Role of the Arabidopsis PIN6 Auxin Transporter in Auxin Homeostasis and Auxin-Mediated Development

Plant-specific PIN-formed (PIN) efflux transporters for the plant hormone auxin are required for tissue-specific directional auxin transport and cellular auxin homeostasis. The Arabidopsis PIN protein family has been shown to play important roles in developmental processes such as embryogenesis, organogenesis, vascular tissue differentiation, root meristem patterning and tropic growth. Here we analyzed roles of the less characterised Arabidopsis PIN6 auxin transporter. PIN6 is auxin-inducible and is expressed during multiple auxin–regulated developmental processes. Loss of pin6 function interfered with primary root growth and lateral root development. Misexpression of PIN6 affected auxin transport and interfered with auxin homeostasis in other growth processes such as shoot apical dominance, lateral root primordia development, adventitious root formation, root hair outgrowth and root waving. These changes in auxin-regulated growth correlated with a reduction in total auxin transport as well as with an altered activity of DR5-GUS auxin response reporter. Overall, the data indicate that PIN6 regulates auxin homeostasis during plant development.     

Ups and downs of tissue and planar polarity in plants

The polar orientation of cells within a tissue is an intensively studied research area in animal cells. The term planar polarity refers to the common polar arrangement of cells within the plane of an epithelium. In plants, the subcellular analysis of tissue polarity has been limited by the lack of appropriate markers. Recently, research on plant tissue polarity has come of age. Advances are based on studies of Arabidopsis patterning, cell polarity and auxin transport mutants employing the coordinated, polar localization of auxin transporters and the planar polarity of root epidermal hairs as markers. These approaches have revealed auxin transport and response, vesicular trafficking, membrane sterol and cytoskeletal requirements of tissue polarity. This review summarizes recent progress in research on vascular tissue and planar epidermal polarity in the Arabidopsis root and compares it to findings on planar polarity in animals and cell polarity in yeast.     

Root System Architecture from Coupling Cell Shape to Auxin Transport

Lateral organ position along roots and shoots largely determines plant architecture, and depends on auxin distribution patterns. Determination of the underlying patterning mechanisms has hitherto been complicated because they operate during growth and division. Here, we show by experiments and computational modeling that curvature of the Arabidopsis root influences cell sizes, which, together with tissue properties that determine auxin transport, induces higher auxin levels in the pericycle cells on the outside of the curve. The abundance and position of the auxin transporters restricts this response to the zone competent for lateral root formation. The auxin import facilitator, AUX1, is up-regulated by auxin, resulting in additional local auxin import, thus creating a new auxin maximum that triggers organ formation. Longitudinal spacing of lateral roots is modulated by PIN proteins that promote auxin efflux, and pin2,3,7 triple mutants show impaired lateral inhibition. Thus, lateral root patterning combines a trigger, such as cell size difference due to bending, with a self-organizing system that mediates alterations in auxin transport.     

Polar auxin transport – old questions and new concepts?

Polar auxin transport controls multiple aspects of plant development including differential growth, embryo and root patterning and vascular tissue differentiation. Identification of proteins involved in this process and availability of new tools enabling `visualization' of auxin and auxin routes in planta largely contributed to the significant progress that has recently been made. New data support classical concepts, but several recent findings are likely to challenge our view on the mechanism of auxin transport. The aim of this review is to provide a comprehensive overview of the polar auxin transport field. It starts with classical models resulting from physiological studies, describes the genetic contributions and discusses the molecular basis of auxin influx and efflux. Finally, selected questions are presented in the context of developmental biology, integrating available data from different fields.     

被子植物的早期胚胎形态建成

被子植物早期胚胎形态建成是其有性生殖过程中一个重要发育阶段。在这一阶段中,被子植物形体基本特征形成,包括顶-基轴极性建立、不同细胞层分化以及分生组织形成。合子极性直接与顶基细胞命运相关,但其极性产生机理仍然不明。研究表明,WOX家族转录因子、生长素定向运输以及生长素响应应答可能参与了早期顶-基模型建成;辐射对称模型的建立可能由细胞与细胞间相互作用来介导;生长素流可能参与胚胎顶端组织形成。该文对近年来被子植物早期胚胎形态建成过程中的合子极性建立与生长、合子分裂及其顶基细胞的形成、胚根原特化及根极的形成、辐射对称模式及表皮原特化、顶端分生组织特化及子叶起始等方面的研究进展进行了综述。     

Polar auxin transport together with AINTEGUMENTA and REVOLUTA coordinate early Arabidopsis gynoecium development

In flowering plants the gynoecium is the female reproductive structure and the site of oogenesis, fertilization, and maturation of the embryo and the seed. Proper development of the gynoecium requires that the early gynoecial primordium be partitioned into distinct spatial domains with divergent fates. Regulated transport of the phytohormone auxin previously has been shown to play a role in the patterning of spatial domains along the apical-basal axis of the gynoecium. Here we establish a role for auxin transport in patterning along the medio-lateral axis of the gynoecial ovary. We demonstrate that auxin transport is required for the development of the medial ovary domain that contains the carpel margin meristem, a vital female reproductive structure. Disruptions in auxin transport enhance the medial domain defects observed in aintegumenta and revoluta mutant genotypes. AINTEGUMENTA and REVOLUTA are likely to function in parallel and partially overlapping pathways required for medial domain development. Our data indicate that different ovary domains are differentially sensitive to the reduction of polar auxin transport and the loss of AINTEGUMENTA and REVOLUTA activity. We suggest that an auxin-mediated positional cue is important for the differential specification of the medial and lateral ovary domains.     

Very-Long-Chain Fatty Acids Are Involved in Polar Auxin Transport and Developmental Patterning in Arabidopsis

Very-long-chain fatty acids (VLCFAs) are essential for many aspects of plant development and necessary for the synthesis of seed storage triacylglycerols, epicuticular waxes, and sphingolipids. Identification of the acetyl-CoA carboxylase PASTICCINO3 and the 3-hydroxy acyl-CoA dehydratase PASTICCINO2 revealed that VLCFAs are important for cell proliferation and tissue patterning. Here, we show that the immunophilin PASTICCINO1 (PAS1) is also required for VLCFA synthesis. Impairment of PAS1 function results in reduction of VLCFA levels that particularly affects the composition of sphingolipids, known to be important for cell polarity in animals. Moreover, PAS1 associates with several enzymes of the VLCFA elongase complex in the endoplasmic reticulum. The pas1 mutants are deficient in lateral root formation and are characterized by an abnormal patterning of the embryo apex, which leads to defective cotyledon organogenesis. Our data indicate that in both tissues, defective organogenesis is associated with the mistargeting of the auxin efflux carrier PIN FORMED1 in specific cells, resulting in local alteration of polar auxin distribution. Furthermore, we show that exogenous VLCFAs rescue lateral root organogenesis and polar auxin distribution, indicating their direct involvement in these processes. Based on these data, we propose that PAS1 acts as a molecular scaffold for the fatty acid elongase complex in the endoplasmic reticulum and that the resulting VLCFAs are required for polar auxin transport and tissue patterning during plant development.     

Root development: cytokinin transport matters, too!

Unlike the plant hormone auxin, the mechanism and function of cytokinin transport is poorly characterised. Two new studies now demonstrate that cytokinins transported from shoot to roots via the phloem are critical for creating mutually exclusive auxin and cytokinin signalling domains that control root vascular patterning.     

Phloem-associated auxin response maxima determine radial positioning of lateral roots in maize

In Arabidopsis thaliana, lateral-root-forming competence of pericycle cells is associated with their position at the xylem poles and depends on the establishment of protoxylem-localized auxin response maxima. In maize, our histological analyses revealed an interruption of the pericycle at the xylem poles, and confirmed the earlier reported proto-phloem-specific lateral root initiation. Phloem-pole pericycle cells were larger and had thinner cell walls compared with the other pericycle cells, highlighting the heterogeneous character of the maize root pericycle. A maize DR5::RFP marker line demonstrated the presence of auxin response maxima in differentiating xylem cells at the root tip and in cells surrounding the proto-phloem vessels. Chemical inhibition of auxin transport indicated that the establishment of the phloem-localized auxin response maxima is crucial for lateral root formation in maize, because in their absence, random divisions of pericycle and endodermis cells occurred, not resulting in organogenesis. These data hint at an evolutionarily conserved mechanism, in which the establishment of vascular auxin response maxima is required to trigger cells in the flanking outer tissue layer for lateral root initiation. It further indicates that lateral root initiation is not dependent on cellular specification or differentiation of the type of vascular tissue.     

NO VEIN Mediates Auxin-Dependent Specification and Patterning in the Arabidopsis Embryo,Shoot, and Root

Local efflux-dependent auxin gradients and maxima mediate organ and tissue development in plants. Auxin efflux is regulated by dynamic expression and subcellular localization of the PIN auxin-efflux proteins, which appears to be established not only through a self-organizing auxin-mediated polarization mechanism, but also through other means, such as cell fate determination and auxin-independent mechanisms. Here, we show that the Arabidopsis thaliana NO VEIN (NOV) gene, encoding a novel, plant-specific nuclear factor, is required for leaf vascular development, cellular patterning and stem cell maintenance in the root meristem, as well as for cotyledon outgrowth and separation. nov mutations affect many aspects of auxin-dependent development without directly affecting auxin perception. NOV is required for provascular PIN1 expression and region-specific expression of PIN7 in leaf primordia, cell type–specific expression of PIN3, PIN4, and PIN7 in the root, and PIN2 polarity in the root cortex. NOV is specifically expressed in developing embryos, leaf primordia, and shoot and root apical meristems. Our data suggest that NOV function underlies cell fate decisions associated with auxin gradients and maxima, thus establishing cell type–specific PIN expression and polarity. We propose that NOV mediates the acquisition of competence to undergo auxin-dependent coordinated cell specification and patterning, thereby eliciting context-dependent auxin-mediated developmental responses.     

Complex regulation of Arabidopsis AGR1/PIN2-mediated root gravitropic response and basipetal auxin transport by cantharidin-sensitive protein phosphatases

Polar auxin transport, mediated by two distinct plasma membrane-localized auxin influx and efflux carrier proteins/complexes, plays an important role in many plant growth and developmental processes including tropic responses to gravity and light, development of lateral roots and patterning in embryogenesis. We have previously shown that the Arabidopsis AGRAVITROPIC 1/PIN2 gene encodes an auxin efflux component regulating root gravitropism and basipetal auxin transport. However, the regulatory mechanism underlying the function of AGR1/PIN2 is largely unknown. Recently, protein phosphorylation and dephosphorylation mediated by protein kinases and phosphatases, respectively, have been implicated in regulating polar auxin transport and root gravitropism. Here, we examined the effects of chemical inhibitors of protein phosphatases on root gravitropism and basipetal auxin transport, as well as the expression pattern of AGR1/PIN2 gene and the localization of AGR1/PIN2 protein. We also examined the effects of inhibitors of vesicle trafficking and protein kinases. Our data suggest that protein phosphatases, sensitive to cantharidin and okadaic acid, are likely involved in regulating AGR1/PIN2-mediated root basipetal auxin transport and gravitropism, as well as auxin response in the root central elongation zone (CEZ). BFA-sensitive vesicle trafficking may be required for the cycling of AGR1/PIN2 between plasma membrane and the BFA compartment, but not for the AGR1/PIN2-mediated root basipetal auxin transport and auxin response in CEZ cells.     

An auxin-dependent distal organizer of pattern and polarity in the Arabidopsis root

Root formation in plants involves the continuous interpretation of positional cues. Physiological studies have linked root formation to auxins. An auxin response element displays a maximum in the Arabidopsis root and we investigate its developmental significance. Auxin response mutants reduce the maximum or its perception, and interfere with distal root patterning. Polar auxin transport mutants affect its localization and distal pattern. Polar auxin transport inhibitors cause dramatic relocalization of the maximum, and associated changes in pattern and polarity. Auxin application and laser ablations correlate root pattern with a maximum adjacent to the vascular bundle. Our data indicate that an auxin maximum at a vascular boundary establishes a distal organizer in the root.