Polar flow of the phytohormone auxin requires plasma membrane-associated PIN proteins and underlies multiple developmental processes in plants. Here we address the importance of the polarity of subcellular PIN localization for the directionality of auxin transport in Arabidopsis thaliana. Expression of different PINs in the root epidermis revealed the importance of PIN polar positions for directional auxin flow and root gravitropic growth. Interfering with sequence-embedded polarity signals directly demonstrates that PIN polarity is a primary factor in determining the direction of auxin flow in meristematic tissues. This finding provides a crucial piece in the puzzle of how auxin flow can be redirected via rapid changes in PIN polarity.

In plants, each developmental process integrates a network of signaling events that are regulated by different phytohormones, and interactions among hormonal pathways are essential to modulate their effect. Continuous growth of roots results from the postembryonic activity of cells within the root meristem that is controlled by the coordinated action of several phytohormones, including auxin and ethylene. Although their interaction has been studied intensively, the molecular and cellular mechanisms underlying this interplay are unknown. We show that the effect of ethylene on root growth is largely mediated by the regulation of the auxin biosynthesis and transport-dependent local auxin distribution. Ethylene stimulates auxin biosynthesis and basipetal auxin transport toward the elongation zone, where it activates a local auxin response leading to inhibition of cell elongation. Consistently, in mutants affected in auxin perception or basipetal auxin transport, ethylene cannot activate the auxin response nor regulate the root growth. In addition, ethylene modulates the transcription of several components of the auxin transport machinery. Thus, ethylene achieves a local activation of the auxin signaling pathway and regulates root growth by both stimulating the auxin biosynthesis and by modulating the auxin transport machinery.

Plant development is governed by signaling molecules called phytohormones. Typically, in certain developmental processes more than 1 hormone is implicated and, thus, coordination of their overlapping activities is crucial for correct plant development. However, molecular mechanisms underlying the hormonal crosstalk are only poorly understood. Multiple hormones including cytokinin and auxin have been implicated in the regulation of root development. Here we dissect the roles of cytokinin in modulating growth of the primary root. We show that cytokinin effect on root elongation occurs through ethylene signaling whereas cytokinin effect on the root meristem size involves ethylene-independent modulation of transport-dependent asymmetric auxin distribution. Exogenous or endogenous modification of cytokinin levels and cytokinin signaling lead to specific changes in transcription of several auxin efflux carrier genes from the PIN family having a direct impact on auxin efflux from cultured cells and on auxin distribution in the root apex. We propose a novel model for cytokinin action in regulating root growth: Cytokinin influences cell-to-cell auxin transport by modification of expression of several auxin transport components and thus modulates auxin distribution important for regulation of activity and size of the root meristem.auxin ͉ auxin transport ͉ cytokinin ͉ hormonal crosstalk ͉ root meristem P lant hormones play a crucial role in regulating plant development and the flexible shaping of the plant architecture in response to variable environmental conditions. The final developmental and physiological output of the hormonal signaling in plants is the typical result of combined actions of several hormonal pathways. However, our knowledge of the mechanisms involved in the hormonal crosstalk is still poor.In the regulation of root development, several hormonal pathways are involved, with auxin and cytokinin being the principal players. The whole process of root organogenesis, starting with the initiation of the root pole in embryos (1), positioning and formation of stem cell niche (2, 3), maintenance of mitotic activity in proximal meristem (4-6), and rapid elongation and differentiation of cells leaving the root meristem (7) has been demonstrated to be controlled by auxin. In this context, the differential auxin distribution between cells is crucial (3,8,9). The auxin gradients or local auxin maxima can be generated by auxin metabolic reactions, mainly by local auxin biosynthesis (6, 10, 11) and intercellular auxin transport dependent on the coordinated action of influx carriers of the AUX/LAX family (12), PIN efflux carriers (13,14), and members of the multidrugresistant/P-glycoprotein (MDR/PGP) subfamily B of ATPbinding cassette (ABCB) proteins (15, 16). Accordingly, interference with the polar auxin transport disrupts the auxin distribution and results in dramatic patterning defects in the root meristem (2, 3, 9).Besides auxin, cytokinin (CK) is also involved in root organogenesis. Increase in CK levels by exogen...

Summary N6‐adenosine methylation (m6A) of mRNA is an essential process in most eukaryotes, but its role and the status of factors accompanying this modification are still poorly understood.Using combined methods of genetics, proteomics and RNA biochemistry, we identified a core set of mRNA m6A writer proteins in Arabidopsis thaliana.The components required for m6A in Arabidopsis included MTA, MTB, FIP37, VIRILIZER and the E3 ubiquitin ligase HAKAI. Downregulation of these proteins led to reduced relative m6A levels and shared pleiotropic phenotypes, which included aberrant vascular formation in the root, indicating that correct m6A methylation plays a role in developmental decisions during pattern formation.The conservation of these proteins amongst eukaryotes and the demonstration of a role in writing m6A for the E3 ubiquitin ligase HAKAI is likely to be of considerable relevance beyond the plant sciences.