Endoreplication as a potential driver of cell wall modifications

Ploidy-specific transcriptomes shed light on the heterogeneous identity and metabolism of developing tomato pericarp cells

Endoreduplication, during which cells increase their DNA content through successive rounds of full genome replication without cell division, is the major source of endopolyploidy in higher plants. Endoreduplication plays pivotal roles in plant growth and development and is associated with the activation of specific transcriptional programmes that are characteristic of each cell type, thereby defining their identity. In plants, endoreduplication is found in numerous organs and cell types, especially in agronomically valuable ones, such as the fleshy fruit (pericarp) of tomato presenting high ploidy levels. We used the tomato pericarp tissue as a model system to explore the transcriptomes associated with endoreduplication progression during fruit growth. We confirmed that expression globally scales with ploidy level and identified sets of differentially expressed genes presenting only developmental-specific, only ploidy-specific expression patterns or profiles resulting from an additive effect of ploidy and development. When comparing ploidy levels at a specific developmental stage, we found that non-endoreduplicated cells are defined by cell division state and cuticle synthesis while endoreduplicated cells are mainly defined by their metabolic activity changing rapidly over time. By combining this dataset with publicly available spatiotemporal pericarp expression data, we proposed a map describing the distribution of ploidy levels within the pericarp. These transcriptome-based predictions were validated by quantifying ploidy levels within the pericarp tissue. This in situ ploidy quantification revealed the dynamic progression of endoreduplication and its cell layer specificity during early fruit development. In summary, the study sheds light on the complex relationship between endoreduplication, cell differentiation and gene expression patterns in the tomato pericarp.

Bombyx moriSuppressor of Hairless is involved in the regulation of the silkworm cell cycle and endoreplication of the silk glands

The Notch signaling pathway is important in cell cycle regulation and cell proliferation. The transcriptional repressor Suppressor of Hairless [Su(H)] is a molecular switch for downstream target genes of the Notch signaling pathway but the regulatory mechanism of the Su(H) gene in the cell cycle is unclear. We determined the function of the Notch signaling pathway and Bombyx mori Su(H) [BmSu(H)] in the regulation of the silkworm cell cycle. Inhibition of Notch signaling promoted the replication of DNA in silkworm gland cells and expression of the BmSu(H) gene was significantly reduced. Overexpression of the BmSu(H) gene inhibited DNA replication and cell proliferation of silkworm cells, whereas knockout of the BmSu(H) gene promoted DNA replication and cell proliferation. Knockout of the BmSu(H) in silkworms improved the efficiency of silk gland cell endoreplication and increased important economic traits. We demonstrated that BmSu(H) protein can directly bind to the promoters of BmCyclinA, BmCyclinE and BmCDK1 genes, inhibiting or promoting their transcription at the cell and individual level. This study identified molecular targets for genetic improvement of the silkworm and also provided insights into the regulatory mechanism of the cell cycle.

Arabidopsis Root Development Regulation by the Endogenous Folate Precursor, Para-Aminobenzoic Acid, via Modulation of the Root Cell Cycle

The continuous growth of roots depends on their ability to maintain a balanced ratio between cell production and cell differentiation at the tip. This process is regulated by the hormonal balance of cytokinin and auxin. However, other important regulators, such as plant folates, also play a regulatory role. In this study, we investigated the impact of the folate precursor para-aminobenzoic acid (PABA) on root development. Using pharmacological, genetic, and imaging approaches, we show that the growth of Arabidopsis thaliana roots is repressed by either supplementing the growth medium with PABA or overexpressing the PABA synthesis gene GAT-ADCS. This is associated with a smaller root meristem consisting of fewer cells. Conversely, reducing the levels of free root endogenous PABA results in longer roots with extended meristems. We provide evidence that PABA represses Arabidopsis root growth in a folate-independent manner and likely acts through two mechanisms: (i) the G2/M transition of cell division in the root apical meristem and (ii) promoting premature cell differentiation in the transition zone. These data collectively suggest that PABA plays a role in Arabidopsis root growth at the intersection between cell division and cell differentiation.

The interplay between cell wall integrity and cell cycle progression in plants

Plant cell walls are dynamic structures that play crucial roles in growth, development, and stress responses. Despite our growing understanding of cell wall biology, the connections between cell wall integrity (CWI) and cell cycle progression in plants remain poorly understood. This review aims to explore the intricate relationship between CWI and cell cycle progression in plants, drawing insights from studies in yeast and mammals. We provide an overview of the plant cell cycle, highlight the role of endoreplication in cell wall composition, and discuss recent findings on the molecular mechanisms linking CWI perception to cell wall biosynthesis and gene expression regulation. Furthermore, we address future perspectives and unanswered questions in the field, such as the identification of specific CWI sensing mechanisms and the role of CWI maintenance in the growth-defense trade-off. Elucidating these connections could have significant implications for crop improvement and sustainable agriculture.

Endoreduplication in plant organogenesis: a means to boost fruit growth

Endoreduplication is the major source of somatic endopolyploidy in higher plants, and leads to variation in cell ploidy levels due to iterative rounds of DNA synthesis in the absence of mitosis. Despite its ubiquitous occurrence in many plant organs, tissues, and cells, the physiological meaning of endoreduplication is not fully understood, although several roles during plant development have been proposed, mostly related to cell growth, differentiation, and specialization via transcriptional and metabolic reprogramming. Here, we review recent advances in our knowledge of the molecular mechanisms and cellular characteristics of endoreduplicated cells, and provide an overview of the multi-scale effects of endoreduplication on supporting growth in plant development. In addition, the effects of endoreduplication in fruit development are discussed, since it is highly prominent during fruit organogenesis where it acts as a morphogenetic factor supporting rapid fruit growth, as illustrated by case of the model fleshy fruit, tomato (Solanum lycopersicum).

Endoreplication-Why Are We Not Using Its Full Application Potential?

Endoreplication-a process that is common in plants and also accompanies changes in the development of animal organisms-has been seen from a new perspective in recent years. In the paper, we not only shed light on this view, but we would also like to promote an understanding of the application potential of this phenomenon in plant cultivation. Endoreplication is a pathway for cell development, slightly different from the classical somatic cell cycle, which ends with mitosis. Since many rounds of DNA synthesis take place within its course, endoreplication is a kind of evolutionary compensation for the relatively small amount of genetic material that plants possess. It allows for its multiplication and active use through transcription and translation. The presence of endoreplication in plants has many positive consequences. In this case, repeatedly produced copies of genes, through the corresponding transcripts, help the plant acquire the favorable properties for which proteins are responsible directly or indirectly. These include features that are desirable in terms of cultivation and marketing: a greater saturation of fruit and flower colors, a stronger aroma, a sweeter fruit taste, an accumulation of nutrients, an increased resistance to biotic and abiotic stress, superior tolerance to adverse environmental conditions, and faster organ growth (and consequently the faster growth of the whole plant and its biomass). The two last features are related to the nuclear-cytoplasmic ratio-the greater the content of DNA in the nucleus, the higher the volume of cytoplasm, and thus the larger the cell size. Endoreplication not only allows cells to reach larger sizes but also to save the materials used to build organelles, which are then passed on to daughter cells after division, thus ending the classic cell cycle. However, the content of genetic material in the cell nucleus determines the number of corresponding organelles. The article also draws attention to the potential practical applications of the phenomenon and the factors currently limiting its use.

SIAMESE-RELATED1 imposes differentiation of stomatal lineage ground cells into pavement cells

The leaf epidermis represents a multifunctional tissue consisting of trichomes, pavement cells and stomata, the specialized cellular pores of the leaf. Pavement cells and stomata both originate from regulated divisions of stomatal lineage ground cells (SLGCs), but whereas the ontogeny of the stomata is well characterized, the genetic pathways activating pavement cell differentiation remain relatively unexplored. Here, we reveal that the cell cycle inhibitor SIAMESE-RELATED1 (SMR1) is essential for timely differentiation of SLGCs into pavement cells by terminating SLGC self-renewal potency, which depends on CYCLIN A proteins and CYCLIN-DEPENDENT KINASE B1. By controlling SLGC-to-pavement cell differentiation, SMR1 determines the ratio of pavement cells to stomata and adjusts epidermal development to suit environmental conditions. We therefore propose SMR1 as an attractive target for engineering climate-resilient plants.

Endoreplication controls cell size via mechanochemical signaling

During hypocotyl development, an asymmetric auxin gradient causes differential cell elongation, leading to tissue bending and apical hook formation. Recently, Ma et al. identified a molecular pathway that links auxin with endoreplication and cell size through cell wall integrity sensing, cell wall remodeling, and regulation of cell wall stiffness.

SCARECROW-LIKE28 modulates organ growth in Arabidopsis by controlling mitotic cell cycle exit, endoreplication, and cell expansion dynamics

The processes that contribute to plant organ morphogenesis are spatial-temporally organized. Within the meristem, mitosis produces new cells that subsequently engage in cell expansion and differentiation programs. The latter is frequently accompanied by endoreplication, being an alternative cell cycle that replicates the DNA without nuclear division, causing a stepwise increase in somatic ploidy. Here, we show that the Arabidopsis SCL28 transcription factor promotes organ growth by modulating cell expansion dynamics in both root and leaf cells. Gene expression studies indicated that SCL28 regulates members of the SIAMESE/SIAMESE-RELATED (SIM/SMR) family, encoding cyclin-dependent kinase inhibitors with a role in promoting mitotic cell cycle (MCC) exit and endoreplication, both in response to developmental and environmental cues. Consistent with this role, mutants in SCL28 displayed reduced endoreplication, both in roots and leaves. We also found evidence indicating that SCL28 co-expresses with and regulates genes related to the biogenesis, assembly, and remodeling of the cytoskeleton and cell wall. Our results suggest that SCL28 controls, not only cell proliferation as reported previously but also cell expansion and differentiation by promoting MCC exit and endoreplication and by modulating aspects of the biogenesis, assembly, and remodeling of the cytoskeleton and cell wall.

Endoreplication mediates cell size control via mechanochemical signaling from cell wall

Endoreplication is an evolutionarily conserved mechanism for increasing nuclear DNA content (ploidy). Ploidy frequently scales with final cell and organ size, suggesting a key role for endoreplication in these processes. However, exceptions exist, and, consequently, the endoreplication-size nexus remains enigmatic. Here, we show that prolonged tissue folding at the apical hook in Arabidopsis requires endoreplication asymmetry under the control of an auxin gradient. We identify a molecular pathway linking endoreplication levels to cell size through cell wall remodeling and stiffness modulation. We find that endoreplication is not only permissive for growth: Endoreplication reduction enhances wall stiffening, actively reducing cell size. The cell wall integrity kinase THESEUS plays a key role in this feedback loop. Our data thus explain the nonlinearity between ploidy levels and size while also providing a molecular mechanism linking mechanochemical signaling with endoreplication-mediated dynamic control of cell growth.

Brassinosteroid signaling restricts root lignification by antagonizing SHORT-ROOT function in Arabidopsis

Cell wall lignification is a key step in forming functional endodermis and protoxylem (PX) in plant roots. Lignified casparian strips (CS) in endodermis and tracheary elements of PX are essential for selective absorption and transport of water and nutrients. Although multiple key regulators of CS and PX have been identified, the spatial information that drives the developmental shift to root lignification remains unknown. Here, we found that brassinosteroid (BR) signaling plays a key role in inhibiting root lignification in the root elongation zone. The inhibitory activity of BR signaling occurs partially through the direct binding of BRASSINAZOLE-RESISTANT 1 (BZR1) to SHORT-ROOT (SHR), repressing the SHR-mediated activation of downstream genes that are involved in root lignification. Upon entering the mature root zone, BR signaling declines rapidly, which releases SHR activity and initiates root lignification. Our results provide a mechanistic view of the developmental transition to cell wall lignification in Arabidopsis thaliana roots.

A Journey to the Core of the Plant Cell Cycle

Production of new cells as a result of progression through the cell division cycle is a fundamental biological process for the perpetuation of both unicellular and multicellular organisms. In the case of plants, their developmental strategies and their largely sessile nature has imposed a series of evolutionary trends. Studies of the plant cell division cycle began with cytological and physiological approaches in the 1950s and 1960s. The decade of 1990 marked a turn point with the increasing development of novel cellular and molecular protocols combined with advances in genetics and, later, genomics, leading to an exponential growth of the field. In this article, I review the current status of plant cell cycle studies but also discuss early studies and the relevance of a multidisciplinary background as a source of innovative questions and answers. In addition to advances in a deeper understanding of the plant cell cycle machinery, current studies focus on the intimate interaction of cell cycle components with almost every aspect of plant biology.

Ploidy and local environment drive intraspecific variation in endoreduplication in Arabidopsis arenosa

PREMISE

Endoreduplication, nonheritable duplication of a nuclear genome, is widespread in plants and plays a role in developmental processes related to cell differentiation. However, neither ecological nor cytological factors influencing intraspecific variation in endoreduplication are fully understood.

METHODS

We cultivated plants covering the range-wide natural diversity of diploid and tetraploid populations of Arabidopsis arenosa in common conditions to investigate the effect of original ploidy level on endoreduplication. We also raised plants from several foothill and alpine populations from different lineages and of both ploidies to test for the effect of elevation. We determined the endoreduplication level in leaves of young plants by flow cytometry. Using RNA-seq data available for our populations, we analyzed gene expression analysis in individuals that differed in endoreduplication level.

RESULTS

We found intraspecific variation in endoreduplication that was mainly driven by the original ploidy level of populations, with significantly higher endoreduplication in diploids. An effect of elevation was also found within each ploidy, yet its direction exhibited rather regional-specific patterns. Transcriptomic analysis comparing individuals with high vs. low endopolyploidy revealed a majority of differentially expressed genes related to the stress and hormone response and to modifications especially in the cell wall and in chloroplasts.

CONCLUSIONS

Our results support the general assumption of higher potential of low-ploidy organisms to undergo endoreduplication and suggest that endoreduplication is further integrated within the stress response pathways for a fine-tune adjustment of the endoreduplication process to their local environment.

Cycling in a crowd: Coordination of plant cell division, growth, and cell fate

The reiterative organogenesis that drives plant growth relies on the constant production of new cells, which remain encased by interconnected cell walls. For these reasons, plant morphogenesis strictly depends on the rate and orientation of both cell division and cell growth. Important progress has been made in recent years in understanding how cell cycle progression and the orientation of cell divisions are coordinated with cell and organ growth and with the acquisition of specialized cell fates. We review basic concepts and players in plant cell cycle and division, and then focus on their links to growth-related cues, such as metabolic state, cell size, cell geometry, and cell mechanics, and on how cell cycle progression and cell division are linked to specific cell fates. The retinoblastoma pathway has emerged as a major player in the coordination of the cell cycle with both growth and cell identity, while microtubule dynamics are central in the coordination of oriented cell divisions. Future challenges include clarifying feedbacks between growth and cell cycle progression, revealing the molecular basis of cell division orientation in response to mechanical and chemical signals, and probing the links between cell fate changes and chromatin dynamics during the cell cycle.

Insertion of a mMoshan transposable element in PpLMI1, is associated with the absence or globose phenotype of extrafloral nectaries in peach [Prunus persica (L.) Batsch]

Most commercial peach [Prunus persica (L.) Batsch] cultivars have leaves with extrafloral nectaries (EFNs). Breeders have selected this character over time, as they observed that the eglandular phenotype resulted in high susceptibility to peach powdery mildew, a major disease of peach trees. EFNs are controlled by a Mendelian locus (E), mapped on chromosome 7. However, the genetic factor underlying E was unknown. In order to address this point, we developed a mapping population of 833 individuals derived from the selfing of "Malo Konare", a Bulgarian peach cultivar, heterozygous for the trait. This progeny was used to investigate the E-locus region, along with additional resources including peach genomic resequencing data, and 271 individuals from various origins used for validation. High-resolution mapping delimited a 40.6 kbp interval including the E-locus and four genes. Moreover, three double-recombinants allowed identifying Prupe.7G121100, a LMI1-like homeodomain leucine zipper (HD-Zip) transcription factor, as a likely candidate for the trait. By comparing peach genomic resequencing data from individuals with contrasted phenotypes, a MITE-like transposable element of the hAT superfamily (mMoshan) was identified in the third exon of Prupe.7G121100. It was associated with the absence or globose phenotype of EFNs. The insertion of the transposon was positively correlated with enhanced expression of Prupe.7G121100. Furthermore, a PCR marker designed from the sequence-variants, allowed to properly assign the phenotypes of all the individuals studied. These findings provide valuable information on the genetic control of a trait poorly known so far although selected for a long time in peach.

A perspective on cell proliferation kinetics in the root apical meristem

Organogenesis in plants is primarily postembryonic and relies on a strict balance between cell division and cell expansion. The root is a particularly well-suited model to study cell proliferation in detail since the two processes are spatially and temporally separated for all the different tissues. In addition, the root is amenable to detailed microscopic analysis to identify cells progressing through the cell cycle. While it is clear that cell proliferation activity is restricted to the root apical meristem (RAM), understanding cell proliferation kinetics and identifying its parameters have required much effort over many years. Here, we review the main concepts, experimental settings, and findings aimed at obtaining a detailed knowledge of how cells proliferate within the RAM. The combination of novel tools, experimental strategies, and mathematical models has contributed to our current view of cell proliferation in the RAM. We also discuss several lines of research that need to be explored in the future.

Regulation of the Plant Cell Cycle in Response to Hormones and the Environment

Developmental and environmental signals converge on cell cycle machinery to achieve proper and flexible organogenesis under changing environments. Studies on the plant cell cycle began 30 years ago, and accumulated research has revealed many links between internal and external factors and the cell cycle. In this review, we focus on how phytohormones and environmental signals regulate the cell cycle to enable plants to cope with a fluctuating environment. After introducing key cell cycle regulators, we first discuss how phytohormones and their synergy are important for regulating cell cycle progression and how environmental factors positively and negatively affect cell division. We then focus on the well-studied example of stress-induced G2 arrest and view the current model from an evolutionary perspective. Finally, we discuss the mechanisms controlling the transition from the mitotic cycle to the endocycle, which greatly contributes to cell enlargement and resultant organ growth in plants.

Transcriptomics of Differential Ripening in 'd'Anjou' Pear (Pyrus communis L.)

Estimating maturity in pome fruits is a critical task that directs virtually all postharvest supply chain decisions. This is especially important for European pear (Pyrus communis) cultivars because losses due to spoilage and senescence must be minimized while ensuring proper ripening capacity is achieved (in part by satisfying a fruit chilling requirement). Reliable methods are lacking for accurate estimation of pear fruit maturity, and because ripening is maturity dependent it makes predicting ripening capacity a challenge. In this study of the European pear cultivar 'd'Anjou', we sorted fruit at harvest based upon on-tree fruit position to build contrasts of maturity. Our sorting scheme showed clear contrasts of maturity between canopy positions, yet there was substantial overlap in the distribution of values for the index of absorbance difference (I _AD ), a non-destructive spectroscopic measurement that has been used as a proxy for pome fruit maturity. This presented an opportunity to explore a contrast of maturity that was more subtle than I _AD could differentiate, and thus guided our subsequent transcriptome analysis of tissue samples taken at harvest and during storage. Using a novel approach that tests for condition-specific differences of co-expressed genes, we discovered genes with a phased character that mirrored our sorting scheme. The expression patterns of these genes are associated with fruit quality and ripening differences across the experiment. Functional profiles of these co-expressed genes are concordant with previous findings, and also offer new clues, and thus hypotheses, about genes involved in pear fruit quality, maturity, and ripening. This work may lead to new tools for enhanced postharvest management based on activity of gene co-expression modules, rather than individual genes. Further, our results indicate that modules may have utility within specific windows of time during postharvest management of 'd'Anjou' pear.

Rice responses to silicon addition at different Fe status and growth pH. Evaluation of ploidy changes

It has been described in rice that Si only plays a physical barrier that does not allow Fe to enter cell apoplast, causing Fe deficiency responses even under Fe sufficiency growth conditions. Most of the conclusions were attained at acidic pH, but rice is also grown at calcareous conditions, which especially induce Fe deficiency in the plants. In this study, we assay the effect of Si in rice suffering both Fe deficiency and sufficiency in hydroponics at two pHs (5.5 and 7.5). Plant biometric parameters, ROS concentration, enzymatic activities, and total phenolic compounds, as well as ploidy levels, have been determined. In general, both pHs promoted similar rice responses under Fe sufficiency and deficiency status, but at pH 7.5, stress was favored. Flow cytometry studies revealed that Fe deficiency increased the percentage of cells in higher ploidy levels. Moreover, under this Fe status, Si addition enhanced this effect. This increase contributed to maintaining chloroplast structure which may have preserved antioxidant activities, and fortified cell walls, diminishing Fe uptake. The first is considered a beneficial effect as plants presented acceptable SPAD values, well chloroplast structure, and qualitatively high fluorescence observed by confocal microscopy, even under Fe deficiency. But contributes to intensify the Fe shortage, by decreasing apoplast Fe pools. In summary, Si addition to rice plants may not only behave as an apoplastic barrier but may also protect plant chloroplast and alter the plant endoreplication cycle, giving a memory effect to cope with present and future stresses.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Plant cell wall integrity maintenance in model plants and crop species-relevant cell wall components and underlying guiding principles

The walls surrounding the cells of all land-based plants provide mechanical support essential for growth and development as well as protection from adverse environmental conditions like biotic and abiotic stress. Composition and structure of plant cell walls can differ markedly between cell types, developmental stages and species. This implies that wall composition and structure are actively modified during biological processes and in response to specific functional requirements. Despite extensive research in the area, our understanding of the regulatory processes controlling active and adaptive modifications of cell wall composition and structure is still limited. One of these regulatory processes is the cell wall integrity maintenance mechanism, which monitors and maintains the functional integrity of the plant cell wall during development and interaction with environment. It is an important element in plant pathogen interaction and cell wall plasticity, which seems at least partially responsible for the limited success that targeted manipulation of cell wall metabolism has achieved so far. Here, we provide an overview of the cell wall polysaccharides forming the bulk of plant cell walls in both monocotyledonous and dicotyledonous plants and the effects their impairment can have. We summarize our current knowledge regarding the cell wall integrity maintenance mechanism and discuss that it could be responsible for several of the mutant phenotypes observed.

Endoreplication - a means to an end in cell growth and stress response

Endoreplication, also called endoreduplication or endopolyploidization, is a cell cycle variant in which the genome is re-replicated in the absence of mitosis causing cellular polyploidization. Despite the common occurrence of endoreplication in plants and the tremendous extent in specific tissues and cell types such as the endosperm, the underlying molecular regulation and the physiological consequences have only now started to be understood. Endoreplication is often associated with cell differentiation and withdrawal from mitotic cycles. Recent studies have underlined the importance of endoreplication as a stress response and we summarize here this progress with particular focus on future perspectives offered by the recent advances in genomics and biotechnology.

Anatomy, Flow Cytometry, and X-Ray Tomography Reveal Tissue Organization and Ploidy Distribution in Long-Term In Vitro Cultures of Melocactus Species

Cacti have a highly specialized stem that enables survival during extended dry periods. Despite the ornamental value of cacti and the fact that stems represent the main source of explants in tissue culture, there are no studies on their morpho-anatomical and cytological characteristics in Melocactus. The present study seeks to address the occurrence of cells with mixed ploidy level in cacti tissues. Specifically, we aim to understand how Melocactus stem tissue is organized, how mixoploidy is distributed when present, and whether detected patterns of ploidy change after long periods of in vitro culture. To analyze tissue organization, Melocactus glaucescens and Melocactus paucispinus plants that had been germinated and cultivated in vitro were analyzed for stem structure using toluidine blue, Xylidine Ponceau, Periodic Acid Schiff, ruthenium red, and acid floroglucin. To investigate patterns of ploidy, apical, medial, and basal zones of the stem, as well as, periphery, cortex, and stele (vascular tissue and pith) regions of the stem and root apexes from four- and ten-year old cultured in vitro were analyzed by flow cytometry. X-ray micro-computed tomography (XRµCT) was performed with fragments of stems from both species. The scarcity of support elements (i.e., sclereids and fibers) indicates that epidermis, hypodermis, and wide-band tracheids present in cortical vascular bundles and stele, as well as water stored in aquifer parenchyma cells along the cortex, provide mechanical support to the stem. Parenchyma cells increase in volume with a four-fold increase in ploidy. M. glaucescens and M. paucispinus exhibit the same pattern of cell ploidy irrespective of topophysical region or age, but there is a marked difference in ploidy between the stem periphery (epidermis and hypodermis), cortex, stele, and roots. Mixoploidy in Melocactus is not related to the age of the culture, but is a developmental trait, whereby endocycles promote cell differentiation to accumulate valuable water.

Developmental Roles of AUX1/LAX Auxin Influx Carriers in Plants

Plant hormone auxin regulates several aspects of plant growth and development. Auxin is predominantly synthesized in the shoot apex and developing leaf primordia and from there it is transported to the target tissues e.g. roots. Auxin transport is polar in nature and is carrier-mediated. AUXIN1/LIKE-AUX1 (AUX1/LAX) family members are the major auxin influx carriers whereas PIN-FORMED (PIN) family and some members of the P-GLYCOPROTEIN/ATP-BINDING CASSETTE B4 (PGP/ABCB) family are major auxin efflux carriers. AUX1/LAX auxin influx carriers are multi-membrane spanning transmembrane proteins sharing similarity to amino acid permeases. Mutations in AUX1/LAX genes result in auxin related developmental defects and have been implicated in regulating key plant processes including root and lateral root development, root gravitropism, root hair development, vascular patterning, seed germination, apical hook formation, leaf morphogenesis, phyllotactic patterning, female gametophyte development and embryo development. Recently AUX1 has also been implicated in regulating plant responses to abiotic stresses. This review summarizes our current understanding of the developmental roles of AUX1/LAX gene family and will also briefly discuss the modelling approaches that are providing new insight into the role of auxin transport in plant development.