development. PvAGO5 immunoprecipitation coupled to small RNA sequencing revealed the small RNAs bound to PvAGO5 during the root nodule symbiosis. Identification of small RNAs associated to PvAGO5 revealed miRNAs previously known to participate in this symbiotic process, further supporting a role for AGO5 in this process. Overall, the data presented shed light on the roles that PvAGO5 plays during the root nodule symbiosis in P. vulgaris.

Global analysis of common bean multidrug and toxic compound extrusion transporters (PvMATEs): PvMATE8 and pinto bean seed coat darkening

In common bean (Phaseolus vulgaris L.), postharvest seed coat darkening is an undesirable trait that affects crop value. The increased accumulation of proanthocyanidins (PAs) in the seed coat results in darker seeds in many market classes of colored beans after harvest. The precursors of PAs are synthesized in the cytoplasm, and subsequently get glycosylated and then transported to the vacuoles where polymerization occurs. Thus, vacuolar transporters play an important role in the accumulation of PAs. Here, we report that common bean genome contains 59 multidrug and toxic compound extrusion genes (PvMATEs). Phylogenetic analysis of putative PvMATEs with functionally characterized MATEs from other plant species categorized them into substrate-specific clades. Our data demonstrate that a vacuolar transporter PvMATE8 is expressed at a higher level in the pinto bean cultivar CDC Pintium (regular darkening) compared to 1533-15 (slow darkening). PvMATE8 localizes in the vacuolar membrane and rescues the PA deficient (tt12) mutant phenotype in Arabidopsis thaliana. Analysis of PA monomers in transgenic seeds together with wild-type and mutants suggests a possible feedback regulation of PA biosynthesis and accumulation. Identification of PvMATE8 will help better understand the mechanism of PA accumulation in common bean.

Highly efficient hairy root genetic transformation and applications in citrus

Highly efficient genetic transformation technology is greatly beneficial for crop gene function analysis and precision breeding. However, the most commonly used genetic transformation technology for woody plants, mediated by Agrobacterium tumefaciens, is time-consuming and inefficient, which limits its utility for gene function analysis. In this study, a simple, universal, and highly efficient genetic transformation technology mediated by A. rhizogenes K599 is described. This technology can be applied to multiple citrus genotypes, and only 2-8 weeks were required for the entire workflow. Genome-editing experiments were simultaneously conducted using 11 plasmids targeting different genomic positions and all corresponding transformants with the target knocked out were obtained, indicating that A. rhizogenes-mediated genome editing was highly efficient. In addition, the technology is advantageous for investigation of specific genes (such as ACD2) for obtaining "hard-to-get" transgenic root tissue. Furthermore, A. rhizogenes can be used for direct viral vector inoculation on citrus bypassing the requirement for virion enrichment in tobacco, which facilitates virus-induced gene silencing and virus-mediated gene expression. In summary, we established a highly efficient genetic transformation technology bypassing tissue culture in citrus that can be used for genome editing, gene overexpression, and virus-mediated gene function analysis. We anticipate that by reducing the cost, required workload, experimental period, and other technical obstacles, this genetic transformation technology will be a valuable tool for routine investigation of endogenous and exogenous genes in citrus.

The aquaporin gene PvXIP1;2 conferring drought resistance identified by GWAS at seedling stage in common bean

A whole-genome resequencing-derived SNP dataset used for genome-wide association analysis revealed 12 loci significantly associated with drought stress based on survival rate after drought stress at seedling stage. We further confirmed the drought-related function of an aquaporin gene (PvXIP1;2) located at Locus_10. A variety of adverse conditions, including drought stress, severely affect common bean production. Molecular breeding for drought resistance has been proposed as an effective and practical way to improve the drought resistance of common bean. A genome-wide association analysis was conducted to identify drought-related loci based on survival rates at the seedling stage using a natural population consisting of 400 common bean accessions and 3,832,340 SNPs. The coefficient of variation ranged from 40.90 to 56.22% for survival rates in three independent experiments. A total of 12 associated loci containing 89 significant SNPs were identified for survival rates at the seedling stage. Four loci overlapped in the region of the QTLs reported to be associated with drought resistance. According to the expression profiles, gene annotations and references of the functions of homologous genes in Arabidopsis, 39 genes were considered potential candidate genes selected from 199 genes annotated within all associated loci. A stable locus (Locus_10) was identified on chromosome 11, which contained LEA, aquaporin, and proline-rich protein genes. We further confirmed the drought-related function of an aquaporin (PvXIP1;2) located at Locus_10 by expression pattern analysis, phenotypic analysis of PvXIP1;2-overexpressing Arabidopsis and Agrobacterium rhizogenes-mediated hairy root transformation systems, indicating that the association results can facilitate the efficient identification of genes related to drought resistance. These loci and their candidate genes provide a foundation for crop improvement via breeding for drought resistance in common bean.

Metallothionein1A Regulates Rhizobial Infection and Nodulation in Phaseolus vulgaris

Metallothioneins (MTs) constitute a heterogeneous family of ubiquitous metal ion-binding proteins. In plants, MTs participate in the regulation of cell growth and proliferation, protection against heavy metal stress, oxidative stress responses, and responses to pathogen attack. Despite their wide variety of functions, the role of MTs in symbiotic associations, specifically nodule-fabacean symbiosis, is poorly understood. Here, we analyzed the role of the PvMT1A gene in Phaseolus vulgaris-Rhizobium tropici symbiosis using bioinformatics and reverse genetics approaches. Using in silico analysis, we identified six genes encoding MTs in P. vulgaris, which were clustered into three of the four classes described in plants. PvMT1A transcript levels were significantly higher in roots inoculated with R. tropici at 7 and 30 days post inoculation (dpi) than in non-inoculated roots. Functional analysis showed that downregulating PvMT1A by RNA interference (RNAi) reduced the number of infection events at 7 and 10 dpi and the number of nodules at 14 and 21 dpi. In addition, nodule development was negatively affected in PvMT1A:RNAi transgenic roots, and these nodules displayed a reduced nitrogen fixation rate at 21 dpi. These results strongly suggest that PvMT1A plays an important role in the infection process and nodule development in P. vulgaris during rhizobial symbiosis.

An NADPH oxidase regulates carbon metabolism and the cell cycle during root nodule symbiosis in common bean (Phaseolus vulgaris)

BACKGROUND

Rhizobium-legume symbiosis is a specific, coordinated interaction that results in the formation of a root nodule, where biological nitrogen fixation occurs. NADPH oxidases, or Respiratory Burst Oxidase Homologs (RBOHs) in plants, are enzymes that generate superoxide (O2 •-). Superoxide produces other reactive oxygen species (ROS); these ROS regulate different stages of mutualistic interactions. For example, changes in ROS levels are thought to induce ROS scavenging, cell wall remodeling, and changes in phytohormone homeostasis during symbiotic interactions. In common bean (Phaseolus vulgaris), PvRbohB plays a key role in the early stages of nodulation.

RESULTS

In this study, to explore the role of PvRbohB in root nodule symbiosis, we analyzed transcriptomic data from the roots of common bean under control conditions (transgenic roots without construction) and roots with downregulated expression of PvRbohB (by RNA interference) non-inoculated and inoculated with R. tropici. Our results suggest that ROS produced by PvRBOHB play a central role in infection thread formation and nodule organogenesis through crosstalk with flavonoids, carbon metabolism, cell cycle regulation, and the plant hormones auxin and cytokinin during the early stages of this process.

CONCLUSIONS

Our findings provide important insight into the multiple roles of ROS in regulating rhizobia-legume symbiosis.

Control of the Rhizobia Nitrogen-Fixing Symbiosis by Common Bean MADS-Domain/AGL Transcription Factors

Plants MADS-domain/AGL proteins constitute a large transcription factor (TF) family that controls the development of almost every plant organ. We performed a phylogeny of (ca. 500) MADS-domain proteins from Arabidopsis and four legume species. We identified clades with Arabidopsis MADS-domain proteins known to participate in root development that grouped legume MADS-proteins with similar high expression in roots and nodules. In this work, we analyzed the role of AGL transcription factors in the common bean (Phaseolus vulgaris) - Rhizobium etli N-fixing symbiosis. Sixteen P. vulgaris AGL genes (PvAGL), out of 93 family members, are expressed - at different levels - in roots and nodules. From there, we selected the PvAGL gene denominated PvFUL-like for overexpression or silencing in composite plants, with transgenic roots and nodules, that were used for phenotypic analysis upon inoculation with Rhizobium etli. Because of sequence identity in the DNA sequence used for RNAi-FUL-like construct, roots, and nodules expressing this construct -referred to as RNAi_AGL- showed lower expression of other five PvAGL genes highly expressed in roots/nodules. Contrasting with PvFUL-like overexpressing plants, rhizobia-inoculated plants expressing the RNAi_AGL silencing construct presented affection in the generation and growth of transgenic roots from composite plants, both under non-inoculated or rhizobia-inoculated condition. Furthermore, the rhizobia-inoculated plants showed decreased rhizobial infection concomitant with the lower expression level of early symbiotic genes and increased number of small, ineffective nodules that indicate an alteration in the autoregulation of the nodulation symbiotic process. We propose that the positive effects of PvAGL TF in the rhizobia symbiotic processes result from its potential interplay with NIN, the master symbiotic TF regulator, that showed a CArG-box consensus DNA sequence recognized for DNA binding of AGL TF and presented an increased or decreased expression level in roots from non-inoculated plants transformed with OE_FUL or RNAi_AGL construct, respectively. Our work contributes to defining novel transcriptional regulators for the common bean - rhizobia N-fixing symbiosis, a relevant process for sustainable agriculture.

Phaseolus vulgaris MIR1511 genotypic variations differentially regulate plant tolerance to aluminum toxicity

The common-bean (Phaseolus vulgaris), a widely consumed legume, originated in Mesoamerica and expanded to South America, resulting in the development of two geographically distinct gene pools. Poor soil condition, including metal toxicity, are often constraints to common-bean crop production. Several P. vulgaris miRNAs, including miR1511, respond to metal toxicity. The MIR1511 gene sequence from the two P. vulgaris model sequenced genotypes revealed that, as opposed to BAT93 (Mesoamerican), the G19833 (Andean) accession displays a 58-bp deletion, comprising the mature and star miR1511 sequences. Genotyping-By-Sequencing data analysis from 87 non-admixed Phaseolus genotypes, comprising different Phaseolus species and P. vulgaris populations, revealed that all the P. vulgaris Andean genotypes and part of the Mesoamerican (MW1) genotypes analyzed displayed a truncated MIR1511 gene. The geographic origin of genotypes with a complete versus truncated MIR1511 showed a distinct distribution. The P. vulgaris ALS3 (Aluminum Sensitive Protein 3) gene, known to be important for aluminum detoxification in several plants, was experimentally validated as the miR1511 target. Roots from BAT93 plants showed decreased miR1511 and increased ALS3 transcript levels at early stages under aluminum toxicity (AlT), while G19833 plants, lacking mature miR1511, showed higher and earlier ALS3 response. Root architecture analyses evidenced higher tolerance of G19833 plants to AlT. However, G19833 plants engineered for miR1511 overexpression showed lower ALS3 transcript level and increased sensitivity to AlT. Absence of miR1511 in Andean genotypes, resulting in a diminished ALS3 transcript degradation, appears to be an evolutionary advantage to high Al levels in soils with increased drought conditions.

Rhizobium etli CE3-DsRed pMP604: a useful biological tool to study initial infection steps in Phaseolus vulgaris nodulation

Rhizobium etli CE3-DsRed pMP604 drives infection 12-24 h earlier than R. etli CE3-DsRed and it is an excellent tool in live-cell imaging studies of IT developement in P. vulgaris roots. The study of the cellular dynamics of nodulation has frequently been limited by the difficulty of performing live-cell imaging in nodule primordia and legume roots, which are constituted by multiple cell layers, such is the case of Phaseolus vulgaris. Seeking conditions to reduce the time it takes for rhizobia to infect P. vulgaris root, we decided to explore the nodulation properties of Rhizobium etli CE3 pMP604, a strain that constitutively produces Nod factors through a flavonoids-independent transcriptional activation which is often used to purify Nod factors. Even though the strain infects 12-24 h earlier than the parental R. etli CE3 strain, infection thread (IT) formation, nodule organogenesis processes and N₂-fixation activity are similar for both strains. Additionally, we have confirmed that R. etli CE3-DsRed pMP604 is an excellent tool to trace IT development in P. vulgaris roots.

A hairy-root transformation protocol for Trigonella foenum-graecum L. as a tool for metabolic engineering and specialised metabolite pathway elucidation

The development of genetic transformation methods is critical for enabling the thorough characterization of an organism and is a key step in exploiting any species as a platform for synthetic biology and metabolic engineering approaches. In this work we describe the development of an Agrobacterium rhizogenes-mediated hairy root transformation protocol for the crop and medicinal legume fenugreek (Trigonella foenum-graecum). Fenugreek has a rich and diverse content in bioactive specialised metabolites, notably diosgenin, which is a common precursor for synthetic human hormone production. This makes fenugreek a prime target for identification and engineering of specific biosynthetic pathways for the production of triterpene and steroidal saponins, phenolics, and galactomanans. Through this transformation protocol, we identified a suitable promoter for robust transgene expression in fenugreek. Finally, we establish the proof of principle for the utility of the fenugreek system for metabolic engineering programs, by heterologous expression of known triterpene saponin biosynthesis regulators from the related legume Medicago truncatula in fenugreek hairy roots.

Inhibition of legume nodulation by Pi deficiency is dependent on the autoregulation of nodulation (AON) pathway

Inhibition of nodule development is one of the main adverse effects of phosphate (Pi) deficiency in legumes. Despite all of the efforts made over the last decades to understand how root nodules cope with Pi deficiency, the molecular mechanisms leading to the reduction in nodule number under Pi deficiency remain elusive. In the present study, we provide experimental evidence indicating that Pi deficiency activates the autoregulation of nodulation (AON) pathway, leading to a reduction in nodule numbers in both common bean and soybean. A transcriptional profile analysis revealed that the expression of the AON-related genes PvNIN, PvRIC1, PvRIC2, and PvTML is upregulated under Pi deficiency conditions. The downregulation of the MYB transcription factor PvPHR1 in common bean roots significantly reduced the expression of these four AON-related genes. Physiological analyses indicated that Pi deficiency does not affect the establishment of the root nodule symbiosis in the supernodulation mutant lines Pvnark and Gmnark. Reciprocal grafting and split-roots analyses determined that the activation of the AON pathway was required for the inhibitory effect of Pi deficiency. Altogether, these data improve our understanding of the genetic mechanisms controlling the establishment of the root nodule symbiosis under Pi deficiency.

Highly diverse root endophyte bacterial community is driven by growth substrate and is plant genotype-independent in common bean (Phaseolus vulgaris L.)

The common bean (Phaseolus vulgaris L.) is the most important grain legume in the human diet with an essential role in sustainable agriculture mostly based on the symbiotic relationship established between this legume and rhizobia, a group of bacteria capable of fixing atmospheric nitrogen in the roots nodules. Moreover, root-associated bacteria play an important role in crop growth, yield, and quality of crop products. This is particularly true for legume crops forming symbiotic relationships with rhizobia, for fixation of atmospheric N₂. The main objective of this work is to assess the substrate and genotype effect in the common bean (Phaseolus vulgaris L.) root bacterial community structure. To achieve this goal, we applied next-generation sequencing coupled with bacterial diversity analysis. The analysis of the bacterial community structures between common bean roots showed marked differences between substrate types regardless of the genotype. Also, we were able to find several phyla conforming to the bacterial community structure of the common bean roots, mainly composed by Proteobacteria, Actinobacteria, Bacteroidetes, Acidobacteria, and Firmicutes. Therefore, we determined that the substrate type was the main factor that influenced the bacterial community structure of the common bean roots, regardless of the genotype, following a substrate-dependent pattern. These guide us to develop efficient and sustainable strategies for crop field management based on the soil characteristics and the bacterial community that it harbors.

ABAS1 from soybean is a 1R-subtype MYB transcriptional repressor that enhances ABA sensitivity

Transcription factors (TFs) help plants respond to environmental stresses by regulating gene expression. Up till now, studies on the MYB family of TFs have mainly focused on the highly abundant R2R3-subtype. While the less well-known 1R-subtype has been generally shown to enhance abscisic acid (ABA) sensitivity by acting as transcriptional activators, the mechanisms of their functions are unclear. Here we identified an ABA sensitivity-associated gene from soybean, ABA-Sensitive 1 (GmABAS1), of the 1R-subtype of MYB. Using the GFP-GmABAS1 fusion protein, we demonstrated that GmABAS1 is localized in the nucleus, and with yeast reporter systems, we showed that it is a transcriptional repressor. We then identified the target gene of GmABAS1 to be Glyma.01G060300, an annotated ABI five-binding protein 3 and showed that GmABAS1 binds to the promoter of Glyma.01G060300 both in vitro and in vivo. Furthermore, Glyma.01G060300 and GmABAS1 exhibited reciprocal expression patterns under osmotic stress, inferring that GmABAS1 is a transcriptional repressor of Glyma.01G060300. As a further confirmation, AtAFP2, an orthologue of Glyma.01G060300, was down-regulated in GmABAS1-transgenic Arabidopsis thaliana, enhancing the plant's sensitivity to ABA. This is the first time a 1R-subtype of MYB from soybean has been reported to enhance ABA sensitivity by acting as a transcriptional repressor.

One-step generation of composite soybean plants with transgenic roots by Agrobacterium rhizogenes-mediated transformation

BACKGROUND

Agrobacterium rhizogenes-mediated (ARM) transformation is a highly efficient technique for generating composite plants composed of transgenic roots and wild-type shoot, providing a powerful tool for studying root biology. The ARM transformation has been established in many plant species, including soybean. However, traditional transformation of soybean, transformation efficiency is low. Additionally, the hairy roots were induced in a medium, and then the generated composite plants were transplanted into another medium for growth. This two-step operation is not only time-consuming, but aggravates contamination risk in the study of plant-microbe interactions.

RESULTS

Here, we report a one-step ARM transformation method with higher transformation efficiency for generating composite soybean plants. Both the induction of hairy roots and continuous growth of the composite plants were conducted in a single growth medium. The primary root of a 7-day-old seedling was decapitated with a slanted cut, the residual hypocotyl (maintained 0.7-1 cm apical portion) was inoculated with A. rhizogenes harboring the gene construct of interest. Subsequently, the infected seedling was planted into a pot with wet sterile vermiculite. Almost 100% of the infected seedlings could produce transgenic positive roots 16 days post-inoculation in 7 tested genotypes. Importantly, the transgenic hairy roots in each composite plant are about three times more than those of the traditional ARM transformation, indicating that the one-step method is simpler in operation and higher efficiency in transformation. The reliability of the one-step method was verified by CRISPR/Cas9 system to knockout the soybean Rfg1, which restricts nodulation in Williams 82 (Nod-) by Sinorhizobium fredii USDA193. Furthermore, we applied this method to analyze the function of Arabidopsis YAO promoter in soybean. The activity of YAO promoter was detected in whole roots and stronger in the root tips. We also extended the protocol to tomato.

CONCLUSIONS

We established a one-step ARM transformation method, which is more convenient in operation and higher efficiency (almost 100%) in transformation for generating composite soybean plants. This method has been validated in promoter functional analysis and rhizobia-legume interactions. We anticipate a broad application of this method to analyze root-related events in tomato and other plant species besides soybean.

Actin Depolymerizing Factor Modulates Rhizobial Infection and Nodule Organogenesis in Common Bean

Actin plays a critical role in the rhizobium-legume symbiosis. Cytoskeletal rearrangements and changes in actin occur in response to Nod factors secreted by rhizobia during symbiotic interactions with legumes. These cytoskeletal rearrangements are mediated by diverse actin-binding proteins, such as actin depolymerization factors (ADFs). We examined the function of an ADF in the Phaseolus vulgaris-rhizobia symbiotic interaction (PvADFE). PvADFE was preferentially expressed in rhizobia-inoculated roots and nodules. PvADFE promoter activity was associated with root hairs harbouring growing infection threads, cortical cell divisions beneath root hairs, and vascular bundles in mature nodules. Silencing of PvADFE using RNA interference increased the number of infection threads in the transgenic roots, resulting in increased nodule number, nitrogen fixation activity, and average nodule diameter. Conversely, overexpression of PvADFE reduced the nodule number, nitrogen fixation activity, average nodule diameter, as well as NODULE INCEPTION (NIN) and EARLY NODULIN2 (ENOD2) transcript accumulation. Hence, changes in ADFE transcript levels affect rhizobial infection and nodulation, suggesting that ADFE is fine-tuning these processes.

Enhancement of Pathogen Resistance in Common Bean Plants by Inoculation With Rhizobium etli

Symbiotic Rhizobium-legume associations are mediated by exchange of chemical signals that eventually result in the development of a nitrogen-fixing nodule. Such signal interactions are thought to be at the center of the plants' capacity either to activate a defense response or to suppress the defense response to allow colonization by symbiotic bacteria. In addition, the colonization of plant roots by rhizobacteria activates an induced condition of improved defensive capacity in plants known as induced systemic resistance, based on "defense priming," which protects unexposed plant tissues from biotic stress.Here, we demonstrate that inoculation of common bean plants with Rhizobium etli resulted in a robust resistance against Pseudomonas syringae pv. phaseolicola. Indeed, inoculation with R. etli was associated with a reduction in the lesion size caused by the pathogen and lower colony forming units compared to mock-inoculated plants. Activation of the induced resistance was associated with an accumulation of the reactive oxygen species superoxide anion (O₂ ^-) and a faster and stronger callose deposition. Transcription of defense related genes in plants treated with R. etli exhibit a pattern that is typical of the priming response. In addition, R. etli-primed plants developed a transgenerational defense memory and could produce offspring that were more resistant to halo blight disease. R. etli is a rhizobacteria that could reduce the proliferation of the virulent strain P. syringae pv. phaseolicola in common bean plants and should be considered as a potentially beneficial and eco-friendly tool in plant disease management.

Molecular and biochemical analysis of XDH from Phaseolus vulgaris suggest that uric acid protects the enzyme against the inhibitory effects of nitric oxide in nodules

Xanthine dehydrogenase (XDH) is essential for the assimilation of symbiotically fixed nitrogen in ureidic legumes. Uric acid, produced in the reaction catalyzed by XDH, is the precursor of the ureides, allantoin and allantoate, which are the main N-transporting molecules in these plants. XDH and uric acid have been reported to be involved in the response to stress, both in plants and animals. However, the physiological role of XDH under stressful conditions in ureidic legumes remains largely unexplored. In vitro assays showed that Phaseolus vulgaris XDH (PvXDH) can behave as a dehydrogenase or as an oxidase. Therefore, it could potentially protect against oxidative radicals or, in contrast, it could increase their production. In silico analysis of the upstream genomic region of XDH coding gene from P. vulgaris revealed the presence of several stress-related cis-regulatory elements. PvXDH mRNA and enzymatic activity in plants treated with stress-related phytohormones or subjected to dehydration and stressful temperatures showed several fold induction. However, PvXDH activity was in vivo and in vitro inhibited by nitric oxide in leaves but not in nodules. In extracts from RNAi PvXDH silenced nodules, with lower levels of uric acid, XDH activity was inhibited by SNP which indicates that uric acid produced by XDH in the nodules of this ureidic legume could help to protect XDH against the inhibitory effects of nitric oxide.

An anthocyanin marker for direct visualization of plant transformation and its use to study nitrogen-fixing nodule development

The development and functioning of the nitrogen fixing symbiosis between legume plants and soil bacteria collectively called rhizobia requires continuous chemical dialogue between the partners using different molecules such as flavonoids, lipo-chitooligosaccharides, polysaccharides and peptides. Agrobacterium rhizogenes mediated hairy root transformation of legumes is widely used to study the function of plant genes involved in the process. The identification of transgenic plant tissues is based on antibiotics/herbicide selection and/or the detection of different reporter genes that usually require special equipment such as fluorescent microscopes or destructive techniques and chemicals to visualize enzymatic activity. Here, we developed and efficiently used in hairy root experiments binary vectors containing the MtLAP1 gene driven by constitutive and tissue-specific promoters that facilitate the production of purple colored anthocyanins in transgenic tissues and thus allowing the identification of transformed roots by naked eye. Anthocyanin producing roots were able to establish effective symbiosis with rhizobia. Moreover, it was shown that species-specific allelic variations and a mutation preventing posttranslational acetyl modification of an essential nodule-specific cysteine-rich peptide, NCR169, do not affect the symbiotic interaction of Medicago truncatula cv. Jemalong with Sinorhizobium medicae strain WSM419. Based on the experiments, it could be concluded that it is preferable to use the vectors with tissue-specific promoters that restrict anthocyanin production to the root vasculature for studying biotic interactions of the roots such as symbiotic nitrogen fixation or mycorrhizal symbiosis.

Differential tetraspanin genes expression and subcellular localization during mutualistic interactions in Phaseolus vulgaris

Arbuscular mycorrhizal fungi and rhizobia association with plants are two of the most successful plant-microbe associations that allow the assimilation of P and N by plants, respectively. These mutualistic interactions require a molecular dialogue, i.e., legume roots exude flavonoids or strigolactones which induce the Nod factors or Myc factors synthesis and secretion from the rhizobia or fungi, respectively. These Nod or Myc factors trigger several responses in the plant root, including calcium oscillations, and reactive oxygen species (ROS). Furthermore, superoxide and H₂O₂ have emerged as key components that regulate the transitions from proliferation to differentiation in the plant meristems. Similar to the root meristem, the nodule meristem accumulates superoxide and H₂O₂. Tetraspanins are transmembrane proteins that organize into tetraspanin web regions, where they recruit specific proteins into platforms required for signal transduction, membrane fusion, cell trafficking and ROS generation. Plant tetraspanins are scaffolding proteins associated with root radial patterning, biotic and abiotic stress responses, cell fate determination, and hormonal regulation and recently have been reported as a specific marker of exosomes in animal and plant cells and key players at the site of plant fungal infection. In this study, we conducted transcriptional profiling of the tetraspanin family in common bean (Phaseolus vulgaris L. var. Negro Jamapa) to determine the specific expression patterns and subcellular localization of tetraspanins during nodulation or under mycorrhizal association. Our results demonstrate that the tetraspanins are transcriptionally modulated during the mycorrhizal association, but are also expressed in the infection thread and nodule meristem development. Subcellular localization indicates that tetraspanins have a key role in vesicular trafficking, cell division, and root hair polar growth.

The Noncanonical Heat Shock Protein PvNod22 Is Essential for Infection Thread Progression During Rhizobial Endosymbiosis in Common Bean

In the establishment of plant-rhizobial symbiosis, the plant hosts express nodulin proteins during root nodule organogenesis. A limited number of nodulins have been characterized, and these perform essential functions in root nodule development and metabolism. Most nodulins are expressed in the nodule and at lower levels in other plant tissues. Previously, we isolated Nodulin 22 (PvNod22) from a common bean (Phaseolus vulgaris L.) cDNA library derived from Rhizobium-infected roots. PvNod22 is a noncanonical, endoplasmic reticulum (ER)-localized, small heat shock protein that confers protection against oxidative stress when overexpressed in Escherichia coli. Virus-induced gene silencing of PvNod22 resulted in necrotic lesions in the aerial organs of P. vulgaris plants cultivated under optimal conditions, activation of the ER-unfolded protein response (UPR), and, finally, plant death. Here, we examined the expression of PvNod22 in common bean plants during the establishment of rhizobial endosymbiosis and its relationship with two cellular processes associated with plant immunity, the UPR and autophagy. In the RNA interference lines, numerous infection threads stopped their progression before reaching the cortex cell layer of the root, and nodules contained fewer nitrogen-fixing bacteroids. Collectively, our results suggest that PvNod22 has a nonredundant function during legume-rhizobia symbiosis associated with infection thread elongation, likely by sustaining protein homeostasis in the ER.

Plant Cellular and Molecular Biotechnology: Following Mariotti's Steps

This review is dedicated to the memory of Prof. Domenico Mariotti, who significantly contributed to establishing the Italian research community in Agricultural Genetics and carried out the first experiments of Agrobacterium-mediated plant genetic transformation and regeneration in Italy during the 1980s. Following his scientific interests as guiding principles, this review summarizes the recent advances obtained in plant biotechnology and fundamental research aiming to: (i) Exploit in vitro plant cell and tissue cultures to induce genetic variability and to produce useful metabolites; (ii) gain new insights into the biochemical function of Agrobacterium rhizogenes rol genes and their application to metabolite production, fruit tree transformation, and reverse genetics; (iii) improve genetic transformation in legume species, most of them recalcitrant to regeneration; (iv) untangle the potential of KNOTTED1-like homeobox (KNOX) transcription factors in plant morphogenesis as key regulators of hormonal homeostasis; and (v) elucidate the molecular mechanisms of the transition from juvenility to the adult phase in Prunus tree species.

Hairy Root Transformation: A Useful Tool to Explore Gene Function and Expression in Salix spp. Recalcitrant to Transformation

Willow (Salix spp. L.) species are fast-growing trees and shrubs that have attracted emergent attention for their potential as feedstocks for bioenergy and biofuel production, as well as for pharmaceutical and phytoremediation applications. This economic and environmental potential has propelled the creation of several genetic and genomic resources for Salix spp. Furthermore, the recent availability of an annotated genome for Salix purpurea has pinpointed novel candidate genes underlying economically relevant traits. However, functional studies have been stalled by the lack of rapid and efficient coupled regeneration-transformation systems for Salix purpurea and Salix spp. in general. In this report, we describe a fast and highly efficient hairy root transformation protocol for S. purpurea. It was effective for different explant sources and S. purpurea genotypes, with efficiencies between 63.4% and 98.7%, and the screening of the transformed hairy roots was easily carried out using the fluorescent marker DsRed. To test the applicability of this hairy root transformation system for gene functional analysis, we transformed hairy roots with the vector pGWAY-SpDRM2, where the gene SpDRM2 encoding a putative Domain Rearranged Methyltransferase (DRM) was placed under the control of the CaMV 35S constitutive promoter. Indeed, the transgenic hairy roots obtained exhibited significantly increased expression of SpDRM2 as compared to controls, demonstrating that this protocol is suitable for the medium/high-throughput functional characterization of candidate genes in S. purpurea and other recalcitrant Salix spp.

A dicistronic precursor encoding miR398 and the legume-specific miR2119 coregulates CSD1 and ADH1 mRNAs in response to water deficit

Plant microRNAs are commonly encoded in transcripts containing a single microRNA precursor. Processing by DICER-LIKE 1 and associated factors results in the production of a small RNA, followed by its incorporation into an AGO-containing protein complex to guide silencing of an mRNA possessing a complementary target sequence. Certain microRNA loci contain more than one precursor stem-loop structure, thus encoding more than one microRNA in the same transcript. Here, we describe a unique case where the evolutionary conserved miR398a is encoded in the same transcript as the legume-specific miR2119. The dicistronic arrangement found in common bean was also observed in other legumes. In Phaseolus vulgaris, mature miR398 and miR2119 are repressed in response to water deficit, and we demonstrate that both are functional as they target the mRNAs for CSD1 and ADH1, respectively. Our results indicate that the repression of miR398 and miR2119 leads to coordinated up-regulation of CSD1 and ADH1 mRNAs in response to water deficit in common bean and possibly in other legumes. Furthermore, we show that miRNA directed CSD1 and ADH1 mRNAs up-regulation also occurs when common bean plants are exposed to flooding, suggesting that plant redox status and fermentation metabolism must be closely coordinated under different adverse conditions.

Down-regulation of PvTRX1h increases nodule number and affects auxin, starch, and metabolic fingerprints in the common bean (Phaseolus vulgaris L.)

The legume-rhizobium symbiotic relationship has been widely studied and characterized. However, little information is available about the role of histone lysine methyltransferases in the legume-rhizobium interaction and in the formation of nitrogen-fixing nodules in the common bean. Thus, this study aimed to gain a better understanding of the epigenetic control of nodulation in the common bean. Specifically, we studied the role of PvTRX1h, a histone lysine methyltransferase coding gene, in nodule development and auxin biosynthesis. Through a reverse genetics approach, we generated common bean composite plants to knock-down PvTRX1h expression. Here we found that the down-regulation of PvTRX1h increased the number of nodules per plant, but reduced the number of colony-forming units recovered from nodules. Genes coding for enzymes involved in the synthesis of the indole-3-acetic acid were up-regulated, as was the concentration of this hormone. In addition, PvTRX1h down-regulation altered starch accumulation as determined by the number of amyloplasts per nodule. Metabolic fingerprinting by direct liquid introduction-electrospray ionization-mass spectrometry (DLI-ESI-MS) revealed that the root nodules were globally affected by PvTRX1h down-regulation. Therefore, PvTRX1h likely acts through chromatin histone modifications that alter the auxin signaling network to determine bacterial colonization, nodule number, starch accumulation, hormone levels, and cell proliferation.

The MicroRNA319d/TCP10 Node Regulates the Common Bean - Rhizobia Nitrogen-Fixing Symbiosis

Micro-RNAs from legume plants are emerging as relevant regulators of the rhizobia nitrogen-fixing symbiosis. In this work we functionally characterized the role of the node conformed by micro-RNA319 (miR319) - TEOSINTE BRANCHED/CYCLOIDEA/PCF (TCP) transcription factor in the common bean (Phaseolus vulgaris) - Rhizobium tropici symbiosis. The miR319d, one of nine miR319 isoforms from common bean, was highly expressed in root and nodules from inoculated plants as compared to roots from fertilized plants. The miR319d targets TCP10 (Phvul.005G067950), identified by degradome analysis, whose expression showed a negative correlation with miR319d expression. The phenotypic analysis of R. tropici-inoculated composite plants with transgenic roots/nodules overexpressing or silencing the function of miR319d demonstrated the relevant role of the miR319d/TCP10 node in the common bean rhizobia symbiosis. Increased miR319d resulted in reduced root length/width ratio, increased rhizobial infection evidenced by more deformed root hairs and infection threads, and decreased nodule formation and nitrogenase activity per plant. In addition, these plants with lower TCP10 levels showed decreased expression level of the jasmonic acid (JA) biosynthetic gene: LOX2. The transcription of LOX2 by TCPs has been demonstrated for Arabidopsis and in several plants LOX2 level and JA content have been associate with TCP levels. On this basis, we propose that in roots/nodules of inoculated common bean plants TCP10 could be the transcriptional regulator of LOX2 and the miR319d/TCP10 node could affect nodulation through JA signaling. However, given the complexity of nodulation, the participation of other signaling pathways in the phenotypes observed cannot be ruled out.

Genotype-independent Agrobacterium rhizogenes-mediated root transformation of chickpea: a rapid and efficient method for reverse genetics studies

BACKGROUND

Chickpea (Cicer arietinum L.), an important legume crop is one of the major source of dietary protein. Developing an efficient and reproducible transformation method is imperative to expedite functional genomics studies in this crop. Here, we present an optimized and detailed procedure for Agrobacterium rhizogenes-mediated root transformation of chickpea.

RESULTS

Transformation positive roots were obtained on selection medium after two weeks of A. rhizogenes inoculation. Expression of green fluorescent protein further confirmed the success of transformation. We demonstrate that our method adequately transforms chickpea roots at early developmental stage with high efficiency. In addition, root transformation was found to be genotype-independent and the efficacy of our protocol was highest in two (Annigiri and JG-62) of the seven tested chickpea genotypes. Next, we present the functional analysis of chickpea hairy roots by expressing Arabidopsis TRANSPARENT TESTA 2 (AtTT2) gene involved in proanthocyanidins biosynthesis. Overexpression of AtTT2 enhanced the level of proanthocyanidins in hairy roots that led to the decreased colonization of fungal pathogen, Fusarium oxysporum. Furthermore, the induction of transgenic roots does not affect functional studies involving infection of roots by fungal pathogen.

CONCLUSIONS

Transgenic roots expressing genes of interest will be useful in downstream functional characterization using reverse genetics studies. It requires 1 day to perform the root transformation protocol described in this study and the roots expressing transgene can be maintained for 3-4 weeks, providing sufficient time for further functional studies. Overall, the current methodology will greatly facilitate the functional genomics analyses of candidate genes in root-rhizosphere interaction in this recalcitrant but economically important legume crop.

Gene Silencing of Argonaute5 Negatively Affects the Establishment of the Legume-Rhizobia Symbiosis

The establishment of the symbiosis between legumes and nitrogen-fixing rhizobia is finely regulated at the transcriptional, posttranscriptional and posttranslational levels. Argonaute5 (AGO5), a protein involved in RNA silencing, can bind both viral RNAs and microRNAs to control plant-microbe interactions and plant physiology. For instance, AGO5 regulates the systemic resistance of Arabidopsis against Potato Virus X as well as the pigmentation of soybean (Glycine max) seeds. Here, we show that AGO5 is also playing a central role in legume nodulation based on its preferential expression in common bean (Phaseolus vulgaris) and soybean roots and nodules. We also report that the expression of AGO5 is induced after 1 h of inoculation with rhizobia. Down-regulation of AGO5 gene in P. vulgaris and G. max causes diminished root hair curling, reduces nodule formation and interferes with the induction of three critical symbiotic genes: Nuclear Factor Y-B (NF-YB), Nodule Inception (NIN) and Flotillin2 (FLOT2). Our findings provide evidence that the common bean and soybean AGO5 genes play an essential role in the establishment of the symbiosis with rhizobia.

Respiratory Burst Oxidase Homolog Gene A Is Crucial for Rhizobium Infection and Nodule Maturation and Function in Common Bean

Reactive oxygen species (ROS) produced by respiratory burst oxidase homologs (RBOHs) regulate numerous plant cell processes, including the symbiosis between legumes and nitrogen-fixing bacteria. Rapid and transient ROS production was reported after Phaseolus vulgaris root hairs were treated with Nod factors, indicating the presence of a ROS-associated molecular signature in the symbiosis signaling pathway. Rboh is a multigene family containing nine members (RbohA-I) in P. vulgaris. RNA interference of RbohB suppresses ROS production and attenuates rhizobial infection thread (IT) progression in P. vulgaris root hairs. However, the roles of other Rboh members in symbiotic interactions are largely unknown. In this study, we characterized the role of the NADPH oxidase-encoding gene RbohA (Phvulv091020621) in the P. vulgaris-Rhizobium tropici symbiosis. The spatiotemporal activity of the RbohA promoter colocalized with growing ITs and was associated with vascular bundles in developing nodules. Subcellular localization studies indicated that RBOHA was localized in the plasma membrane of P. vulgaris root hairs. After rhizobial inoculation, PvRBOHA was mainly distributed in the infection pocket and, to a lesser extent, throughout the IT. In PvRbohA RNAi lines, the rhizobial infection events were significantly reduced and, in successful infections, IT progression was arrested within the root hair, but did not impede cortical cell division. PvRbohA-RNAi nodules failed to fix nitrogen, since the infected cells in the few nodules formed were empty. RbohA-dependent ROS production and upregulation of several antioxidant enzymes was attenuated in rhizobia-inoculated PvRbohA-RNAi roots. These combined results indicate that PvRbohA is crucial for effective Rhizobium infection and its release into the nodule cells. This oxidase is partially or indirectly required to promote nodule organogenesis, altering the expression of auxin- and cyclin-related genes and genes involved in cell growth and division.

The riddle of mitochondrial alkaline/neutral invertases: A novel Arabidopsis isoform mainly present in reproductive tissues and involved in root ROS production

Alkaline/neutral invertases (A/N-Inv), glucosidases that irreversibly hydrolyze sucrose into glucose and fructose, play significant roles in plant growth, development, and stress adaptation. They occur as multiple isoforms located in the cytosol or organelles. In Arabidopsis thaliana, two mitochondrial A/N-Inv genes (A/N-InvA and A/N-InvC) have already been investigated. In this study, we functionally characterized A/N-InvH, a third Arabidopsis gene coding for a mitochondrial-targeted protein. The phenotypic analysis of knockout mutant plants (invh) showed a severely reduced shoot growth, while root development was not affected. The emergence of the first floral bud and the opening of the first flower were the most affected stages, presenting a significant delay. A/N-InvH transcription is markedly active in reproductive tissues. It is also expressed in the elongation and apical meristem root zones. Our results show that A/N-InvH expression is not evident in photosynthetic tissues, despite being of relevance in developmental processes and mitochondrial functional status. NaCl and mannitol treatments increased A/N-InvH expression twofold in the columella root cap. Moreover, the absence of A/N-InvH prevented ROS formation, not only in invh roots of salt- and ABA-treated seedlings but also in invh control roots. We hypothesize that this isoform may take part in the ROS/sugar (sucrose or its hydrolysis products) signaling pathway network, involved in reproductive tissue development, cell elongation, and abiotic stress responses.

Hairy root transgene expression analysis of a secretory peroxidase (PvPOX1) from common bean infected by Fusarium wilt

Plant peroxidases (POXs) are one of the most important redox enzymes in the defense responses. However, the large number of different plant POX genes makes it necessary to carefully confirm the function of each paralogous POX gene in specific tissues and disease interactions. Fusarium wilt is a devastating disease of common bean caused by Fusarium oxysporum f. sp. phaseoli. In this study, we evaluated a peroxidase gene, PvPOX1, from a resistant common bean genotype, CAAS260205 and provided direct evidence for PvPOX1's role in resistance by transforming the resistant allele into a susceptible common bean genotype, BRB130, via hairy root transformation using Agrobacterium rhizogenes. Analysis of PvPOX1 gene over-expressing hairy roots showed it increased resistance to Fusarium wilt both in the roots and the rest of transgenic plants. Meanwhile, the PvPOX1 expressive level, the peroxidase activity and hydrogen peroxide (H₂O₂) accumulation were also enhanced in the interaction. The result showed that the PvPOX1 gene played an essential role in Fusarium wilt resistance through the occurrence of reactive oxygen species (ROS) induced hypersensitive response. Therefore, PvPOX1 expression was proven to be a valuable gene for further analysis which can strengthen host defense response against Fusarium wilt through a ROS activated resistance mechanism.

Ex vitro hairy root induction in detached peanut leaves for plant-nematode interaction studies

BACKGROUND

Peanut (Arachis hypogaea) production is largely affected by a variety of abiotic and biotic stresses, including the root-knot nematode (RKN) Meloidogyne arenaria that causes yield losses worldwide. Transcriptome studies of wild Arachis species, which harbor resistance to a number of pests and diseases, disclosed several candidate genes for M. arenaria resistance. Peanut is recalcitrant to genetic transformation, so the use of Agrobacterium rhizogenes-derived hairy roots emerged as an alternative for in-root functional characterization of these candidate genes.

RESULTS

The present report describes an ex vitro methodology for hairy root induction in detached leaves based on the well-known ability of peanut to produce roots spontaneously from its petiole, which can be maintained for extended periods under high-humidity conditions. Thirty days after infection with the A. rhizogenes 'K599' strain, 90% of the detached leaves developed transgenic hairy roots with 5 cm of length in average, which were then inoculated with M. arenaria. For improved results, plant transformation, and nematode inoculation parameters were adjusted, such as bacterial cell density and growth stage; moist chamber conditions and nematode inoculum concentration. Using this methodology, a candidate gene for nematode resistance, AdEXLB8, was successfully overexpressed in hairy roots of the nematode-susceptible peanut cultivar 'Runner', resulting in 98% reduction in the number of galls and egg masses compared to the control, 60 days after M. arenaria infection.

CONCLUSIONS

This methodology proved to be more practical and cost-effective for functional validation of peanut candidate genes than in vitro and composite plant approaches, as it requires less space, reduces analysis costs and displays high transformation efficiency. The reduction in the number of RKN galls and egg masses in peanut hairy roots overexpressing AdEXLB8 corroborated the use of this strategy for functional characterization of root expressing candidate genes. This approach could be applicable not only for peanut-nematode interaction studies but also to other peanut root diseases, such as those caused by fungi and bacteria, being also potentially extended to other crop species displaying similar petiole-rooting competence.

The monomeric GTPase RabA2 is required for progression and maintenance of membrane integrity of infection threads during root nodule symbiosis

Progression of the infection canal that conducts rhizobia to the nodule primordium requires a functional Rab GTPase located in Golgi/trans-Golgi that also participate in root hair polar growth. Common bean (Phaseolus vulgaris) symbiotically associates with its partner Rhizobium etli, resulting in the formation of root nitrogen-fixing nodules. Compatible bacteria can reach cortical cells in a tightly regulated infection process, in which the specific recognition of signal molecules is a key step to select the symbiotic partner. In this work, we show that RabA2, a monomeric GTPase from common bean, is required for the progression of the infection canal, referred to as the infection thread (IT), toward the cortical cells. Expression of miss-regulated mutant variants of RabA2 resulted in an increased number of abortive infection events, including bursting of ITs and a reduction in the number of nodules. Nodules formed in these plants were small and contained infected cells with disrupted symbiosome membranes, indicating either early senescence of these cells or defects in the formation of the symbiosome membrane during bacterial release. RabA2 localized to mobile vesicles around the IT, but mutations that affect GTP hydrolysis or GTP/GDP exchange modified this localization. Colocalization of RabA2 with ArfA1 and a Golgi marker indicates that RabA2 localizes in Golgi stacks and the trans-Golgi network. Our results suggest that RabA2 is part of the vesicle transport events required to maintain the integrity of the membrane during IT progression.

The legume miR1514a modulates a NAC transcription factor transcript to trigger phasiRNA formation in response to drought

Recent studies have identified microRNAs as post-transcriptional regulators involved in stress responses. miR1514a is a legume microRNA that is induced in response to drought stress in Phaseolus vulgaris (common bean) and shows differential accumulation levels in roots during water deficit in two cultivars with different drought tolerance phenotypes. A recent degradome analysis revealed that miR1514a targets the transcripts of two NAC transcription factors (TFs), Phvul.010g121000 and Phvul.010g120700. Furthermore, expression studies and small RNA-seq data indicate that only Phvul.010g120700 generates phasiRNAs, which also accumulate under water deficit conditions. To confirm these results, we over-expressed miR1514a in transgenic hairy roots, and observed a reduced accumulation of Phvul.010g120700 and an increase in NAC-derived phasiRNAs; inhibition of miR1514a activity resulted in the opposite effect. Moreover, we determined that a NAC-derived phasiRNA associates with ARGONAUTE 1 (AGO1), suggesting that it is functional. In addition, a transcriptome analysis of transgenic hairy roots with reduced miR1514a levels revealed several differentially expressed transcripts, mainly involved in metabolism and stress responses, suggesting they are regulated by the NAC TF and/or by phasiRNAs. This work therefore demonstrates the participation of miR1514 in the regulation of a NAC transcription factor transcript through phasiRNA production during the plant response to water deficit.

Small RNAs Derived from the T-DNA of Agrobacterium rhizogenes in Hairy Roots of Phaseolus vulgaris

Agrobacterium rhizogenes is a pathogenic bacteria that causes hairy root disease by transferring bacterial DNA into the plant genome. It is an essential tool for industry and research due to its capacity to produce genetically modified roots and whole organisms. Here, we identified and characterized small RNAs generated from the transfer DNA (T-DNA) of A. rhizogenes in hairy roots of common bean (Phaseolus vulgaris). Distinct abundant A. rhizogenes T-DNA-derived small RNAs (ArT-sRNAs) belonging to several oncogenes were detected in hairy roots using high-throughput sequencing. The most abundant and diverse species of ArT-sRNAs were those of 21- and 22-nucleotides in length. Many T-DNA encoded genes constituted phasiRNA producing loci (PHAS loci). Interestingly, degradome analysis revealed that ArT-sRNAs potentially target genes of P. vulgaris. In addition, we detected low levels of ArT-sRNAs in the A. rhizogenes-induced calli generated at the wound site before hairy root emergence. These results suggest that RNA silencing targets several genes from T-DNA of A. rhizogenes in hairy roots of common bean. Therefore, the role of RNA silencing observed in this study has implications in our understanding and usage of this unique plant-bacteria interaction.

The Class II Trehalose 6-phosphate Synthase Gene PvTPS9 Modulates Trehalose Metabolism in Phaseolus vulgaris Nodules

Legumes form symbioses with rhizobia, producing nitrogen-fixing nodules on the roots of the plant host. The network of plant signaling pathways affecting carbon metabolism may determine the final number of nodules. The trehalose biosynthetic pathway regulates carbon metabolism and plays a fundamental role in plant growth and development, as well as in plant-microbe interactions. The expression of genes for trehalose synthesis during nodule development suggests that this metabolite may play a role in legume-rhizobia symbiosis. In this work, PvTPS9, which encodes a Class II trehalose-6-phosphate synthase (TPS) of common bean (Phaseolus vulgaris), was silenced by RNA interference in transgenic nodules. The silencing of PvTPS9 in root nodules resulted in a reduction of 85% (± 1%) of its transcript, which correlated with a 30% decrease in trehalose contents of transgenic nodules and in untransformed leaves. Composite transgenic plants with PvTPS9 silenced in the roots showed no changes in nodule number and nitrogen fixation, but a severe reduction in plant biomass and altered transcript profiles of all Class II TPS genes. Our data suggest that PvTPS9 plays a key role in modulating trehalose metabolism in the symbiotic nodule and, therefore, in the whole plant.

Functional specialization of one copy of glutamine phosphoribosyl pyrophosphate amidotransferase in ureide production from symbiotically fixed nitrogen in Phaseolus vulgaris

Purines are essential molecules formed in a highly regulated pathway in all organisms. In tropical legumes, the nitrogen fixed in the nodules is used to generate ureides through the oxidation of de novo synthesized purines. Glutamine phosphoribosyl pyrophosphate amidotransferase (PRAT) catalyses the first committed step of de novo purine synthesis. In Phaseolus vulgaris there are three genes coding for PRAT. The three full-length sequences, which are intron-less genes, were cloned, and their expression levels were determined under conditions that affect the synthesis of purines. One of the three genes, PvPRAT3, is highly expressed in nodules and protein amount and enzymatic activity in these tissues correlate with nitrogen fixation activity. Inhibition of PvPRAT3 gene expression by RNAi-silencing and subsequent metabolomic analysis of the transformed roots shows that PvPRAT3 is essential for the synthesis of ureides in P. vulgaris nodules.

Protoplast isolation, transient transformation of leaf mesophyll protoplasts and improved Agrobacterium-mediated leaf disc infiltration of Phaseolus vulgaris: tools for rapid gene expression analysis

BACKGROUND

Phaseolus vulgaris is one of the most extensively studied model legumes in the world. The P. vulgaris genome sequence is available; therefore, the need for an efficient and rapid transformation system is more imperative than ever. The functional characterization of P. vulgaris genes is impeded chiefly due to the non-amenable nature of Phaseolus sp. to stable genetic transformation. Transient transformation systems are convenient and versatile alternatives for rapid gene functional characterization studies. Hence, the present work focuses on standardizing methodologies for protoplast isolation from multiple tissues and transient transformation protocols for rapid gene expression analysis in the recalcitrant grain legume P. vulgaris.

RESULTS

Herein, we provide methodologies for the high-throughput isolation of leaf mesophyll-, flower petal-, hypocotyl-, root- and nodule-derived protoplasts from P. vulgaris. The highly efficient polyethylene glycol-mannitol magnesium (PEG-MMG)-mediated transformation of leaf mesophyll protoplasts was optimized using a GUS reporter gene. We used the P. vulgaris SNF1-related protein kinase 1 (PvSnRK1) gene as proof of concept to demonstrate rapid gene functional analysis. An RT-qPCR analysis of protoplasts that had been transformed with PvSnRK1-RNAi and PvSnRK1-OE vectors showed the significant downregulation and ectopic constitutive expression (overexpression), respectively, of the PvSnRK1 transcript. We also demonstrated an improved transient transformation approach, sonication-assisted Agrobacterium-mediated transformation (SAAT), for the leaf disc infiltration of P. vulgaris. Interestingly, this method resulted in a 90 % transformation efficiency and transformed 60-85 % of the cells in a given area of the leaf surface. The constitutive expression of YFP further confirmed the amenability of the system to gene functional characterization studies.

CONCLUSIONS

We present simple and efficient methodologies for protoplast isolation from multiple P. vulgaris tissues. We also provide a high-efficiency and amenable method for leaf mesophyll transformation for rapid gene functional characterization studies. Furthermore, a modified SAAT leaf disc infiltration approach aids in validating genes and their functions. Together, these methods help to rapidly unravel novel gene functions and are promising tools for P. vulgaris research.

Eucalyptus hairy roots, a fast, efficient and versatile tool to explore function and expression of genes involved in wood formation

Eucalyptus are of tremendous economic importance being the most planted hardwoods worldwide for pulp and paper, timber and bioenergy. The recent release of the Eucalyptus grandis genome sequence pointed out many new candidate genes potentially involved in secondary growth, wood formation or lineage-specific biosynthetic pathways. Their functional characterization is, however, hindered by the tedious, time-consuming and inefficient transformation systems available hitherto for eucalypts. To overcome this limitation, we developed a fast, reliable and efficient protocol to obtain and easily detect co-transformed E. grandis hairy roots using fluorescent markers, with an average efficiency of 62%. We set up conditions both to cultivate excised roots in vitro and to harden composite plants and verified that hairy root morphology and vascular system anatomy were similar to wild-type ones. We further demonstrated that co-transformed hairy roots are suitable for medium-throughput functional studies enabling, for instance, protein subcellular localization, gene expression patterns through RT-qPCR and promoter expression, as well as the modulation of endogenous gene expression. Down-regulation of the Eucalyptus cinnamoyl-CoA reductase1 (EgCCR1) gene, encoding a key enzyme in lignin biosynthesis, led to transgenic roots with reduced lignin levels and thinner cell walls. This gene was used as a proof of concept to demonstrate that the function of genes involved in secondary cell wall biosynthesis and wood formation can be elucidated in transgenic hairy roots using histochemical, transcriptomic and biochemical approaches. The method described here is timely because it will accelerate gene mining of the genome for both basic research and industry purposes.

Use of BABA and INA As Activators of a Primed State in the Common Bean (Phaseolus vulgaris L.)

To survive in adverse conditions, plants have evolved complex mechanisms that "prime" their defense system to respond and adapt to stresses. Their competence to respond to such stresses fundamentally depends on its capacity to modulate the transcriptome rapidly and specifically. Thus, chromatin dynamics is a mechanism linked to transcriptional regulation and enhanced defense in plants. For example, in Arabidopsis, priming of the SA-dependent defense pathway is linked to histone lysine methylation. Such modifications could create a memory of the primary infection that is associated with an amplified gene response upon exposure to a second stress-stimulus. In addition, the priming status of a plant for induced resistance can be inherited to its offspring. However, analyses on the molecular mechanisms of generational and transgenerational priming in the common bean (Phaseolus vulagris L.), an economically important crop, are absent. Here, we provide evidence that resistance to P. syringae pv. phaseolicola infection was induced in the common bean with the synthetic priming activators BABA and INA. Resistance was assessed by evaluating symptom appearance, pathogen accumulation, changes in gene expression of defense genes, as well as changes in the H3K4me3 and H3K36me3 marks at the promoter-exon regions of defense-associated genes. We conclude that defense priming in the common bean occurred in response to BABA and INA and that these synthetic activators primed distinct genes for enhanced disease resistance. We hope that an understanding of the molecular changes leading to defense priming and pathogen resistance will provide valuable knowledge for producing disease-resistant crop varieties by exposing parental plants to priming activators, as well as to the development of novel plant protection chemicals that stimulate the plant's inherent disease resistance mechanisms.