Arabidopsis UNHINGED encodes a VPS51 homolog and reveals a role for the GARP complex in leaf shape and vein patterning.

Asymmetric localization of PIN proteins controls directionality of auxin transport and many aspects of plant development. Directionality of PIN1 within the marginal epidermis and the presumptive veins of developing leaf primordia is crucial for establishing leaf vein pattern. One mechanism that controls PIN protein distribution within the cell membranes is endocytosis and subsequent transport to the vacuole for degradation. The Arabidopsis mutant unhinged-1 (unh-1) has simpler leaf venation with distal non-meeting of the secondary veins and fewer higher order veins, a narrower leaf with prominent serrations, and reduced root and shoot growth. We identify UNH as the Arabidopsis vacuolar protein sorting 51 (VPS51) homolog, a member of the Arabidopsis Golgi-associated retrograde protein (GARP) complex, and show that UNH interacts with VPS52, another member of the complex and colocalizes with trans Golgi network and pre-vacuolar complex markers. The GARP complex in yeast and metazoans retrieves vacuolar sorting receptors to the trans-Golgi network and is important in sorting proteins for lysosomal degradation. We show that vacuolar targeting is reduced in unh-1. In the epidermal cells of unh-1 leaf margins, PIN1 expression is expanded. The unh-1 leaf phenotype is partially suppressed by pin1 and cuc2-3 mutations, supporting the idea that the phenotype results from expanded PIN1 expression in the marginal epidermis. Our results suggest that UNH is important for reducing expression of PIN1 within margin cells, possibly by targeting PIN1 to the lytic vacuole.

A novel, semi-dominant allele of MONOPTEROS provides insight into leaf initiation and vein pattern formation.

Leaf vein pattern is proposed to be specified by directional auxin transport through presumptive vein cells. Activation of auxin response, which induces downstream genes that entrain auxin transport and lead to vascular differentiation, occurs through a set of transcription factors, the auxin response factors. In the absence of auxin, auxin response factors are inactive because they interact with repressor proteins, the Aux/IAA proteins. One member of the auxin response factor protein family, Auxin Response Factor 5/MONOPTEROS (MP), is critical to vein formation as indicated by reduced vein formation in loss-of-function MP alleles. We have identified a semi-dominant, gain-of-function allele of MP, autobahn or mp ( abn ), which results in vein proliferation in leaves and cotyledons. mp ( abn ) is predicted to encode a truncated product that lacks domain IV required for interaction with its Aux/IAA repressor BODENLOS (BDL). We show that the truncated product fails to interact with BDL in yeast two-hybrid assays. Ectopic expression of MP targets including the auxin efflux protein PINFORMED1 (PIN1) further supports the irrepressible nature of mp ( abn ). Asymmetric PIN1:GFP cellular localization does not occur within the enlarged PIN1:GFP expression domains, suggesting the asymmetry requires differential auxin response in neighbouring cells. Organ initiation from mp ( abn ) meristems is altered, consistent with disruption to source/sink relationships within the meristem and possible changes in gene expression. Finally, mp ( abn ) anthers fail to dehisce and their indehiscence can be relieved by jasmonic acid treatment, suggesting a specific role for MP in late anther development.