Vesicle trafficking: ROP-RIC roundabout

In silico identification and experimental validation of amino acid motifs required for the Rho-of-plants GTPase-mediated activation of receptor-like cytoplasmic kinases

Several amino acid motifs required for Rop-dependent activity were found to form a common surface on RLCKVI_A kinases. This indicates a unique mechanism for Rho-type GTPase-mediated kinase activation in plants. Rho-of-plants (Rop) G-proteins are implicated in the regulation of various cellular processes, including cell growth, cell polarity, hormonal and pathogen responses. Our knowledge about the signalling pathways downstream of Rops is continuously increasing. However, there are still substantial gaps in this knowledge. One reason for this is that these pathways are considerably different from those described for yeast and/or animal Rho-type GTPases. Among others, plants lack all Rho/Rac/Cdc42-activated kinase families. Only a small group of plant-specific receptor-like cytoplasmic kinases (RLCK VI_A) has been shown to exhibit Rop-binding-dependent in vitro activity. These kinases do not carry any known GTPase-binding motifs. Based on the sequence comparison of the Rop-activated RLCK VI_A and the closely related but constitutively active RLCK VI_B kinases, several distinguishing amino acid residues/motifs were identified. All but one of these were found to be required for the Rop-mediated regulation of the in vitro activity of two RLCK VI_A kinases. Structural modelling indicated that these motifs might form a common Rop-binding surface. Based on in silico data mining, kinases that have the identified Rop-binding motifs are present in Embryophyta but not in unicellular green algae. It can, therefore, be supposed that Rops recruited these plant-specific kinases for signalling at an early stage of land plant evolution.

Keeping it all together: auxin-actin crosstalk in plant development

Polar auxin transport and the action of the actin cytoskeleton are tightly interconnected, which is documented by the finding that auxin transporters reach their final destination by active movement of secretory vesicles along F-actin tracks. Moreover, auxin transporter polarity and flexibility is thought to depend on transporter cycling that requires endocytosis and exocytosis of vesicles. In this context, we have reviewed the current literature on an involvement of the actin cytoskeleton in polar auxin transport and identify known similarities and differences in its structure, function and dynamics in comparison to non-plant organisms. By describing how auxin modulates actin expression and actin organization and how actin and its stability affects auxin-transporter endocytosis and recycling, we discuss the current knowledge on regulatory auxin-actin feedback loops. We focus on known effects of auxin and of auxin transport inhibitors on the stability and organization of actin and examine the functionality of auxin and/or auxin transport inhibitor-binding proteins with respect to their suitability to integrate auxin/auxin transport inhibitor action. Finally, we indicate current difficulties in the interpretation of organ, time and concentration-dependent auxin/auxin transport inhibitor treatments and formulate simple future experimental guidelines.

Focusing on the focus: what else beyond the master switches for polar cell growth?

Cell polarity, often associated with polarized cell expansion/growth in plants, describes the uneven distribution of cellular components, such as proteins, nucleic acids, signaling molecules, vesicles, cytoskeletal elements, and organelles, which may ultimately modulate cell shape, structure, and function. Pollen tubes and root hairs are model cell systems for studying the molecular mechanisms underlying sustained tip growth. The formation of intercalated epidermal pavement cells requires excitatory and inhibitory pathways to coordinate cell expansion within single cells and between cells in contact. Strictly controlled cell expansion is linked to asymmetric cell division in zygotes and stomatal lineages, which require integrated processes of pre-mitotic cellular polarization and division asymmetry. While small GTPase ROPs are recognized as fundamental signaling switches for cell polarity in various cellular and developmental processes in plants, the broader molecular machinery underpinning polarity establishment required for asymmetric division remains largely unknown. Here, we review the widely used ROP signaling pathways in cell polar growth and the recently discovered feedback loops with auxin signaling and PIN effluxers. We discuss the conserved phosphorylation and phospholipid signaling mechanisms for regulating uneven distribution of proteins, as well as the potential roles of novel proteins and MAPKs in the polarity establishment related to asymmetric cell division in plants.

Ca(2+)-Transport through Plasma Membrane as a Test of Auxin Sensitivity

Auxin is one of the crucial regulators of plant growth and development. The discovered auxin cytosolic receptor (TIR1) is not involved in the perception of the hormone signal at the plasma membrane. Instead, another receptor, related to the ABP1, auxin binding protein1, is supposed to be responsible for the perception at the plasma membrane. One of the fast and sensitive auxin-induced reactions is an increase of Ca(2+) cytosolic concentration, which is suggested to be dependent on the activation of Ca(2+) influx through the plasma membrane. This investigation was carried out with a plasmalemma enriched vesicle fraction, obtained from etiolated maize coleoptiles. The magnitude of Ca(2+) efflux through the membrane vesicles was estimated according to the shift of potential dependent fluorescent dye diS-C₃-(5). The obtained results showed that during coleoptiles ageing (3rd, 4th and 5th days of seedling etiolated growth) the magnitude of Ca(2+) efflux from inside-out vesicles was decreased. Addition of ABP1 led to a recovery of Ca(2+) efflux to the level of the youngest and most sensitive cells. Moreover, the efflux was more sensitive, responding from 10(-8) to 10(-6) M 1-NAA, in vesicles containing ABP1, whereas native vesicles showed the highest efflux at 10(-6) M 1-NAA. We suggest that auxin increases plasma membrane permeability to Ca(2+) and that ABP1 is involved in modulation of this reaction.

Telocytes in the human kidney cortex

Renal interstitial cells play an important role in the physiology and pathology of the kidneys. As a novel type of interstitial cell, telocytes (TCs) have been described in various tissues and organs, including the heart, lung, skeletal muscle, urinary tract, etc. (www.telocytes.com). However, it is not known if TCs are present in the kidney interstitium. We demonstrated the presence of TCs in human kidney cortex interstitium using primary cell culture, transmission electron microscopy (TEM) and in situ immunohistochemistry (IHC). Renal TCs were positive for CD34, CD117 and vimentin. They were localized in the kidney cortex interstitial compartment, partially covering the tubules and vascular walls. Morphologically, renal TCs resemble TCs described in other organs, with very long telopodes (Tps) composed of thin segments (podomers) and dilated segments (podoms). However, their possible roles (beyond intercellular signalling) as well as their specific phenotype in the kidney remain to be established.

N-MYC down-regulated-like proteins regulate meristem initiation by modulating auxin transport and MAX2 expression

Background

N-MYC DOWN-REGULATED-LIKE (NDL) proteins interact with the Gβ subunit (AGB1) of the heterotrimeric G protein complex and play an important role in AGB1-dependent regulation of lateral root formation by affecting root auxin transport, auxin gradients and the steady-state levels of mRNA encoding the PIN-FORMED 2 and AUXIN 1 auxin transport facilitators. Auxin transport in aerial tissue follows different paths and utilizes different transporters than in roots; therefore, in the present study, we analyzed whether NDL proteins play an important role in AGB1-dependent, auxin-mediated meristem development.

Methodology/Principal Findings

Expression levels of NDL gene family members need to be tightly regulated, and altered expression (both over-expression and down-regulation) confers ectopic growth. Over-expression of NDL1 disrupts vegetative and reproductive organ development. Reduced expression of the NDL gene family members results in asymmetric leaf emergence, twinning of rosette leaves, defects in leaf formation, and abnormal silique distribution. Reduced expression of the NDL genes in the agb1-2 (null allele) mutant rescues some of the abnormal phenotypes, such as silique morphology, silique distribution, and peduncle angle, suggesting that proper levels of NDL proteins are maintained by AGB1. We found that all of these abnormal aerial phenotypes due to altered NDL expression were associated with increases in basipetal auxin transport, altered auxin maxima and altered MAX2 expression within the inflorescence stem.

Conclusion/Significance

NDL proteins, together with AGB1, act as positive regulators of meristem initiation and branching. AGB1 and NDL1 positively regulate basipetal inflorescence auxin transport and modulate MAX2 expression in shoots, which in turn regulates organ and lateral meristem formation by the establishment and maintenance of auxin gradients.

From perception to attenuation: auxin signalling and responses

The plant hormone auxin is essential for growth, development, and responses to environmental factors. Recently, Auxin Binding Protein 1 was shown to mediate non-transcriptional auxin signalling at the cell periphery. This has provoked reexamination of the paradigm that all auxin perception is intracellular and is mediated by the TIR1/AFB-Aux/IAA co-receptors for which auxin functions as a concentration-dependent molecular glue. Further, another F-box protein, SKP2a, was shown to bind auxin in the same way as TIR1/AFB, which provides a link to the role of auxin in the cell cycle. New work on auxin signalling and homeostasis include D6 PROTEIN KINASE activation of PINFORMED (PIN) auxin carriers, ROP-GTPase mediation of PIN localization, endoplasmic reticulum localization PIN and PIN-LIKES auxin carriers, and auxin biosynthesis and metabolism.