Strigolactone- and Karrikin-Independent SMXL Proteins Are Central Regulators of Phloem Formation

Plant stem cell niches, the meristems, require long-distance transport of energy metabolites and signaling molecules along the phloem tissue. However, currently it is unclear how specification of phloem cells is controlled. Here we show that the genes SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4, and SMXL5 act as cell-autonomous key regulators of phloem formation in Arabidopsis thaliana. The three genes form an uncharacterized subclade of the SMXL gene family that mediates hormonal strigolactone and karrikin signaling. Strigolactones are endogenous signaling molecules regulating shoot and root branching [1] whereas exogenous karrikin molecules induce germination after wildfires [2]. Both activities depend on the F-box protein and SCF (Skp, Cullin, F-box) complex component MORE AXILLARY GROWTH2 (MAX2) [3-5]. Strigolactone and karrikin perception leads to MAX2-dependent degradation of distinct SMXL protein family members, which is key for mediating hormonal effects [6-12]. However, the nature of events immediately downstream of SMXL protein degradation and whether all SMXL proteins mediate strigolactone or karrikin signaling is unknown. In this study we demonstrate that, within the SMXL gene family, specifically SMXL3/4/5 deficiency results in strong defects in phloem formation, alteredsugar accumulation, and seedling lethality. By comparing protein stabilities, we show that SMXL3/4/5 proteins function differently to canonical strigolactone and karrikin signaling mediators, although being functionally interchangeable with those under low strigolactone/karrikin signaling conditions. Our observations reveal a fundamental mechanism of phloem formation and indicate that diversity of SMXL protein functions is essential for a steady fuelling of plant meristems.Peer reviewe

Targeted combination prevention to support female sex workers in Zimbabwe accessing and adhering to antiretrovirals for treatment and prevention of HIV (SAPPH-IRe): a cluster-randomised trial.

BACKGROUND: Strengthening engagement of female sex workers with health services is needed to eliminate HIV. We assessed the efficacy of a targeted combination intervention for female sex workers in Zimbabwe. METHODS: We did a cluster-randomised trial from 2014 to 2016. Clusters were areas surrounding female sex worker clinics and were enrolled in matched pairs. Sites were randomly assigned (1:1) to receive usual care (free sexual-health services supported by peer educators, including HIV testing on demand, referral for antiretroviral therapy [ART], and health education) or an intervention that supported additional regular HIV testing, on-site initiation of ART, pre-exposure prophylaxis, adherence, and intensified community mobilisation. The primary outcome was the proportion of all female sex workers with HIV viral load 1000 copies per mL or greater, assessed through respondent-driven sampling surveys. We used an adapted cluster-summary approach to estimate risk differences. This trial is registered with Pan African Clinical Trials Registry, number PACTR201312000722390. RESULTS: We randomly assigned 14 clusters to usual care or the intervention (seven in each group). 3612 female sex workers attended clinics in the usual-care clusters and 4619 in the intervention clusters during the study. Half as many were tested (1151 vs 2606) and diagnosed as being HIV positive (546 vs 1052) in the usual-care clusters. The proportion of all female sex workers with viral loads of 1000 copies per mL or greater fell in both study groups (from 421 [30%] of 1363 to 279 [19%] of 1443 in the usual-care group and from 399 [30%] of 1303 to 240 [16%] of 1439 in the intervention group), but with a risk difference at the end of the assessment period of only -2·8% (95% CI -8·1 to 2·5, p=0·23). Among HIV-positive women, the proportions with viral loads less than 1000 copies per mL were 590 (68%) of 869 in the usual-care group and 588 (72%) of 828 in the intervention group at the end of the assessment period, adjusted risk difference of 5·3% (95% CI -4·0 to 14·6, p=0·20). There were no adverse events. INTERPRETATION: Our intervention of a dedicated programme for female sex workers led to high levels of HIV diagnosis and treatment. Further research is needed to optimise programme content and intensity for the broader population. FUNDING: UN Population Fund (through Zimbabwe's Integrated Support Fund funded by UK Department for International Development, Irish Aid, and Swedish International Development Cooperation Agency)