Generation of effective zinc-deficient agar-solidified media allows identification of root morphology changes in response to zinc limitation

Agar and agarose used for Staphylococcus aureus biofilm cultivation impact fluoroquinolone tolerance

AIMS

Staphylococcus aureus is an opportunistic pathogen whose treatment is further complicated by its ability to form biofilms. In this study, we examine the impact of growing S. aureus biofilms on different polymerizing surfaces, specifically agar and agarose, on the pathogen's tolerance to fluoroquinolones.

METHODS AND RESULTS

Biofilms of two methicillin-resistant strains of S. aureus were grown on agar or agarose in the presence of the same added nutrients, and their antibiotic susceptibility to two fluoroquinolones, moxifloxacin (MXF) and delafloxacin (DLX), were measured. We also compared the metabolism and extracellular polymeric substances (EPS) production of biofilms that were grown on agar and agarose.

CONCLUSIONS

Biofilms that were grown on agarose were consistently more susceptible to antibiotics than those grown on agar. We found that in biofilms that were grown on agar, extracellular protein composition was higher, and adding EPS to agarose-grown biofilms increased their tolerance to DLX to levels that were comparable to agar-grown biofilms.

Two related families of metal transferases, ZNG1 and ZNG2, are involved in acclimation to poor Zn nutrition in Arabidopsis

Metal homeostasis has evolved to tightly modulate the availability of metals within the cell, avoiding cytotoxic interactions due to excess and protein inactivity due to deficiency. Even in the presence of homeostatic processes, however, low bioavailability of these essential metal nutrients in soils can negatively impact crop health and yield. While research has largely focused on how plants assimilate metals, acclimation to metal-limited environments requires a suite of strategies that are not necessarily involved in metal transport across membranes. The identification of these mechanisms provides a new opportunity to improve metal-use efficiency and develop plant foodstuffs with increased concentrations of bioavailable metal nutrients. Here, we investigate the function of two distinct subfamilies of the nucleotide-dependent metallochaperones (NMCs), named ZNG1 and ZNG2, that are found in plants, using Arabidopsis thaliana as a reference organism. AtZNG1 (AT1G26520) is an ortholog of human and fungal ZNG1, and like its previously characterized eukaryotic relatives, localizes to the cytosol and physically interacts with methionine aminopeptidase type I (AtMAP1A). Analysis of AtZNG1, AtMAP1A, AtMAP2A, and AtMAP2B transgenic mutants are consistent with the role of Arabidopsis ZNG1 as a Zn transferase for AtMAP1A, as previously described in yeast and zebrafish. Structural modeling reveals a flexible cysteine-rich loop that we hypothesize enables direct transfer of Zn from AtZNG1 to AtMAP1A during GTP hydrolysis. Based on proteomics and transcriptomics, loss of this ancient and conserved mechanism has pleiotropic consequences impacting the expression of hundreds of genes, including those involved in photosynthesis and vesicle transport. Members of the plant-specific family of NMCs, ZNG2A1 (AT1G80480) and ZNG2A2 (AT1G15730), are also required during Zn deficiency, but their target protein(s) remain to be discovered. RNA-seq analyses reveal wide-ranging impacts across the cell when the genes encoding these plastid-localized NMCs are disrupted.

BRUTUS-LIKE (BTSL) E3 ligase-mediated fine-tuning of Fe regulation negatively affects Zn tolerance of Arabidopsis

The mineral micronutrients zinc (Zn) and iron (Fe) are essential for plant growth and human nutrition, but interactions between the homeostatic networks of these two elements are not fully understood. Here we show that loss of function of BTSL1 and BTSL2, which encode partially redundant E3 ubiquitin ligases that negatively regulate Fe uptake, confers tolerance to Zn excess in Arabidopsis thaliana. Double btsl1 btsl2 mutant seedlings grown on high Zn medium accumulated similar amounts of Zn in roots and shoots to the wild type, but suppressed the accumulation of excess Fe in roots. RNA-sequencing analysis showed that roots of mutant seedlings had relatively higher expression of genes involved in Fe uptake (IRT1, FRO2, and NAS) and in Zn storage (MTP3 and ZIF1). Surprisingly, mutant shoots did not show the transcriptional Fe deficiency response which is normally induced by Zn excess. Split-root experiments suggested that within roots the BTSL proteins act locally and downstream of systemic Fe deficiency signals. Together, our data show that constitutive low-level induction of the Fe deficiency response protects btsl1 btsl2 mutants from Zn toxicity. We propose that BTSL protein function is disadvantageous in situations of external Zn and Fe imbalances, and formulate a general model for Zn-Fe interactions in plants.

Current understanding of plant zinc homeostasis regulation mechanisms

The essential nature of Zn and widespread Zn deficiency in plants under field conditions underlie the great interest of researchers in the regulation of plant Zn homeostasis. Here, the current knowledge of plant Zn homeostasis regulation, mainly in A. thaliana, is reviewed. The plant Zn homeostasis machinery is regulated largely at the transcriptional level. Local regulation in response to changes in cellular Zn status is based on the transcription factors bZIP19 and bZIP23, which sense changes in free Zn²⁺ concentrations in the cell. However, there are likely other unidentified ways to sense cellular free Zn²⁺ concentrations in addition to the well-known bZIP19 and bZIP23 factors. In recent years, the existence of a shoot-derived systemic Zn deficiency signal, which is involved in the upregulation of Zn transport from roots to shoots, was demonstrated. Additionally, rates of mRNA degradation of Zn homeostasis genes are likely regulated by changes in cellular Zn status. In addition to the regulation of Zn transport, other mechanisms for the regulation of plant Zn homeostasis exist. "Zn sparing" mechanisms could be involved in the decrease in plant Zn requirements under Zn deficiency. Additionally, autophagy is probably regulated by local Zn status and involved in Zn reutilization at the cellular level. Current issues related to studying Zn homeostasis regulation are discussed.