A Theoretical Model for ROP Localisation by Auxin in Arabidopsis Root Hair Cells

Local activation of Rho GTPases is important for many functions including cell polarity, morphology, movement, and growth. Although a number of molecules affecting Rho-of-Plants small GTPase (ROP) signalling are known, it remains unclear how ROP activity becomes spatially organised. Arabidopsis root hair cells produce patches of ROP at consistent and predictable subcellular locations, where root hair growth subsequently occurs.We present a mathematical model to show how interaction of the plant hormone auxin with ROPs could spontaneously lead to localised patches of active ROP via a Turing or Turing-like mechanism. Our results suggest that correct positioning of the ROP patch depends on the cell length, low diffusion of active ROP, a gradient in auxin concentration, and ROP levels. Our theory provides a unique explanation linking the molecular biology to the root hair phenotypes of multiple mutants and transgenic lines, including OX-ROP, CA-rop, aux1, axr3, tip1, eto1, etr1, and the triple mutant aux1 ein2 gnom(eb).We show how interactions between Rho GTPases (in this case ROPs) and regulatory molecules (in this case auxin) could produce characteristic subcellular patterning that subsequently affects cell shape. This has important implications for research on the morphogenesis of plants and other eukaryotes. Our results also illustrate how gradient-regulated Turing systems provide a particularly robust and flexible mechanism for pattern formation

Acquisition of HIV by African-born residents of Victoria, Australia : insights from molecular epidemiology

African-born Australians are a recognised "priority population" in Australia's Sixth National HIV/AIDS Strategy. We compared exposure location and route for African-born people living with HIV (PLHIV) in Victoria, Australia, with HIV-1 pol subtype from drug resistance assays and geographical origin suggested by phylogenetic analysis of env gene. Twenty adult HIV positive African-born Victorian residents were recruited via treating doctors. HIV exposure details were obtained from interviews and case notes. Viral RNA was extracted from participant stored plasma or whole blood. The env V3 region was sequenced and compared to globally representative reference HIV-1 sequences in the Los Alamos National Library HIV Database. Twelve participants reported exposure via heterosexual sex and two via iatrogenic blood exposures; four were men having sex with men (MSM); two were exposed via unknown routes. Eight participants reported exposure in their countries of birth, seven in Australia, three in other countries and two in unknown locations. Genotype results (pol) were available for ten participants. HIV env amplification was successful in eighteen cases. HIV-1 subtype was identified in all participants: eight both pol and env; ten env alone and two pol alone. Twelve were subtype C, four subtype B, three subtype A and one subtype CRF02-AG. Reported exposure location was consistent with the phylogenetic clustering of env sequences. African Australians are members of multiple transnational social and sexual networks influencing their exposure to HIV. Phylogenetic analysis may complement traditional surveillance to discern patterns of HIV exposure, providing focus for HIV prevention programs in mobile populations

Safety, immunogenicity, and reactogenicity of BNT162b2 and mRNA-1273 COVID-19 vaccines given as fourth-dose boosters following two doses of ChAdOx1 nCoV-19 or BNT162b2 and a third dose of BNT162b2 (COV-BOOST): a multicentre, blinded, phase 2, randomised trial

Background Some high-income countries have deployed fourth doses of COVID-19 vaccines, but the clinical need, effectiveness, timing, and dose of a fourth dose remain uncertain. We aimed to investigate the safety, reactogenicity, and immunogenicity of fourth-dose boosters against COVID-19.Methods The COV-BOOST trial is a multicentre, blinded, phase 2, randomised controlled trial of seven COVID-19 vaccines given as third-dose boosters at 18 sites in the UK. This sub-study enrolled participants who had received BNT162b2 (Pfizer-BioNTech) as their third dose in COV-BOOST and randomly assigned them (1:1) to receive a fourth dose of either BNT162b2 (30 µg in 0·30 mL; full dose) or mRNA-1273 (Moderna; 50 µg in 0·25 mL; half dose) via intramuscular injection into the upper arm. The computer-generated randomisation list was created by the study statisticians with random block sizes of two or four. Participants and all study staff not delivering the vaccines were masked to treatment allocation. The coprimary outcomes were safety and reactogenicity, and immunogenicity (antispike protein IgG titres by ELISA and cellular immune response by ELISpot). We compared immunogenicity at 28 days after the third dose versus 14 days after the fourth dose and at day 0 versus day 14 relative to the fourth dose. Safety and reactogenicity were assessed in the per-protocol population, which comprised all participants who received a fourth-dose booster regardless of their SARS-CoV-2 serostatus. Immunogenicity was primarily analysed in a modified intention-to-treat population comprising seronegative participants who had received a fourth-dose booster and had available endpoint data. This trial is registered with ISRCTN, 73765130, and is ongoing.Findings Between Jan 11 and Jan 25, 2022, 166 participants were screened, randomly assigned, and received either full-dose BNT162b2 (n=83) or half-dose mRNA-1273 (n=83) as a fourth dose. The median age of these participants was 70·1 years (IQR 51·6–77·5) and 86 (52%) of 166 participants were female and 80 (48%) were male. The median interval between the third and fourth doses was 208·5 days (IQR 203·3–214·8). Pain was the most common local solicited adverse event and fatigue was the most common systemic solicited adverse event after BNT162b2 or mRNA-1273 booster doses. None of three serious adverse events reported after a fourth dose with BNT162b2 were related to the study vaccine. In the BNT162b2 group, geometric mean anti-spike protein IgG concentration at day 28 after the third dose was 23 325 ELISA laboratory units (ELU)/mL (95% CI 20 030–27 162), which increased to 37 460 ELU/mL (31 996–43 857) at day 14 after the fourth dose, representing a significant fold change (geometric mean 1·59, 95% CI 1·41–1·78). There was a significant increase in geometric mean anti-spike protein IgG concentration from 28 days after the third dose (25 317 ELU/mL, 95% CI 20 996–30 528) to 14 days after a fourth dose of mRNA-1273 (54 936 ELU/mL, 46 826–64 452), with a geometric mean fold change of 2·19 (1·90–2·52). The fold changes in anti-spike protein IgG titres from before (day 0) to after (day 14) the fourth dose were 12·19 (95% CI 10·37–14·32) and 15·90 (12·92–19·58) in the BNT162b2 and mRNA-1273 groups, respectively. T-cell responses were also boosted after the fourth dose (eg, the fold changes for the wild-type variant from before to after the fourth dose were 7·32 [95% CI 3·24–16·54] in the BNT162b2 group and 6·22 [3·90–9·92] in the mRNA-1273 group).Interpretation Fourth-dose COVID-19 mRNA booster vaccines are well tolerated and boost cellular and humoral immunity. Peak responses after the fourth dose were similar to, and possibly better than, peak responses after the third dose

Examples of Auxin and ROP mutants.

<p>In all figures the basal cell end is to the left. Each panel shows an actual root hair and, beneath, a simulation output. All parameters are as for wildtype, except as noted. <b>A</b> Auxin mutant <i>aux1</i>-7, <i>k</i>₂ = 0.2 conc.²s⁻¹. <b>B</b> Auxin mutant <i>aux1</i>-22, <i>k</i>₂ = 0.8 conc.²s⁻¹. <b>C</b> ROP mutant OX-ROP, <i>b</i> = 0.03 conc.s⁻¹. <b>D</b> ROP mutant <i>tip1</i>, <i>D</i>₁ = 0.5 µm²s⁻¹, <i>L</i> = 30 µm.</p

Wildtype Root Hair Formation.

<p>Basal cell ends are to the left. <b>A</b> Position of root hair on a wildtype root hair cell. Arrow indicates root hair outgrowth, arrowheads denote end walls. <b>B</b> Fluorescence image of ROP2-Green Fluorescent Protein in a young root hair cell approximately 45 µm long, showing the site from which a hair will begin to grow within the next few minutes. <b>C</b> Simulation plot of concentration of active ROP, <i>u(x,t)</i>, against distance for a cell 50 µm long (% distance relative to cell length); a high concentration represents ROP localisation and thus indicates the position where a hair will subsequently develop; see main text for parameters.</p

Mutant genotypes and phenotypes discussed in the Results.

<p>Items marked (?) are more speculative.</p

Model Parameters and Interpretation.

<p><b>A</b> The model considers two types of Rho of Plants (ROPs) molecules: active, GTP-bound ROPs, represented by variable <i>u(x,t)</i>, which are S-acylated, strongly associated with the cell membrane, and diffuse slowly (<i>D₁</i>), and inactive GDP-bound ROPs, represented by variable <i>v(x,t)</i>, which are not S-acylated, associate weakly with the membrane or not at all, and diffuse much more quickly (<i>D₂</i>). For the other parameters, see main text. <b>B</b> Schematic interpretation of events near the basal end of the cell. At the outset (0 minutes) active and inactive ROPs are assumed randomly distributed around the cell periphery, and active ROPs spend most of their time attached to the membrane via their S-acyl groups. As time passes a ROP patch self-assembles towards the basal end of the cell (10 minutes), causing local root hair growth (30 minutes).</p

The Effects of Timing and Cell Length.

<p>In all figures the basal cell end is to the left. All parameters are as for wildtype, except as noted. <b>A</b> If the time until the bulge appears is slower than wild type then the hair is shifted apically: simulation plot assuming time until bulge forms is 15 minutes (solid line) versus 30 minutes (dashed line). <b>B</b> Cell length at time of hair initiation shifts the final relative hair position: Solid line <i>L</i> = 40 µm; shorter cell shifts towards basal end (dashed line, <i>L</i> = 20 µm); longer cell shifts towards the apical end (dash-dot line, <i>L</i> = 60 µm).</p