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1. A temperature-sensitive FERONIA mutant allele that alters root hair growth

   https://doi.org/10.1093/plphys/kiaa051  

   Kim, Daewon; Yang, Jiyuan; Gu, Fangwei; Park, Sungjin; Combs, Jonathon; Adams, Alexander; Mayes, Heather B; Jeon, Su Jeong; Bahk, Jeong Dong; Nielsen, Erik (December 2020, Plant Physiology)

   null (Ed.)

   Abstract In plants, root hairs undergo a highly polarized form of cell expansion called tip-growth, in which cell wall deposition is restricted to the root hair apex. In order to identify essential cellular components that might have been missed in earlier genetic screens, we identified conditional temperature-sensitive (ts) root hair mutants by ethyl methanesulfonate mutagenesis in Arabidopsis thaliana. Here, we describe one of these mutants, feronia-temperature sensitive (fer-ts). Mutant fer-ts seedlings were unaffected at normal temperatures (20°C), but failed to form root hairs at elevated temperatures (30°C). Map based-cloning and whole-genome sequencing revealed that fer-ts resulted from a G41S substitution in the extracellular domain of FERONIA (FER). A functional fluorescent fusion of FER containing the fer-ts mutation localized to plasma membranes, but was subject to enhanced protein turnover at elevated temperatures. While tip-growth was rapidly inhibited by addition of rapid alkalinization factor 1 (RALF1) peptides in both wild-type and fer-ts mutants at normal temperatures, root elongation of fer-ts seedlings was resistant to added RALF1 peptide at elevated temperatures. Additionally, at elevated temperatures fer-ts seedlings displayed altered reactive oxygen species (ROS) accumulation upon auxin treatment and phenocopied constitutive fer mutant responses to a variety of plant hormone treatments. Molecular modeling and sequence comparison with other Catharanthus roseus receptor-like kinase 1L (CrRLK1L) receptor family members revealed that the mutated glycine in fer-ts is highly conserved, but is not located within the recently characterized RALF23 and LORELI-LIKE-GLYCOPROTEIN 2 binding domains, perhaps suggesting that fer-ts phenotypes may not be directly due to loss of binding to RALF1 peptides. 

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2. VPS45 is required for both diffuse and tip growth of Arabidopsis thaliana cells

   https://doi.org/10.3389/fpls.2023.1120307  

   Mugume, Yosia; Roy, Rahul; Agbemafle, William; Shepard, Gabriella N.; Vue, Yee; Bassham, Diane C. (February 2023, Frontiers in Plant Science)

   IntroductionVPS45 belongs to the Sec1/Munc18 family of proteins, which interact with and regulate Qa-SNARE function during membrane fusion. We have shown previously thatArabidopsis thalianaVPS45 interacts with the SYP61/SYP41/VTI12 SNARE complex, which locates on thetrans-Golgi network (TGN). It is required for SYP41 stability, and it functions in cargo trafficking to the vacuole and in cell expansion. It is also required for correct auxin distribution during gravitropism and lateral root growth. ResultsAsvps45knockout mutation is lethal in Arabidopsis, we identified a mutant,vps45-3, with a point mutation in theVPS45gene causing a serine 284-to-phenylalanine substitution. The VPS45-3 protein is stable and maintains interaction with SYP61 and SYP41. However,vps45-3plants display severe growth defects with significantly reduced organ and cell size, similar tovps45RNAi transgenic lines that have reduced VPS45 protein levels. Root hair and pollen tube elongation, both processes of tip growth, are highly compromised invps45-3. Mutant root hairs are shorter and thicker than those of wild-type plants, and are wavy. These root hairs have vacuolar defects, containing many small vacuoles, compared with WT root hairs with a single large vacuole occupying much of the cell volume. Pollen tubes were also significantly shorter invps45-3compared to WT. DiscussionWe thus show that VPS45 is essential for proper tip growth and propose that the observed vacuolar defects lead to loss of the turgor pressure needed for tip growth. 

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3. Tip Growth

   https://doi.org/10.1002/9780470015902.a0023746  

   Gilroy, Simon; Swanson, Sarah J (April 2017, Encyclopedia of life sciences)

   Pollen tubes and root hairs grow by a highly focused deposition of new wall and membrane materials at their growing apex. Comparison of the machinery that localises such growth between these cell types has revealed common components, providing important insight into how plant cells control cell expansion. Such elements include the small GTPases (e.g. ROPs and RABs), gradients and intricate spatial patterning in the fluxes of ions (e.g. Ca2+ and H+) and partitioning of membrane lipids (such as the phosphoinositides). These regulators are coupled to focused action of the secretory machinery (e.g. the exocyst) and cytoskeletal dynamics, with integral roles emerging for actin, tubulin and their associated motor proteins. These components form an integrated regulatory network that imposes robust spatial localisation of the growth machinery and so supports the production of an elongating tube-like growth form where cell expansion is limited to the very apex, that is, tip growth. 

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4. WALLFLOWER, a RLK, simultaneously localizes to opposite sides of root hair cells & functions to position hairs

   https://doi.org/10.1101/2023.03.16.533027  

   Toth, Jessica N; Rodriguez-Furlan, Cecilia; Van_Norman, Jaimie M (March 2023, bioRxiv)

   ABSTRACT Polarized cells are frequently partitioned into subdomains with unique features or functions. As plant cells are surrounded by walls, polarized cell shape and protein polarity in the plasma membrane are particularly important for normal physiology and development. We have identified WALLFLOWER (WFL), a transmembrane receptor kinase that is asymmetrically distributed at the inner face of epidermal cells and this localization is maintained independent of cell type. In epidermal hair (H) cells in the elongation and differentiation zones, WFL exhibits a dual polar localization, accumulating at the inner domain as well as at the root hair initiation domain (RHID). Furthermore, overexpression of WFL leads to a downward shift in root hair (RH) position suggesting WFL operates in a signaling pathway that functions across H cells to inform RH position. WFL asymmetric distribution and function is affected by deletion of the intracellular domains resulting in its mislocalization to the outer polar domain of H cells and exclusion from RHIDs and bulges. Thus, our results demonstrate that in epidermal H cells the WFL intracellular domains are required to direct its dual polar localization and influence RH position. ONE SENTENCE SUMMARYA receptor kinase with dual polar localization, to the inner polar domain and root hair initiation domain, in root epidermal cells, requires its intracellular domain for localization and function. 

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5. O -glycosylation of the extracellular domain of pollen class I formins modulates their plasma membrane mobility

   https://doi.org/10.1093/jxb/erac131  

   Lara-Mondragón, Cecilia M.; Dorchak, Alexandria; MacAlister, Cora A.; Dobritsa, ed., Anna (April 2022, Journal of Experimental Botany)

   Abstract In plant cells, linkage between the cytoskeleton, plasma membrane, and cell wall is crucial for maintaining cell shape. In highly polarized pollen tubes, this coordination is especially important to allow rapid tip growth and successful fertilization. Class I formins contain cytoplasmic actin-nucleating formin homology domains as well as a proline-rich extracellular domain and are candidate coordination factors. Here, using Arabidopsis, we investigated the functional significance of the extracellular domain of two pollen-expressed class I formins: AtFH3, which does not have a polar localization, and AtFH5, which is limited to the growing tip region. We show that the extracellular domain of both is necessary for their function, and identify distinct O-glycans attached to these sequences, AtFH5 being hydroxyproline-arabinosylated and AtFH3 carrying arabinogalactan chains. Loss of hydroxyproline arabinosylation altered the plasma membrane localization of AtFH5 and disrupted actin cytoskeleton organization. Moreover, we show that O-glycans differentially affect lateral mobility in the plasma membrane. Together, our results support a model of protein sub-functionalization in which AtFH5 and AtFH3, restricted to specific plasma membrane domains by their extracellular domains and the glycans attached to them, organize distinct subarrays of actin during pollen tube elongation. 

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