Microtubule-based perception of mechanical conflicts controls plant organ morphogenesis.

Precise coordination between cells and tissues is essential for differential growth in plants. During lateral root formation in Arabidopsis thaliana, the endodermis is actively remodeled to allow outgrowth of the new organ. Here, we show that microtubule arrays facing lateral root founder cells display a higher order compared to arrays on the opposite side of the same cell, and this asymmetry is required for endodermal remodeling and lateral root initiation. We identify that MICROTUBULE ASSOCIATED PROTEIN 70-5 (MAP70-5) is necessary for the establishment of this spatially defined microtubule organization and endodermis remodeling and thus contributes to lateral root morphogenesis. We propose that MAP70-5 and cortical microtubule arrays in the endodermis integrate the mechanical signals generated by lateral root outgrowth, facilitating the channeling of organogenesis.

VAPYRIN-like is required for development of the moss Physcomitrella patens.

The VAPYRIN (VPY) gene in Medicago truncatula and Petunia hybrida is required for arbuscular mycorrhizal (AM) symbiosis. The moss Physcomitrella patens has a close homolog (VPY-like, VPYL), although it does not form AM. Here, we explore the phylogeny of VPY and VPYL in land plants, and study the expression and developmental function of VPYL in Ppatens We show that VPYL is expressed primarily in the protonema, the early filamentous stage of moss development, and later in rhizoids arising from the leafy gametophores and in adult phyllids. Knockout mutants have specific phenotypes in branching of the protonema and in cell division of the leaves (phyllids) in gametophores. The mutants are responsive to auxin and strigolactone, which are involved in regulation of protonemal branching, indicating that hormonal signaling in the mutants is not affected in hormonal signaling. Taken together, these results suggest that VPYL exerts negative regulation of protonemal branching and cell division in phyllids. We discuss VPY and VPYL phylogeny and function in land plants in the context of AM symbiosis in angiosperms and development in the moss.

RMR (Receptor Membrane RING-H2) type 1 and 2 show different promoter activities and subcellular localizations in Arabidopsis thaliana.

Soluble vacuolar proteins reach their compartments of final accumulation through the binding with specific transmembrane cargo receptors. In Arabidopsis thaliana two different families of receptors have been characterized. The AtVSRs (Vacuolar Sorting Receptor), which are known to be involved in the protein sorting to lytic vacuoles (LV), and the AtRMRs (Receptor Membrane RING-H2), for which there is less evidence for a role in the traffic to the protein storage vacuole (PSV). In this study we investigated the localization and tissue expression of two RMRs (AtRMR1 and 2) in their species of origin, A. thaliana. Our experiments using leaf protoplasts and transgenic plants supported previous results of subcellular localization in Nicotiana benthamiana that visualized AtRMR1 and 2 in the cisternae of endoplasmic reticulum (ER) and in the trans-Golgi network (TGN), respectively. The promoter activities of AtRMR1 and AtRMR2 detected in transgenic A. thaliana lines suggest that the expression of these two receptors only partially overlap in some organs and tissues. These results suggest that AtRMR1 and 2 are not functionally redundant, but could also interact and participate in the same cellular process in tissues with an overlapping expression.

The destination for single-pass membrane proteins is influenced markedly by the length of the hydrophobic domain.

The tonoplast was proposed as a default destination of membrane-bound proteins without specific targeting signals. To investigate the nature of this targeting, we created type I fusion proteins with green fluorescent protein followed by the transmembrane domain of the human lysosomal protein LAMP1. We varied the length of the transmembrane domain from 23 to either 20 or 17 amino acids by deletion within the hydrophobic domain. The resulting chimeras, called TM23, TM20, and TM17, were expressed either transiently or stably in tobacco. TM23 clearly accumulated in the plasmalemma, as confirmed by immunoelectron microscopy. In contrast, TM17 clearly was retained in the endoplasmic reticulum, and TM20 accumulated in small mobile structures. The nature of the TM20-labeled compartments was investigated by coexpression with a marker localized mainly in the Golgi apparatus, AtERD2, fused to a yellow fluorescent protein. The strict colocalization of both fluorescent proteins indicated that TM20 accumulated in the Golgi apparatus. To further test the default destination of type I membrane proteins, green fluorescent protein was fused to the 19-amino acid transmembrane domain of the plant vacuolar sorting receptor BP-80. The resulting chimera also accumulated in the Golgi instead of in post-Golgi compartments, where native BP-80 localized. Additionally, when the transmembrane domain of BP-80 was lengthened to 22 amino acids, the reporter escaped the Golgi and accumulated in the plasma membrane. Thus, the tonoplast apparently is not a favored default destination for type I membrane proteins in plants. Moreover, the target membrane where the chimera concentrates is not unique and depends at least in part on the length of the membrane-spanning domain.