ROTUNDA3 function in plant development by phosphatase 2A-mediated regulation of auxin transporter recycling.

The shaping of organs in plants depends on the intercellular flow of the phytohormone auxin, of which the directional signaling is determined by the polar subcellular localization of PIN-FORMED (PIN) auxin transport proteins. Phosphorylation dynamics of PIN proteins are affected by the protein phosphatase 2A (PP2A) and the PINOID kinase, which act antagonistically to mediate their apical-basal polar delivery. Here, we identified the ROTUNDA3 (RON3) protein as a regulator of the PP2A phosphatase activity in Arabidopsis thaliana. The RON3 gene was map-based cloned starting from the ron3-1 leaf mutant and found to be a unique, plant-specific gene coding for a protein with high and dispersed proline content. The ron3-1 and ron3-2 mutant phenotypes [i.e., reduced apical dominance, primary root length, lateral root emergence, and growth; increased ectopic stages II, IV, and V lateral root primordia; decreased auxin maxima in indole-3-acetic acid (IAA)-treated root apical meristems; hypergravitropic root growth and response; increased IAA levels in shoot apices; and reduced auxin accumulation in root meristems] support a role for RON3 in auxin biology. The affinity-purified PP2A complex with RON3 as bait suggested that RON3 might act in PIN transporter trafficking. Indeed, pharmacological interference with vesicle trafficking processes revealed that single ron3-2 and double ron3-2 rcn1 mutants have altered PIN polarity and endocytosis in specific cells. Our data indicate that RON3 contributes to auxin-mediated development by playing a role in PIN recycling and polarity establishment through regulation of the PP2A complex activity.

The ang3 mutation identified the ribosomal protein gene RPL5B with a role in cell expansion during organ growth.

The role of translation in the regulation of higher plant growth and development is not well understood. Mutational analysis is a powerful tool to identify and study the function of genes related to a biological process, such as growth. Here we analyzed functionally the angusta3 (ang3) narrow leaf mutant. The AG3 gene was cloned by fine mapping combined with candidate gene sequencing and it corresponded to the ribosomal protein gene RPL5B. Based on amino acid sequence homology, promoter DNA sequence homology and in silico gene expression analysis, RPL5B was found to be putatively functionally redundant with RPL5A. The morphological analysis of ang3 mutants showed that the leaf lamina area was significantly reduced from the third rosette leaf on, mainly because of decreased width. Cellular analysis of the abaxial epidermal cell layer of the third leaf indicated that the cell number in the mutant was similar to that of the wild type, but the cell size was significantly reduced. We postulate that the reduced cell expansion in the epidermis contributes to the narrow shape of ang3 leaves. Growth was also significantly impaired in hypocotyls and primary roots, hinting at a general role for RPL5B in organ growth, unrelated to dorsiventral axis formation. Comparison of the transcriptome of the shoot apices of the mutant and the wild type revealed a limited number of differentially expressed genes, such as MYB23 and MYB5, of which the lower expression in the ang3 mutant correlated with reduced trichome density. Our data suggest that translation is an important level of control of growth and development in plants.