Arabidillo Proteins Promote Root Branching

ARABIDILLO-1 and -2 are the Arabidopsis proteins most similar to animal and Dictyostelium P-catenins (Coates 2003). Loss-of-function arabidillo-1/-2 mutants form fewer lateral roots than wild-type seedlings. Conversely, ARABIDILLO-1 overexpression promotes lateral root formation (Coates et al. 2006).

CHRKt

S-Iocus receptor kinase

U-box Arm protein i mir

U-box Arm protein

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growth, GA and light signalling (PH0R1)

cell proliferation differentiation (NtPUB4) •-cell death,SI response (ARC1) cell death, hypersensitive response (ACREs) Cell death, basal pathogen response (SPI11)

CUL3?

^RBXj other responses? E2 ligase rrmm

ASA responsive gene expression glucose-and other stress responses?

i. Trichorne branching Endoredupiication (DMA replication) Hypocotyl elongation microtubules glucose-and other stress responses?

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5,1 KINASE

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Root hair tip growth direction; root hair cell morphology phragmoplast formation,cell division

Lateral roots initiate from coordinated cell divisions of selected pericycle cells in the centre of the primary root of the plant. This generates a lateral root primordium, and subsequently a new root meristem. This in turn generates a fully-formed root that emerges through the outer cell layers of the primary root (Casimiro et al. 2003). Arabidillo-1/-2 mutants are defective in early stages of lateral root formation, when lateral roots first initiate (Coates et al. 2006). The hormone auxin is critical for lateral root development (Casimiro et al. 2003); however, arabidillo-1 and -2 appear to regulate lateral root formation in an auxin-independent manner (Coates et al. 2006), suggesting the existence of a novel lateral root regulatory pathway.

ARABIDILLO proteins localise to the nucleus, and this is dependent in part on the Arm repeats. Certain ARABIDILLO protein fragments, including the F-box region, are cytosolic (Coates et al. 2006). It is not yet known how ARA-BIDILLO subcellular localisation is regulated in vivo. ARABIDILLO-1 and -2 are F-box proteins but it is unclear whether ARABIDILLOs function as ubiq-uitin ligases. Preliminary data suggest that ARABIDILLO-1 can interact with Arabidopsis SKP1-related proteins, which are components of SCF ubiquitin ligases (DJ Gibbs and JC Coates, unpublished).

The current hypothesis is that ARABIDILLO proteins target an inhibitor of lateral root development (for example, an inhibitor of cell division) for degradation. Studies are currently underway to identify ARABIDILLO-interacting proteins.

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