Joining forces: feedback and integration in plant development

Feedback and integration of information are of paramount importance for the robust functioning and dynamics of biological systems. In plant developmental biology, experimentation is increasingly combined with computational modeling to obtain a better understanding of how such regulatory interactions shape the systems' behavior. Here we highlight experimental and modeling studies on feedback loops and integration mechanisms involved in plant development. These studies have substantially expanded our understanding of previously characterized gene regulatory networks (GRNs). In addition, they illustrate the pervasiveness of regulatory interactions between seemingly unrelated processes and levels of organization. Modelers in plant development will increasingly face the challenges of what level of detail, which processes and how many levels of organization to incorporate when trying to understand a particular process.     

Distinct cell-autonomous functions of RETINOBLASTOMA-RELATED in Arabidopsis stem cells revealed by the Brother of Brainbow clonal analysis system

Mutations that cause lethality in the gametophyte phase pose a major challenge for studying postfertilization gene function. When both male and female haploid cells require a functional gene copy, null alleles cause developmental arrest before the formation of the zygote, precluding further investigation. The Arabidopsis thaliana Rb homolog RETINOBLASTOMA-RELATED (RBR) has an important function in the stem cell niche, but its requirement in both male and female gametophytes has prevented full loss-of-function studies. To circumvent this obstacle, we designed a clonal deletion system named BOB (Brother of Brainbow) in which null mutant sectors marked by double fluorescence are generated in a fully complemented wild-type background. In this system, both copies of a complementing RBR transgene are eliminated by tissue-specific and inducible CRE expression, and homozygous mutant clones can be distinguished visually. Since mutant sectors can be produced in a homozygous, rather than a heterozygous, background, this system facilitates clonal deletion analysis not only for gametophytic lethal alleles but also for any type of mutation. Using the BOB system, we show that RBR has unique cell-autonomous functions in different cell types within the root stem cell niche.     

Increased incidence of pregnancy complications in women who later develop scleroderma: a case control study

Introduction  

Studies have shown that fetal progenitor cells persist in maternal blood or bone marrow for more than 30 years after delivery. Increased trafficking of fetal cells occurs during pregnancy complications, such as hypertension, preeclampsia, miscarriage and intra-uterine growth restriction (IUGR). Women with these pregnancy complications are significantly more often HLA-class II compatible with their spouses. Women who later develop scleroderma also give birth to an HLA-class II child more often. From these prior studies we hypothesized that preeclampsia and other pregnancy complications could be associated with increased levels of fetal cell trafficking, and later be involved in the development of scleroderma.     

RETRACTED ARTICLE: Mast cells are the main interleukin 17-positive cells in anticitrullinated protein antibody-positive and -negative rheumatoid arthritis and osteoarthritis synovium

Introduction  

Mast cells have been implicated to play a functional role in arthritis, especially in autoantibody-positive disease. Among the cytokines involved in rheumatoid arthritis (RA), IL-17 is an important inflammatory mediator. Recent data suggest that the synovial mast cell is a main producer of IL-17, although T cells have also been implicated as prominent IL-17 producers as well. We aimed to identify IL-17 expression by mast cells and T cells in synovium of arthritis patients.     

Membrane localization diversity of TPK channels and their physiological role

Potassium (K) is one of the major nutrients that is essential for plant growth and development. The majority of cellular K⁺ resides in the vacuole and tonoplast K⁺ channels of the TPK (Two Pore K) family are main players in cellular K⁺ homeostasis. All TPK channels were previously reported to be expressed in the tonoplast of the large central lytic vacuole (LV) except for one isoform in Arabidopsis that resides in the plasma membrane. However, plant cells often contain more than one type of vacuole that coexist in the same cell. We recently showed that two TPK isoforms (OsTPKa and OsTPKb) from Oryza sativa localize to different vacuoles with OsTPKa predominantly found in the LV tonoplast and OsTPKb primarily in smaller compartments that resemble small vacuoles (SVs). Our study further revealed that it is the C-terminal domain that determines differential targeting of OsTPKa and OsTPKb. Three C-terminal amino acids were particularly relevant for targeting TPKs to their respective endomembranes. In this addendum we further evaluate how the different localization of TPKa and TPKb impact on their physiological role and how TPKs provide a potential tool to study the physiology of different types of vacuole.Key words: TPK channels, small vacuoles, vacuolar targeting, potassiumThe roles of plant vacuolar K⁺ channels are diverse and include potassium homeostasis, turgor regulation and responses to abiotic stress. Vacuolar K⁺-selective channels belong to two-pore K⁺ (TPK) channel families which have been found in genomes of many plant species such as Arabidopsis, poplar, Physcomitrella, Eucalyptus, barley, potato, rice and tobacco (Fig. 1). TPKs have structural similarity to mammalian “tandem P domain” channels with a secondary structure that contains four transmembrane domains and two pore regions (Fig. 2).^1–5 TPK channels have pore regions with a GYGD signature that endows K⁺ selectivity and a variable number of Ca²⁺ binding EF domains in the C terminus.^3–8 One of the best characterized members of the TPK family is AtTPK1 from Arabidopsis thaliana. AtTPK1 activity is voltage independent but sensitive to cytosolic Ca²⁺, cytosolic pH and N-terminal phosphorylation by 14-3-3 proteins.^5,6,8,9 In Arabidopsis, AtTPK1 expresses in the large lytic vacuole (LV) and plays roles in cellular K⁺ homeostasis, K⁺-release during stomatal closure and seed germination.^4,5 Other members of the Arabidopsis TPK family (AtTPK2, AtTPK3, AtTPK5) have been shown to localize to the LV but also showed some expression in smaller, vesicle-like, compartments.⁴ However, none of these isoforms appears to form functional channels in planta although our experiments with heterologous expression of AtTPK3 and AtTPK5 in the K⁺ uptake deficient E. coli LB2003 demonstrates complementation of bacterial growth phenotype (Isayenkov S, et al. unpublished results). Equally intriguing, is the plasma membrane localization of the Arabidopsis TPK4 isoform, in spite of its sequence being very similar to that of other TPKs.¹⁰Open in a separate windowFigure 1Phylogenetic tree of plant TPKs. The three main clusters of TPKs comprise: Cluster 1 with AtTPK1-like channels; Cluster 2 with AtTPK3/TPK5-like channels; Cluster 3 with barley HvTPKb. Bootstrap analysis was performed using ‘Molecular Evolutionary Genetics Analysis, MEGA4’ software available at www.megasoftware.net/mega4/megaOpen in a separate windowFigure 2Two-pore potassium channel secondary structure. TPK channels comprise four transmembrane domains (1–4) and two pore regions (P) per subunit. Functional channels are formed from two subunits. In most TPKs, both P regions contain a K⁺ selectivity signature, GYGD. However, the tobacco NtTPKa isoform has different motifs in the second P domain. In the N terminal region, TPKs have a 14-3-3 binding domain that impact on channel activity, with the binding of 14-3-3 protein leading to channel activation. C-termini of TPKs show a varying number of putative Ca²⁺ binding “EF hands” which may vary from zero to two.     

The RETINOBLASTOMA-RELATED gene regulates stem cell maintenance in Arabidopsis roots

The maintenance of stem cells in defined locations is crucial for all multicellular organisms. Although intrinsic factors and signals for stem cell fate have been identified in several species, it has remained unclear how these connect to the ability to reenter the cell cycle that is one of the defining properties of stem cells. We show that local reduction of expression of the RETINOBLASTOMA-RELATED (RBR) gene in Arabidopsis roots increases the amount of stem cells without affecting cell cycle duration in mitotically active cells. Conversely, induced RBR overexpression dissipates stem cells prior to arresting other mitotic cells. Overexpression of D cyclins, KIP-related proteins, and E2F factors also affects root stem cell pool size, and genetic interactions suggest that these factors function in a canonical RBR pathway to regulate somatic stem cells. Expression analysis and genetic interactions position RBR-mediated regulation of the stem cell state downstream of the patterning gene SCARECROW.     

Dissection of Arabidopsis ADP-RIBOSYLATION FACTOR 1 function in epidermal cell polarity

Vesicle trafficking is essential for the generation of asymmetries, which are central to multicellular development. Core components of the vesicle transport machinery, such as ADP-ribosylation factor (ARF) GTPases, have been studied primarily at the single-cell level. Here, we analyze developmental functions of the ARF1 subclass of the Arabidopsis thaliana multigene ARF family. Six virtually identical ARF1 genes are ubiquitously expressed, and single loss-of-function mutants in these genes reveal no obvious developmental phenotypes. Fluorescence colocalization studies reveal that ARF1 is localized to the Golgi apparatus and endocytic organelles in both onion (Allium cepa) and Arabidopsis cells. Apical-basal polarity of epidermal cells, reflected by the position of root hair outgrowth, is affected when ARF1 mutants are expressed at early stages of cell differentiation but after they exit mitosis. Genetic interactions during root hair tip growth and localization suggest that the ROP2 protein is a target of ARF1 action, but its localization is slowly affected upon ARF1 manipulation when compared with that of Golgi and endocytic markers. Localization of a second potential target of ARF1 action, PIN2, is also affected with slow kinetics. Although extreme redundancy precludes conventional genetic dissection of ARF1 functions, our approach separates different ARF1 downstream networks involved in local and specific aspects of cell polarity.