Direct activation of EXPANSIN14 by LBD18 in the gene regulatory network of lateral root formation in Arabidopsis

The Expansin Gene CsLIEXP1 Is a Direct Target of CsLOB1 in Citrus

Transcription activator-like effectors are key virulence factors of Xanthomonas. They are secreted into host plant cells and mimic transcription factors inducing the expression of host susceptibility (S) genes. In citrus, CsLOB1 is a direct target of PthA4, the primary effector associated with citrus canker symptoms. CsLOB1 is a transcription factor, and its expression is required for canker symptoms induced by Xanthomonas citri subsp. citri. Several genes are up-regulated by PthA4; however, only CsLOB1 was described as an S gene induced by PthA4. Here, we investigated whether other up-regulated genes could be direct targets of PthA4 or CsLOB1. Seven up-regulated genes by PthA4 were investigated; however, an expansin-coding gene was more induced than CsLOB1. In Nicotiana benthamiana transient expression experiments, we demonstrate that the expansin-coding gene, referred here to as CsLOB1-INDUCED EXPANSIN 1 (CsLIEXP1), is not a direct target of PthA4, but CsLOB1. Interestingly, CsLIEXP1 was induced by CsLOB1 even without the predicted CsLOB1 binding site, which suggested that CsLOB1 has other unknown binding sites. We also investigated the minimum promoter regulated by CsLOB1, and this region and LOB1 domain were conserved among citrus species and relatives, which suggests that the interaction PthA4-CsLOB1-CsLIEXP1 is conserved in citrus species and relatives. This is the first study that experimentally demonstrated a CsLOB1 downstream target and lays the foundation to identify other new targets. In addition, we demonstrated that the CsLIEXP1 is a putative S gene indirectly induced by PthA4, which may serve as the target for genome editing to generate citrus canker-resistant varieties.

LBD18 and IAA14 antagonistically interact with ARF7 via the invariant Lys and acidic residues of the OPCA motif in the PB1 domain

MAIN CONCLUSION

LBD18 and IAA14 antagonistically interact with ARF7 through the electrostatic faces in the ARF7PB1 domain, modulating ARF7 transcriptional activity. Auxin Response Factor 7 (ARF7)/ARF19 control lateral root development by directly activating Lateral Organ Boundaries Domain 16 (LBD16)/LBD18 genes in Arabidopsis. LBD18 upregulates ARF19 expression by binding to the ARF19 promoter. It also interacts with ARF7 through the Phox and Bem1 (PB1) domain to enhance the ARF7 transcriptional activity, forming a dual mode of positive feedback loop. LBD18 competes with the repressor indole-3-acetic acid 14 (IAA14) for ARF7 binding through the PB1 domain. In this study, we examined the molecular determinant of the ARF7 PB1 domain for interacting with LBD18 and showed that the electronic faces in the ARF7 PB1 domain are critical for interacting with LBD18 and IAA14/17. We used a luminescence complementation imaging assay to determine protein-protein interactions. The results showed that mutation of the invariant lysine residue and the OPCA motif in the PB1 domain in ARF7 significantly reduces the protein interaction between ARF7 and LBD18. Transient gene expression assays with Arabidopsis protoplasts showed that IAA14 suppressed transcription-enhancing activity of LBD18 on the LUC reporter gene fused to the ARF19 promoter harboring an auxin response element, but mutation of the invariant lysine residue and OPCA motif in the PB1 domain of IAA14 reduced the repression capability of IAA14 for transcription-enhancing activity of LBD18. We further showed that the same mutation in the PB1 domain of IAA14 reduces its repression capability, thereby increasing the LUC activity induced by both ARF7 and LBD18 compared with IAA14. These results suggest that LBD18 competes with IAA14 for ARF7 binding via the electrostatic faces of the ARF7 PB1 domain to modulate ARF7 transcriptional activity.

Genome-Wide Analysis of LBD Transcription Factor Genes in Dendrobiumcatenatum

The LATERAL ORGAN BOUNDARIES DOMAIN (LBD) gene family comprises plant-specific transcription factors that control cell proliferation and differentiation during growth and development in many plant species. However, to date, no studies of the LBD gene family in Dendrobium catenatum have been reported. In this study, a genome-wide analysis of LBD genes was performed in D. catenatum and 24 LBD genes were identified. The genes were classified into two classes (I and II) based on phylogenetic relationships and motif structure. Subcellular localization analysis for DcaLBD6 and DcaLBD18 from class I and DcaLBD37 and DcaLBD41 from class II revealed that the proteins were localized in the nucleus. Transient expression analysis of DcaLBD6, DcaLBD18, DcaLBD37, and DcaLBD41 indicated that class I and class II members have opposite roles in regulating VASCULAR-RELATED NAC-DOMAIN 7 (VND7) expression. DcaLBD genes showed diverse expression patterns in response to different phytohormone treatments. Heat maps revealed diverse patterns of DcaLBD gene expression in different organs. These results lay the foundation for further detailed studies of the LBD gene family in D. catenatum.

HD-ZIP IV gene Roc8 regulates the size of bulliform cells and lignin content in rice

The morphology of bulliform cells located on the upper epidermis of leaves is one of the most important cell structures affecting leaf shape. Although many mechanisms regulating the development of bulliform cells have been reported, the fine regulatory mechanisms governing this process have rarely been described. To identify novel components regulating rice leaf morphology, a mutant showing a constitutively rolling phenotype from the seedling stage to flowering, known as crm1-D, was selected for further analysis. Anatomical analyses in crm1-D were attributable to the size reduction of bulliform cells. The crm1-D was controlled by a single dominant nuclear gene. Map-based cloning revealed that Roc8, an HD zipper class IV family member, was responsible for the crm1-D phenotype. Notably, the 50-bp sequence in the 3'-untranslated region (3'-UTR) of the Roc8 gene represses Roc8 at the translational level. Moreover, the roc8 knockdown lines notably increased the size of bulliform cells. A series of assays revealed that Roc8 negatively regulates the size of bulliform cells. Unexpectedly, Roc8 was also observed to positively mediate lignin biosynthesis without incurring a production penalty. The above results show that Roc8 may have a practical application in cultivating materials with high photosynthetic efficiency and low lignin content.

PRH1 mediates ARF7-LBD dependent auxin signaling to regulate lateral root development in Arabidopsis thaliana

The development of lateral roots in Arabidopsis thaliana is strongly dependent on signaling directed by the AUXIN RESPONSE FACTOR7 (ARF7), which in turn activates LATERAL ORGAN BOUNDARIES DOMAIN (LBD) transcription factors (LBD16, LBD18 and LBD29). Here, the product of PRH1, a PR-1 homolog annotated previously as encoding a pathogen-responsive protein, was identified as a target of ARF7-mediated auxin signaling and also as participating in the development of lateral roots. PRH1 was shown to be strongly induced by auxin treatment, and plants lacking a functional copy of PRH1 formed fewer lateral roots. The transcription of PRH1 was controlled by the binding of both ARF7 and LBDs to its promoter region.

In Arabidopsis thaliana AUXIN RESPONSE FACTOR7 (ARF7)-mediated auxin signaling plays a key role in lateral roots (LRs) development. The LATERAL ORGAN BOUNDARIES DOMAIN (LBD) transcription factors (LBD16, LBD18 and LBD29) act downstream of ARF7-mediated auxin signaling to control LRs formation. Here, the PR-1 homolog PRH1 was identified as a novel target of both ARF7 and LBDs (especially the LBD29) during auxin induced LRs formation, as both ARF7 and LBDs were able to bind to the PRH1 promoter. This study provides new insights about how auxin regulates lateral root development.

Genome-wide identification and characterization of the lateral organ boundaries domain gene family in Brassica rapa var. rapa

The Lateral Organ Boundaries Domain (LBD) genes encode highly conserved plant-specific LOB domain proteins which regulate growth and development in various species. However, members of the LBD gene family have yet to be identified in Brassica rapa var. rapa. In the present study, fifty-nine LBD genes were identified and distributed on 10 chromosomes. The BrrLBD proteins are predicted to encode hydrophobic polypeptides between 118 and 394 amino acids in length and with molecular weights ranging from 13.31 to 44.24 kDa; the theoretical pI for these proteins varies from 4.83 to 9.68. There were 17 paralogous gene pairs in the BrrLBD family, suggesting that the amplification of the BrrLBD gene family involved large-scale gene duplication events. Members of the BrrLBD family were divided into 7 subclades (class I a to e, class II a and b). Analysis of gene structure and conserved domains revealed that most BrrLBD genes of the same subclade had similar gene structures and protein motifs. The expression profiles of 59 BrrLBD genes were determined through Quantitative Real-time fluorescent PCR (qRT-PCR). Most BrrLBD genes in the same subclade had similar gene expression profiles. However, the expression patterns of 7 genes differed from their duplicates, indicating that although the gene function of most BrrLBD genes has been conserved, some BrrLBD genes may have undergone evolutionary change.

The Systems Biology of Lateral Root Formation: Connecting the Dots

The root system is a major determinant of a plant's access to water and nutrients. The architecture of the root system to a large extent depends on the repeated formation of new lateral roots. In this review, we discuss lateral root development from a systems biology perspective. We focus on studies combining experiments with computational modeling that have advanced our understanding of how the auxin-centered regulatory modules involved in different stages of lateral root development exert their specific functions. Moreover, we discuss how these regulatory networks may enable robust transitions from one developmental stage to the next, a subject that thus far has received limited attention. In addition, we analyze how environmental factors impinge on these modules, and the different manners in which these environmental signals are being integrated to enable coordinated developmental decision making. Finally, we provide some suggestions for extending current models of lateral root development to incorporate multiple processes and stages. Only through more comprehensive models we can fully elucidate the cooperative effects of multiple processes on later root formation, and how one stage drives the transition to the next.

LBD18 uses a dual mode of a positive feedback loop to regulate ARF expression and transcriptional activity in Arabidopsis

A hierarchy of transcriptional regulators controlling lateral root formation in Arabidopsis thaliana has been identified, including the AUXIN RESPONSE FACTOR 7 (ARF7)/ARF19-LATERAL ORGAN BOUNDARIES DOMAIN 16 (LBD16)/LBD18 transcriptional network; however, their feedback regulation mechanisms are not known. Here we show that LBD18 controls ARF activity using the dual mode of a positive feedback loop. We showed that ARF7 and ARF19 directly bind AuxRE in the LBD18 promoter. A variety of molecular and biochemical experiments demonstrated that LBD18 binds a specific DNA motif in the ARF19 promoter to regulate its expression in vivo as well as in vitro. LBD18 interacts with ARFs including ARF7 and ARF19 via the Phox and Bem1 domain of ARF to enhance the transcriptional activity of ARF7 on AuxRE, and competes with auxin/indole-3-acetic acid (IAA) repressors for ARF binding, overriding the negative feedback loop exerted by Aux/IAA repressors. Taken together, these results show that LBD18 and ARFs form a double positive feedback loop, and that LBD18 uses the dual mode of a positive feedback loop by binding directly to the ARF19 promoter and through the protein-protein interactions with ARF7 and ARF19. This novel mechanism of feedback loops may constitute a robust feedback mechanism that ensures continued lateral root growth in response to auxin in Arabidopsis.

Lateral root formation and the multiple roles of auxin

Root systems can display variable architectures that contribute to survival strategies of plants. The model plant Arabidopsis thaliana possesses a tap root system, in which the primary root and lateral roots (LRs) are major architectural determinants. The phytohormone auxin fulfils multiple roles throughout LR development. In this review, we summarize recent advances in our understanding of four aspects of LR formation: (i) LR positioning, which determines the spatial distribution of lateral root primordia (LRP) and LRs along primary roots; (ii) LR initiation, encompassing the activation of nuclear migration in specified lateral root founder cells (LRFCs) up to the first asymmetric cell division; (iii) LR outgrowth, the 'primordium-intrinsic' patterning of de novo organ tissues and a meristem; and (iv) LR emergence, an interaction between LRP and overlaying tissues to allow passage through cell layers. We discuss how auxin signaling, embedded in a changing developmental context, plays important roles in all four phases. In addition, we discuss how rapid progress in gene network identification and analysis, modeling, and four-dimensional imaging techniques have led to an increasingly detailed understanding of the dynamic regulatory networks that control LR development.

The LATERAL ORGAN BOUNDARIES Domain gene family in grapevine: genome-wide characterization and expression analyses during developmental processes and stress responses

LATERAL ORGAN BOUNDARIES (LOB) DOMAIN (LBD) constitute a family of plant-specific transcription factors with key roles in the regulation of plant organ development, pollen development, plant regeneration, pathogen response, and anthocyanin and nitrogen metabolisms. However, the role of LBDs in fruit ripening and in grapevine (Vitis vinifera L.) development and stress responses is poorly documented. By performing a model curation of LBDs in the latest genome annotation 50 genes were identified. Phylogenetic analysis showed that LBD genes can be grouped into two classes mapping on 16 out of the 19 V. vinifera chromosomes. New gene subclasses were identified that have not been characterized in other species. Segmental and tandem duplications contributed significantly to the expansion and evolution of the LBD gene family in grapevine as noticed for other species. The analysis of cis-regulatory elements and transcription factor binding sites in the VviLBD promoter regions suggests the involvement of several hormones in the regulation of LBDs expression. Expression profiling suggest the involvement of LBD transcription factors in grapevine development, berry ripening and stress responses. Altogether this study provides valuable information and robust candidate genes for future functional analysis aiming to clarify mechanisms responsible for the onset of fruit ripening and fruit defense strategies.

Genomewide analysis of the lateral organ boundaries domain gene family in Vitis vinifera

In plants, the transcription factor families have been implicated in many important biological processes. These processes include morphogenesis, signal transduction and environmental stress responses. Proteins containing the lateral organ boundaries domain (LBD), which encodes a zinc finger-like domain are only found in plants. This finding indicates that this unique gene family regulates only plant-specific biological processes. LBD genes play crucial roles in the growth and development of plants such as Arabidopsis, Oryza sativa, Zea mays, poplar, apple and tomato. However, relatively little is known about the LBD genes in grape (Vitis vinifera). In this study, we identified 40 LBD genes in the grape genome. A complete overview of the chromosomal locations, phylogenetic relationships, structures and expression profiles of this gene family during development in grape is presented here. Phylogenetic analysis showed that the LBD genes could be divided into classes I and II, together with LBDs from Arabidopsis. We mapped the 40 LBD genes on the grape chromosomes (chr1-chr19) and found that 37 of the predicted grape LBD genes were distributed in different densities across 12 chromosomes. Grape LBDs were found to share a similar intron/exon structure and gene length within the same class. The expression profiles of grape LBD genes at different developmental stages were analysed using microarray data. Results showed that 21 grape LBD genes may be involved in grape developmental processes, including preveraison, veraison and ripening. Finally, we analysed the expression patterns of six LBD genes through quantitative real-time polymerase chain reation analysis. The six LBD genes showed differential expression patterns among the three representative grape tissues, and five of these genes were found to be involved in responses to mannitol, sodium chloride, heat stress and low temperature treatments. To our knowledge, this is the first study to analyse the LBD gene family in grape and provides valuable information for classification and functional investigation of this gene family.

Periodic Lateral Root Priming: What Makes It Tick?

Conditioning small groups of root pericycle cells for future lateral root formation has a major impact on overall plant root architecture. This priming of lateral roots occurs rhythmically, involving temporal oscillations in auxin response in the root tip. During growth, this process generates a spatial pattern of prebranch sites, an early stage in lateral root formation characterized by a stably maintained high auxin response. To date, the molecular mechanism behind this rhythmicity has remained elusive. Some data implicate a cell-autonomous oscillation in gene expression, while others strongly support the importance of tissue-level modulations in auxin fluxes. Here, we summarize the experimental data on periodic lateral root priming. We present a theoretical framework that distinguishes between a priming signal and its subsequent memorization and show how major roles for auxin fluxes and gene expression naturally emerge from this framework. We then discuss three mechanisms that could potentially induce oscillations of auxin response: cell-autonomous oscillations, Turing-type patterning, and tissue-level oscillations in auxin fluxes, along with specific properties of lateral root priming that may be used to discern which type of mechanism is most likely to drive lateral root patterning. We conclude with suggestions for future experiments and modeling studies.

Growth Stimulatory Effects and Genome-Wide Transcriptional Changes Produced by Protein Hydrolysates in Maize Seedlings

Protein hydrolysates are an emerging class of crop management products utilized for improving nutrient assimilation and mitigating crop stress. They generally consist of a mixture of peptides and free amino acids derived from the hydrolysis of plant or animal sources. The present work was aimed at studying the effects and the action mechanisms of a protein hydrolysate derived from animal residues on maize root growth and physiology in comparison with the effects induced by either free amino acids or inorganic N supply. The application of the protein hydrolysate caused a remarkable enhancement of root growth. In particular, in the protein hydrolysate-treated plants the length and surface area of lateral roots were about 7 and 1.5 times higher than in plants treated with inorganic N or free amino acids, respectively. The root growth promoting effect of the protein hydrolysate was associated with an increased root accumulation of K, Zn, Cu, and Mn when compared with inorganic N and amino acids treatments. A microarray analysis allowed to dissect the transcriptional changes induced by the different treatments demonstrating treatment-specific effects principally on cell wall organization, transport processes, stress responses and hormone metabolism.

Identification of a stable major-effect QTL (Parth 2.1) controlling parthenocarpy in cucumber and associated candidate gene analysis via whole genome re-sequencing

BACKGROUND

Parthenocarpy is an important trait for yield and quality in many plants. But due to its complex interactions with genetic and physiological factors, it has not been adequately understood and applied to breeding and production. Finding novel and effective quantitative trait loci (QTLs) is a critical step towards understanding its genetic mechanism. Cucumber (Cucumis sativus L.) is a typical parthenocarpic plant but the QTLs controlling parthenocarpy in cucumber were not mapped on chromosomes, and the linked markers were neither user-friendly nor confirmed by previous studies. Hence, we conducted a two-season QTL study of parthenocarpy based on the cucumber genome with 145 F2:3 families derived from a cross between EC1 (a parthenocarpic inbred line) and 8419 s-1 (a non-parthenocarpic inbred line) in order to map novel QTLs. Whole genome re-sequencing was also performed both to develop effective linked markers and to predict candidate genes.

RESULTS

A genetic linkage map, employing 133 Simple Sequence Repeats (SSR) markers and nine Insertion/Deletion (InDel) markers spanning 808.1 cM on seven chromosomes, was constructed from an F2 population. Seven novel QTLs were identified on chromosomes 1, 2, 3, 5 and 7. Parthenocarpy 2.1 (Parth2.1), a QTL on chromosome 2, was a major-effect QTL with a logarithm of odds (LOD) score of 9.0 and phenotypic variance explained (PVE) of 17.0 % in the spring season and with a LOD score of 6.2 and PVE of 10.2 % in the fall season. We confirmed this QTL using a residual heterozygous line97-5 (RHL97-5). Effectiveness of linked markers of the Parth2.1 was validated in F3:4 population and in 21 inbred lines. Within this region, there were 57 genes with nonsynonymous SNPs/InDels in the coding sequence. Based on further combined analysis with transcriptome data between two parents, CsARF19, CsWD40, CsEIN1, CsPPR, CsHEXO3, CsMDL, CsDJC77 and CsSMAX1 were predicted as potential candidate genes controlling parthenocarpy.

CONCLUSIONS

A major-effect QTL Parth2.1 and six minor-effect QTLs mainly contribute to the genetic architecture of parthenocarpy in cucumber. SSR16226 and Indel-T-39 can be used in marker-assisted selection (MAS) of cucumber breeding. Whole genome re-sequencing enhances the efficiency of polymorphic marker development and prediction of candidate genes.

LOB Domain Proteins: Beyond Lateral Organ Boundaries

LATERAL ORGAN BOUNDARIES DOMAIN (LBD) proteins defined by a conserved LATERAL ORGAN BOUNDARIES (LOB) domain are key regulators of plant organ development. Recent studies have expanded their functional diversity beyond the definition of lateral organ boundaries to pollen development, plant regeneration, photomorphogenesis, pathogen response, and specific developmental functions in non-model plants, such as poplar and legumes. The identification of a range of upstream regulators, protein partners, and downstream targets of LBD family members has unraveled the molecular networks of LBD-dependent processes. Moreover, it has been demonstrated that LBD proteins have essential roles in integrating developmental changes in response to phytohormone signaling or environmental cues. As we discuss here, these novel discoveries of LBD functions and their molecular contexts promote a better understanding of this plant-specific transcription factor family.

Lateral Organ Boundaries Domain16 and 18 Act Downstream of the AUXIN1 and LIKE-AUXIN3 Auxin Influx Carriers to Control Lateral Root Development in Arabidopsis

Several members of the Lateral Organ Boundaries Domain (LBD)/Asymmetric Leaves2-Like (ASL) gene family have been identified to play important roles in Arabidopsis (Arabidopsis thaliana) lateral root (LR) development during auxin response, but their functional relationship with auxin transporters has not been established yet. Here, we show that the AUXIN1 (AUX1) and LIKE-AUXIN3 (LAX3) auxin influx carriers are required for auxin signaling that activates LBD16/ASL18 and LBD18/ASL20 to control LR development. The lax3 mutant phenotype was not significantly enhanced when combined with lbd16 or lbd18. However, LBD18 overexpression could rescue the defects in LR emergence in lax3 with concomitant expression of the LBD18 target genes. Genetic and gene expression analyses indicated that LBD16 and LBD18 act with AUX1 to regulate LR initiation and LR primordium development, and that AUX1 and LAX3 are needed for auxin-responsive expression of LBD16 and LBD18. LBD18:SUPERMAN REPRESSIVE DOMAIN X in the lbd18 mutant inhibited LR initiation and LR primordium development in response to a gravitropic stimulus and suppressed promoter activities of the cell cycle genes Cyclin-Dependent Kinase A1;1 and CYCLINB1;1. Taken together, these results suggest that LBD16 and LBD18 are important regulators of LR initiation and development downstream of AUX1 and LAX3.

Identification of CROWN ROOTLESS1-regulated genes in rice reveals specific and conserved elements of postembryonic root formation

In monocotyledons, the root system is mostly composed of postembryonic shoot-borne roots called crown roots. In rice (Oryza sativa), auxin promotes crown root initiation via the LOB-domain transcription factor (LBD) transcription factor CROWN ROOTLESS1 (CRL1); however, the gene regulatory network downstream of CRL1 remains largely unknown. We tested CRL1 transcriptional activity in yeast and in planta, identified CRL1-regulated genes using an inducible gene expression system and a transcriptome analysis, and used in situ hybridization to demonstrate coexpression of a sample of CRL1-regulated genes with CRL1 in crown root primordia. We show that CRL1 positively regulates 277 genes, including key genes involved in meristem patterning (such as QUIESCENT-CENTER SPECIFIC HOMEOBOX; QHB), cell proliferation and hormone homeostasis. Many genes are homologous to Arabidopsis genes involved in lateral root formation, but about a quarter are rice-specific. Our study reveals that several genes acting downstream of LBD transcription factors controlling postembryonic root formation are conserved between monocots and dicots. It also provides evidence that specific genes are involved in the formation of shoot-derived roots in rice.

Genome-wide identification and expression profiling analysis of the Aux/IAA gene family in Medicago truncatula during the early phase of Sinorhizobium meliloti infection

Background

Auxin/indoleacetic acid (Aux/IAA) genes, coding a family of short-lived nuclear proteins, play key roles in wide variety of plant developmental processes, including root system regulation and responses to environmental stimulus. However, how they function in auxin signaling pathway and symbiosis with rhizobial in Medicago truncatula are largely unknown. The present study aims at gaining deeper insight on distinctive expression and function features of Aux/IAA family genes in Medicago truncatula during nodule formation.

Principal Findings

Using the latest updated draft of the full Medicago truncatula genome, a comprehensive identification and analysis of IAA genes were performed. The data indicated that MtIAA family genes are distributed in all the M. truncatula chromosomes except chromosome 6. Most of MtIAA genes are responsive to exogenous auxin and express in tissues-specific manner. To understand the biological functions of MtIAA genes involved in nodule formation, quantitative real-time polymerase chain reaction (qRT-PCR) was used to test the expression profiling of MtIAA genes during the early phase of Sinorhizobium meliloti (S. meliloti) infection. The expression patterns of most MtIAA genes were down-regulated in roots and up-regulated in shoots by S. meliloti infection. The differences in expression responses between roots and shoots caused by S. meliloti infection were alleviated by 1-NOA application.

Conclusion

The genome-wide identification, evolution and expression pattern analysis of MtIAA genes were performed in this study. The data helps us to understand the roles of MtIAA-mediated auxin signaling in nodule formation during the early phase of S. meliloti infection.

How boundaries control plant development

Continuous growth and organ development from the shoot apical meristem (SAM) requires a precise coordination of stem cell proliferation, commitment of stem cell descendants to diverse differentiation pathways and establishment of morphological meristem-to-organ boundaries. These complex biological processes require extensive integration of several components of cell-to-cell signaling and gene regulatory networks whose coordinated actions have an impact on cell division and growth. Here we review the current knowledge of gene networks involved in organogenesis from the SAM in higher plants. We focus on recent advances to show how the interaction between transcriptional regulators, hormonal crosstalk and physical stress regulates the establishment and maintenance of meristem-to-organ boundaries. Continuous growth and organ development from the shoot apical meristem (SAM) requires a precise coordination of stem cell proliferation, commitment of stem cell descendants to diverse differentiation pathways and establishment of morphological meristem-to-organ boundaries. These complex biological processes require extensive integration of several components of cell-to-cell signaling and gene regulatory networks whose coordinated actions have an impact on cell division and growth. Here we review the current knowledge of gene networks involved in organogenesis from the SAM in higher plants. We focus on recent advances to show how the interaction between transcriptional regulators, hormonal crosstalk and physical stress regulates the establishment and maintenance of meristem-to-organ boundaries.

Lateral organ boundaries 1 is a disease susceptibility gene for citrus bacterial canker disease

Citrus bacterial canker (CBC) disease occurs worldwide and incurs considerable costs both from control measures and yield losses. Bacteria that cause CBC require one of six known type III transcription activator-like (TAL) effector genes for the characteristic pustule formation at the site of infection. Here, we show that Xanthomonas citri subspecies citri strain Xcc306, with the type III TAL effector gene pthA4 or with the distinct yet biologically equivalent gene pthAw from strain XccA(w), induces two host genes, CsLOB1 and CsSWEET1, in a TAL effector-dependent manner. CsLOB1 is a member of the Lateral Organ Boundaries (LOB) gene family of transcription factors, and CsSWEET1 is a homolog of the SWEET sugar transporter and rice disease susceptibility gene. Both TAL effectors drive expression of CsLOB1 and CsSWEET1 promoter reporter gene fusions when coexpressed in citrus or Nicotiana benthamiana. Artificially designed TAL effectors directed to sequences in the CsLOB1 promoter region, but not the CsSWEET1 promoter, promoted pustule formation and higher bacterial leaf populations. Three additional distinct TAL effector genes, pthA*, pthB, and pthC, also direct pustule formation and expression of CsLOB1. Unlike pthA4 and pthAw, pthB and pthC do not promote the expression of CsSWEET1. CsLOB1 expression was associated with the expression of genes associated with cell expansion. The results indicate that CBC-inciting species of Xanthomonas exploit a single host disease susceptibility gene by altering the expression of an otherwise developmentally regulated gene using any one of a diverse set of TAL effector genes in the pathogen populations.