Auxin transport, which is mediated by specialized influx and efflux carriers, plays a major role in many aspects of plant growth and development. AUXIN1 (AUX1) has been demonstrated to encode a high-affinity auxin influx carrier. In Arabidopsis thaliana, AUX1 belongs to a small multigene family comprising four highly conserved genes (i.e., AUX1 and LIKE AUX1 [LAX] genes LAX1, LAX2, and LAX3). We report that all four members of this AUX/LAX family display auxin uptake functions. Despite the conservation of their biochemical function, AUX1, LAX1, and LAX3 have been described to regulate distinct auxindependent developmental processes. Here, we report that LAX2 regulates vascular patterning in cotyledons. We also describe how regulatory and coding sequences of AUX/LAX genes have undergone subfunctionalization based on their distinct patterns of spatial expression and the inability of LAX sequences to rescue aux1 mutant phenotypes, respectively. Despite their high sequence similarity at the protein level, transgenic studies reveal that LAX proteins are not correctly targeted in the AUX1 expression domain. Domain swapping studies suggest that the N-terminal half of AUX1 is essential for correct LAX localization. We conclude that Arabidopsis AUX/LAX genes encode a family of auxin influx transporters that perform distinct developmental functions and have evolved distinct regulatory mechanisms.

The transport of auxin controls developmental events in plants. Here, we report that in addition to maintaining vacuolar pH, the H+-pyrophosphatase, AVP1, controls auxin transport and consequently auxin-dependent development. AVP1 overexpression results in increased cell division at the onset of organ formation, hyperplasia, and increased auxin transport. In contrast, avp1-1 null mutants have severely disrupted root and shoot development and reduced auxin transport. Changes in the expression of AVP1 affect the distribution and abundance of the P-adenosine triphosphatase and Pinformed 1 auxin efflux facilitator, two proteins implicated in auxin distribution. Thus, AVP1 facilitates the auxin fluxes that regulate organogenesis.

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It is well accepted that lateral redistribution of the phytohormone auxin underlies the bending of plant organs towards light. In monocots, photoreception occurs at the shoot tip above the region of differential growth. Despite more than a century of research, it is still unresolved how light regulates auxin distribution and where this occurs in dicots. Here, we establish a system in Arabidopsis thaliana to study hypocotyl phototropism in the absence of developmental events associated with seedling photomorphogenesis. We show that auxin redistribution to the epidermal sites of action occurs at and above the hypocotyl apex, not at the elongation zone. Within this region, we identify the auxin efflux transporter ATP-BINDING CASSETTE B19 (ABCB19) as a substrate target for the photoreceptor kinase PHOTOTROPIN 1 (phot1). Heterologous expression and physiological analyses indicate that phosphorylation of ABCB19 by phot1 inhibits its efflux activity, thereby increasing auxin levels in and above the hypocotyl apex to halt vertical growth and prime lateral fluxes that are subsequently channeled to the elongation zone by PIN-FORMED 3 (PIN3). Together, these results provide new insights into the roles of ABCB19 and PIN3 in establishing phototropic curvatures and demonstrate that the proximity of light perception and differential phototropic growth is conserved in angiosperms.

SUMMARYHeterologous expression systems based on tobacco BY-2 cells, Arabidopsis cell cultures, Xenopus oocytes, Saccharomyces cerevisiae, and human HeLa cells have been used to express and characterize PIN, ABCB (PGP), and AUX/LAX auxin transporters from Arabidopsis. However, no single system has been identified that can be used for effective comparative analyses of these proteins. We have developed an accessible Schizosaccharomyces pombe system for comparative studies of plant transport proteins. The system includes knockout mutants in all ABC and putative auxin transport genes and Gateway Ò -compatible expression vectors for functional analysis and subcellular localization of recombinant proteins. We expressed Arabidopsis ABCB1 and ABCB19 in mam1pdr1 host lines under the inducible nmt41 promoter. ABCB19 showed a higher 3 H-IAA export activity than ABCB1. Arabidopsis PIN proteins were expressed in a mutant lacking the auxin effluxer like 1 (AEL1) gene. PIN1 showed higher activity than PIN2 with similar protein expression levels. Expression of AUX1 in a permease-deficient vat3 mutant resulted in increased net auxin uptake activity. Finally, ABCB4 expressed in mam1pdr1 displayed a concentration-dependent reversal of 3 H-IAA transport that is consistent with its observed activity in planta. Structural modelling suggests that ABCB4 has three substrate interaction sites rather than the two found in ABCB19, thus providing a rationale for the observed substrate activation. Taken together, these results suggest that the S. pombe system described here can be employed for comparative analyses and subsequent structural characterizations of plant transport proteins.