Flowering also has to end: knowns and unknowns of reproductive arrest in monocarpic plants

All flowering plants adjust their reproductive period for successful reproduction. Flowering initiation is controlled by a myriad of intensively studied factors, so it can occur in the most favorable conditions. But the end of flowering is also a controlled process, required to optimize the size of the offspring and to maximize resource allocation. Flowering arrest was described and mainly studied in the last century by physiological approaches, but it is much less understood at the genetic or molecular level. In this review, we present an overview of recent progress in this topic, fueled by highly complementary studies that are beginning to provide an integrated view of how the end of flowering is regulated. In this emerging picture, we also highlight key missing aspects that will guide future research and may provide new biotechnological avenues to improve crop yield in annual plants.

Laser Microdissection: A High-Precision Approach to Isolate Specific Cell Types from Any Plant Species for Downstream Molecular Analyses

Plants display a great diversity of particular cell types that obviously perform functions and regulations that are essential for successful growth and development, whether under optimal or adverse conditions. The functions performed by each of these particular cell types must be associated with specific transcriptomic, proteomic, and metabolic profiles that cannot be disentangled by analyzing whole plant organs and tissues. Laser microdissection is a technique for the collection of specific cell types in plant organs and tissues comprising heterogeneous cell populations. It has been successfully used for physiological and molecular studies. Laser microdissection can be applied to any plant species as long as it is possible to reliably identify the cell types of interest. Here, we describe step by step, using citrus as a model plant, a fast, simple, easy to perform, and experimentally validated protocol to collect cells from the abscission zone, a specific tissue that is difficult to access and whose activity is important in the response of plants to adverse environmental conditions.

A cellular analysis of meristem activity at the end of flowering points to cytokinin as a major regulator of proliferative arrest in Arabidopsis

In monocarpic plants, all reproductive meristem activity arrests and flower production ceases after the production of a certain number of fruits. This proliferative arrest (PA) is an evolutionary adaptation that ensures nutrient availability for seed production. Moreover, PA is a process of agronomic interest because it affects the duration of the flowering period and therefore fruit production. While our knowledge of the inputs and genetic factors controlling the initiation of the flowering period is extensive, little is known about the regulatory pathways and cellular events that participate in the end of flowering and trigger PA. Here, we characterize with high spatiotemporal resolution the cellular and molecular changes related to cell proliferation and meristem activity in the shoot apical meristem throughout the flowering period and PA. Our results suggest that cytokinin (CK) signaling repression precedes PA and that this hormone is sufficient to prevent and revert the process. We have also observed that repression of known CK downstream factors, such as type B cyclins and WUSCHEL (WUS), correlates with PA. These molecular changes are accompanied by changes in cell size and number likely caused by the cessation of cell division and WUS activity during PA. Parallel assays in fruitfull (ful) mutants, which do not undergo PA, have revealed that FUL may promote PA via repression of these CK-dependent pathways. Moreover, our data allow to define two phases, based on the relative contribution of FUL, that lead to PA: an early reduction of CK-related events and a late blocking of these events.

The shady side of leaf development: the role of the REVOLUTA/KANADI1 module in leaf patterning and auxin-mediated growth promotion

Leaves are present in all land plants and are specialized organs for light harvesting. They arise at the flanks of the shoot apical meristem (SAM), and develop into lamina structures that exhibit adaxial/abaxial (upper/lower side of the leaf) polarity. At the molecular level, an intricate regulatory network determines ad-/abaxial polarity in Arabidopsis thaliana leaves, where the Class III Homeodomain Leucine Zipper (HD-ZIPIII) and KANADI (KAN) proteins are key mediators. The HD-ZIPIII REVOLUTA (REV) is expressed in the adaxial domain of lateral organs, whereas KAN1 is involved in abaxial differentiation. The REV/KAN1 module directly and antagonistically regulates the expression of several genes involved in shade-induced growth and auxin biosynthetic enzymes.

Cell Wall Remodeling in Abscission Zone Cells during Ethylene-Promoted Fruit Abscission in Citrus

Abscission is a cell separation process by which plants can shed organs such as fruits, leaves, or flowers. The process takes place in specific locations termed abscission zones. In fruit crops like citrus, fruit abscission represents a high percentage of annual yield losses. Thus, understanding the molecular regulation of abscission is of capital relevance to control production. To identify genes preferentially expressed within the citrus fruit abscission zone (AZ-C), we performed a comparative transcriptomics assay at the cell type resolution level between the AZ-C and adjacent fruit rind cells (non-abscising tissue) during ethylene-promoted abscission. Our strategy combined laser microdissection with microarray analysis. Cell wall modification-related gene families displayed prominent representation in the AZ-C. Phylogenetic analyses of such gene families revealed a link between phylogenetic proximity and expression pattern during abscission suggesting highly conserved roles for specific members of these families in abscission. Our transcriptomic data was validated with (and strongly supported by) a parallel approach consisting on anatomical, histochemical and biochemical analyses on the AZ-C during fruit abscission. Our work identifies genes potentially involved in organ abscission and provides relevant data for future biotechnology approaches aimed at controlling such crucial process for citrus yield.

Corrigendum: Cell Wall Remodeling in Abscission Zone Cells during Ethylene-Promoted Fruit Abscission in Citrus

[This corrects the article on p. 126 in vol. 8, PMID: 28228766.].

Novel insights into the Citrus sinensis nonhost response suggest photosynthesis decline, abiotic stress networks and secondary metabolism modifications

Plants are constantly exposed to stress factors. Biotic stress is produced by living organisms such as pathogens, whereas abiotic stress by unfavourable environmental conditions. In Citrus species, one of the most important fruit crops in the world, these stresses generate serious limitations in productivity. Through biochemical and transcriptomic assays, we had previously characterised the Citrus sinensis (L.) Osbeck nonhost response to Xanthomonas campestris pv. vesicatoria (Doidge), in contrast to Asiatic citrus canker infection caused by Xanthomonas citri subsp. citri (Hasse). A hypersensitive response (HR) including changes in the expression of several transcription factors was reported. Here, a new exhaustive analysis of the Citrus sinensis transcriptomes previously obtained was performed, allowing us to detect the over-representation of photosynthesis, abiotic stress and secondary metabolism processes during the nonhost HR. The broad downregulation of photosynthesis-related genes was correlated with an altered photosynthesis physiology. The high number of heat shock proteins and genes related to abiotic stress, including aquaporins, suggests that stresses crosstalk. Additionally, the secondary metabolism exhibited lignin and carotenoid biosynthesis modifications and expression changes in the cell rescue GSTs. In conclusion, novel features of the Citrus nonhost HR, an important part of the plants' defence against disease that has yet to be fully exploited in plant breeding programs, are presented.

The LOV protein of Xanthomonas citri subsp. citri plays a significant role in the counteraction of plant immune responses during citrus canker

Pathogens interaction with a host plant starts a set of immune responses that result in complex changes in gene expression and plant physiology. Light is an important modulator of plant defense response and recent studies have evidenced the novel influence of this environmental stimulus in the virulence of several bacterial pathogens. Xanthomonas citri subsp. citri is the bacterium responsible for citrus canker disease, which affects most citrus cultivars. The ability of this bacterium to colonize host plants is influenced by bacterial blue-light sensing through a LOV-domain protein and disease symptoms are considerably altered upon deletion of this protein. In this work we aimed to unravel the role of this photoreceptor during the bacterial counteraction of plant immune responses leading to citrus canker development. We performed a transcriptomic analysis in Citrus sinensis leaves inoculated with the wild type X. citri subsp. citri and with a mutant strain lacking the LOV protein by a cDNA microarray and evaluated the differentially regulated genes corresponding to specific biological processes. A down-regulation of photosynthesis-related genes (together with a corresponding decrease in photosynthesis rates) was observed upon bacterial infection, this effect being more pronounced in plants infected with the lov-mutant bacterial strain. Infection with this strain was also accompanied with the up-regulation of several secondary metabolism- and defense response-related genes. Moreover, we found that relevant plant physiological alterations triggered by pathogen attack such as cell wall fortification and tissue disruption were amplified during the lov-mutant strain infection. These results suggest the participation of the LOV-domain protein from X. citri subsp. citri in the bacterial counteraction of host plant defense response, contributing in this way to disease development.

Genome-wide identification of KANADI1 target genes

Plant organ development and polarity establishment is mediated by the action of several transcription factors. Among these, the KANADI (KAN) subclade of the GARP protein family plays important roles in polarity-associated processes during embryo, shoot and root patterning. In this study, we have identified a set of potential direct target genes of KAN1 through a combination of chromatin immunoprecipitation/DNA sequencing (ChIP-Seq) and genome-wide transcriptional profiling using tiling arrays. Target genes are over-represented for genes involved in the regulation of organ development as well as in the response to auxin. KAN1 affects directly the expression of several genes previously shown to be important in the establishment of polarity during lateral organ and vascular tissue development. We also show that KAN1 controls through its target genes auxin effects on organ development at different levels: transport and its regulation, and signaling. In addition, KAN1 regulates genes involved in the response to abscisic acid, jasmonic acid, brassinosteroids, ethylene, cytokinins and gibberellins. The role of KAN1 in organ polarity is antagonized by HD-ZIPIII transcription factors, including REVOLUTA (REV). A comparison of their target genes reveals that the REV/KAN1 module acts in organ patterning through opposite regulation of shared targets. Evidence of mutual repression between closely related family members is also shown.

Characterization of Citrus sinensis transcription factors closely associated with the non-host response to Xanthomonas campestris pv. vesicatoria

Plants, when exposed to certain pathogens, may display a form of genotype-independent resistance, known as non-host response. In this study, the response of Citrus sinensis (sweet orange) leaves to Xanthomonas campestris pv. vesicatoria (Xcv), a pepper and tomato pathogenic bacterium, was analyzed through biochemical assays and cDNA microarray hybridization and compared with Asiatic citrus canker infection caused by Xanthomonas citri subsp. citri. Citrus leaves exposed to the non-host bacterium Xcv showed hypersensitive response (HR) symptoms (cell death), a defense mechanism common in plants but poorly understood in citrus. The HR response was accompanied by differentially expressed genes that are associated with biotic stress and cell death. Moreover, 58 transcription factors (TFs) were differentially regulated by Xcv in citrus leaves, including 26 TFs from the stress-associated families AP2-EREBP, bZip, Myb and WRKY. Remarkably, in silico analysis of the distribution of expressed sequence tags revealed that 10 of the 58 TFs, belonging to C2C2-GATA, C2H2, CCAAT, HSF, NAC and WRKY gene families, were specifically over-represented in citrus stress cDNA libraries. This study identified candidate TF genes for the regulation of key steps during the citrus non-host HR. Furthermore, these TFs might be useful in future strategies of molecular breeding for citrus disease resistance.

Elucidating mechanisms underlying organ abscission

Abscission consists in the detachment of entire vegetative and reproductive organs due to cell separation processes occurring at the abscission zones (AZs) at specific positions of the plant body. From an evolutionary point of view, abscission is a highly advantageous process resulting into fruit and seed dispersal as well as the shedding of no longer useful organs. In an agricultural context, however, abscission may become a major limiting factor for crop productivity. Domestication of major crops included the selection of plants that did not naturally shed ripe fruits or seeds. The understanding of abscission is of great importance to control seed and fruit production and to improve breeding and harvesting practices. Thus, advances made on model plants and crops are of major importance since they may provide potential candidate genes for further biotechnological applications. Here, we review the current knowledge of the physiological, genetic and genomic aspects related to abscission including the most recently disclosed putative regulators that appear to be implicated in the development and/or activation of the AZs.

Early gene expression events in the laminar abscission zone of abscission-promoted citrus leaves after a cycle of water stress/rehydration: involvement of CitbHLH1

Leaf abscission is a common response of plants to drought stress. Some species, such as citrus, have evolved a specific behaviour in this respect, keeping their leaves attached to the plant body during water stress until this is released by irrigation or rain. This study successfully reproduced this phenomenon under controlled conditions (24h of water stress followed by 24h of rehydration) and used it to construct a suppression subtractive hybridization cDNA library enriched in genes involved in the early stages of rehydration-promoted leaf abscission after water stress. Sequencing of the library yielded 314 unigenes, which were spotted onto nylon membranes. Membrane hybridization with petiole (Pet)- and laminar abscission zone (LAZ)-enriched RNA samples corresponding to early steps in leaf abscission revealed an almost exclusive preferential gene expression programme in the LAZ. The data identified major processes such as protein metabolism, cell-wall modification, signalling, control of transcription and vesicle production, and transport as the main biological processes activated in LAZs during the early steps of rehydration-promoted leaf abscission after water stress. Based on these findings, a model for the early steps of citrus leaf abscission is proposed. In addition, it is suggested that CitbHLH1, the putative citrus orthologue of Arabidopsis BIGPETAL, may play major roles in the control of abscission-related events in citrus abscission zones.

Comparative transcriptome analysis of stylar canal cells identifies novel candidate genes implicated in the self-incompatibility response of Citrus clementina

BACKGROUND

Reproductive biology in citrus is still poorly understood. Although in recent years several efforts have been made to study pollen-pistil interaction and self-incompatibility, little information is available about the molecular mechanisms regulating these processes. Here we report the identification of candidate genes involved in pollen-pistil interaction and self-incompatibility in clementine (Citrus clementina Hort. ex Tan.). These genes have been identified comparing the transcriptomes of laser-microdissected stylar canal cells (SCC) isolated from two genotypes differing for self-incompatibility response ('Comune', a self-incompatible cultivar and 'Monreal', a self- compatible mutation of 'Comune').

RESULTS

The transcriptome profiling of SCC indicated that the differential regulation of few specific, mostly uncharacterized transcripts is associated with the breakdown of self-incompatibility in 'Monreal'. Among them, a novel F-box gene showed a drastic up-regulation both in laser microdissected stylar canal cells and in self-pollinated whole styles with stigmas of 'Comune' in concomitance with the arrest of pollen tube growth. Moreover, we identify a non-characterized gene family as closely associated to the self-incompatibility genetic program activated in 'Comune'. Three different aspartic-acid rich (Asp-rich) protein genes, located in tandem in the clementine genome, were over-represented in the transcriptome of 'Comune'. These genes are tightly linked to a DELLA gene, previously found to be up-regulated in the self-incompatible genotype during pollen-pistil interaction.

CONCLUSION

The highly specific transcriptome survey of the stylar canal cells identified novel genes which have not been previously associated with self-pollen rejection in citrus and in other plant species. Bioinformatic and transcriptional analyses suggested that the mutation leading to self-compatibility in 'Monreal' affected the expression of non-homologous genes located in a restricted genome region. Also, we hypothesize that the Asp-rich protein genes may act as Ca2+ "entrapping" proteins, potentially regulating Ca2+ homeostasis during self-pollen recognition.

Comparative transcriptional survey between laser-microdissected cells from laminar abscission zone and petiolar cortical tissue during ethylene-promoted abscission in citrus leaves

BACKGROUND

Abscission is the cell separation process by which plants are able to shed organs. It has a great impact on the yield of most crop plants. At the same time, the process itself also constitutes an excellent model to study cell separation processes, since it occurs in concrete areas known as abscission zones (AZs) which are composed of a specific cell type. However, molecular approaches are generally hampered by the limited area and cell number constituting the AZ. Therefore, detailed studies at the resolution of cell type are of great relevance in order to accurately describe the process and to identify potential candidate genes for biotechnological applications.

RESULTS

Efficient protocols for the isolation of specific citrus cell types, namely laminar abscission zone (LAZ) and petiolar cortical (Pet) cells based on laser capture microdissection (LCM) and for RNA microextraction and amplification have been developed. A comparative transcriptome analysis between LAZ and Pet from citrus leaf explants subjected to an in-vitro 24 h ethylene treatment was performed utilising microarray hybridization and analysis. Our analyses of gene functional classes differentially represented in ethylene-treated LAZ revealed an activation program dominated by the expression of genes associated with protein synthesis, protein fate, cell type differentiation, development and transcription. The extensive repertoire of genes associated with cell wall biosynthesis and metabolism strongly suggests that LAZ layers activate both catabolic and anabolic wall modification pathways during the abscission program. In addition, over-representation of particular members of different transcription factor families suggests important roles for these genes in the differentiation of the effective cell separation layer within the many layers contained in the citrus LAZ. Preferential expression of stress-related and defensive genes in Pet reveals that this tissue is likely to be reprogrammed to prevent pathogen attacks and general abiotic stresses after organ shedding.

CONCLUSION

The LCM-based data generated in this survey represent the most accurate description of the main biological processes and genes involved in organ abscission in citrus. This study provides novel molecular insight into ethylene-promoted leaf abscission and identifies new putative target genes for characterization and manipulation of organ abscission in citrus.

Ethylene-induced differential gene expression during abscission of citrus leaves

The main objective of this work was to identify and classify genes involved in the process of leaf abscission in Clementina de Nules (Citrus clementina Hort. Ex Tan.). A 7 K unigene citrus cDNA microarray containing 12 K spots was used to characterize the transcriptome of the ethylene-induced abscission process in laminar abscission zone-enriched tissues and the petiole of debladed leaf explants. In these conditions, ethylene induced 100% leaf explant abscission in 72 h while, in air-treated samples, the abscission period started later and took 240 h. Gene expression monitored during the first 36 h of ethylene treatment showed that out of the 12 672 cDNA microarray probes, ethylene differentially induced 725 probes distributed as follows: 216 (29.8%) probes in the laminar abscission zone and 509 (70.2%) in the petiole. Functional MIPS classification and manual annotation of differentially expressed genes highlighted key processes regulating the activation and progress of the cell separation that brings about abscission. These included cell-wall modification, lipid transport, protein biosynthesis and degradation, and differential activation of signal transduction and transcription control pathways. Expression data associated with the petiole indicated the occurrence of a double defensive strategy mediated by the activation of a biochemical programme including scavenging ROS, defence and PR genes, and a physical response mostly based on lignin biosynthesis and deposition. This work identifies new genes probably involved in the onset and development of the leaf abscission process and suggests a different but co-ordinated and complementary role for the laminar abscission zone and the petiole during the process of abscission.