Vacuolar SNAREs Function in the Formation of the Leaf Vascular Network by Regulating Auxin Distribution

Abstract

In normal leaf development, a two-dimensional pattern of leaf veins is known to form by differentiation of vascular cells from ground meristem cells in a manner that is regulated by the polar flow of auxin. However, the mechanisms regulating the distribution of auxin in the leaf primordium are largely unknown. Here we show that vacuolar SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), VAM3 and VTI11, are required for the formation of the leaf vascular network in a dosage-dependent manner. This is the first report to show that the pre-vacuolar compartment (PVC)-vacuole traffic pathway is required for the formation of the leaf vascular network. vam3-4, a VAM3-defective mutant, was found to have an immature vascular network. An analysis of the DR5 reporter in vam3-4 indicated that VAM3 is involved in the proper pattern formation of auxin maxima in the leaf primordium. This suggests that the immature vascular network in vam3-4 was mainly determined at the stage of procambium formation in the leaf primordium. The abnormal distribution of auxin maxima was caused by the non-polarized localization of the auxin efflux carrier PIN1 (PIN-FORMED 1) in leaf primordium cells. VAM3 is the first key protein which is required for the proper localization of PIN1 in leaf cells. Finally, we found that PIN1 proteins were constitutively transported to vacuoles in leaf and roots cells. Our findings demonstrate that the PVC-vacuole pathway is required for the formation of auxin maxima, which regulates the polar localization of PIN1, which, in turn, is required for the formation of the leaf vascular network.

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

“…A second route carries PIN from early endosomes to the lytic vacuole through late endosomes also known as multi-vesicular bodies (MVBs) or pre-vacuolar complexes (PVCs) (Kleine-Vehn et al, 2008;Laxmi et al, 2008;Oliviusson et al, 2006;Spitzer et al, 2009). Mutation in genes encoding endosomal sorting complex required for transport (ESCRTs) proteins (Spitzer et al, 2009), adaptor protein 3 (AP3) subunits (Feraru et al, 2010) and vacuolar morphology 3 (VAM3) (Shirakawa et al, 2009), which are all orthologs of lysosomal targeting factors in other eukaryotes, result in defects to the localization of PIN protein in the plasma membrane that are often accompanied by its ectopic accumulation within the cytoplasm. The mutations cause defects to various developmental processes, indicating an important regulatory role for vacuolar targeting of PIN.…”

Increase in vascular pattern complexity caused by mutations in LHY and CCA1 in Arabidopsis thaliana under continuous light

“…VAM3 protein is required for proper localization of PIN1 in leaf cells. There was a report on correlation between a decrease of vascular network complexity and a decrease in flatness of leaves in vam3 mutant plants (Shirakawa et al 2009). Therefore, we thought that a change (increase or decrease) of vascular pattern complexity might be responsible for the decrease in flatness of lhy;cca1 leaves under LL.…”

Shedding light on auxin movement: Light-regulation of polar auxin transport in the photocontrol of plant development

“…Other auxin transporters, such as PIN1, PIN3 and PIN7 are also targeted to the vacuole in dark-grown RAM, but genetic analyses suggest that only PIN1 and PIN2 are essential for the regulation of root growth in response to light. 28,34 Dark-dependent vacuolar targeting of PIN1 was also observed in organs other than the RAM, such as leaf primordia and SAM, both in adult plants 37,38 and in etiolated seedlings (Fig. 1).…”

“…23,24,31,42 On the other hand, in undifferentiated tissues such as meristems and leaf primordia, light-mediated control of PIN protein abundance through the regulation of vacuolar degradation is likely to specifically and reversibly regulate auxin fluxes and as a result growth, without affecting meristem patterning. 28,34,35,37,38 Another interesting aspect that is surfacing from these studies is that the environmental control of plant growth relies on common auxin transport-dependent regulatory mechanisms. For instance both phototropin-and phytochrome-mediated light response impinge on PIN3 relocalization to modify hypocotyl elongation, 31,42 whereas light and gravity affect PIN2 levels to modulate root growth.…”