Growth Hormones in Broad Bean (Vicia faba L.) and Radish (Raphanus raphanistrum subsp. sativus L.) Are Associated with Accumulated Concentrations of Perfluoroalkyl Substances.

In this study, we grew radish (Raphanus raphanistrum subsp. sativus L.) and broad beans (Vicia faba L.) in a greenhouse on soils spiked with a mixture of 15 per- and polyfluoroalkyl substances (PFASs) and investigated the association between accumulated ∑PFAS concentrations, growth, and hormone levels. Short-chained PFASs dominated aboveground tissues, whereas long-chained PFASs were most abundant in the plant roots. Our results showed that the presence or absence of exodermal Casparian strips, as well as the hydrophobicity and anion exchange capacities of PFASs, could explain the translocation of PFASs within plants. Significant associations found between accumulated PFAS concentrations and levels of gibberellins (GA1 and GA15), methionine, and indole-3-acetic acid (IAA) imply potential effects of PFASs on plant development and growth. This study provides the first evidence of associations between PFAS accumulation in plants and growth hormone levels, possibly leading to growth reduction of the apical dome and effects on the cell cycle in pericycle cells and methionine metabolism in plants.

Accumulation and effects of perfluoroalkyl substances in Arabidopsis thaliana in a temperature-dependent manner: an in vitro study.

The replacement of long-chained per- and polyfluoroalkyl substances (PFAS) with their short-chained homologues may have an impact on the accumulation in plants. The extent to which PFAS are absorbed by plants may differ among species and may depend on environmental factors, including temperature. The effect of an increased temperature on root uptake and translocation of PFAS in plants has been poorly studied. In addition, very few studies have examined toxicity of environmentally realistic PFAS concentrations to plants. Here, we investigated the bioaccumulation and tissue-distribution of fifteen PFAS in Arabidopsis thaliana L. grown in vitro at two different temperatures. Additionally, we examined the combined effects of temperature and PFAS accumulation on plant growth. Short-chained PFAS mainly accumulated in the leaves. The perfluorocarboxylic acid (PFCA) concentrations in roots and leaves, and the relative contribution of PFCAs to the ΣPFAS concentrations increased with carbon chain length regardless of temperature, with the exception of perfluorobutanoic acid (PFBA). An increased uptake of PFAS in leaves and roots at higher temperatures was observed for PFAS containing either eight or nine carbon atoms and could hence potentially result in higher risks for human intake. Leaf:root ratios of PFCAs followed a U-shaped pattern with carbon chain length, which is attributed to both hydrophobicity and anion exchange. Overall, no combined effects of realistic PFAS concentrations and temperature on the growth of A. thaliana were observed. PFAS exposure positively affected early root growth rates and root hair lengths, indicating a potential effect on factors involved in root hair morphogenesis. However, this effect on root growth rate became negligible later on in the exposure, and solely a temperature effect was observed after 6 days. Temperature also affected the leaf surface area. The underlying mechanisms on how PFAS stimulates root hair growth require further examination.

The Arabidopsis thaliana RNA editing factor SLO2, which affects the mitochondrial electron transport chain, participates in multiple stress and hormone responses.

Recently, we reported that the novel mitochondrial RNA editing factor SLO2 is essential for mitochondrial electron transport, and vital for plant growth through regulation of carbon and energy metabolism. Here, we show that mutation in SLO2 causes hypersensitivity to ABA and insensitivity to ethylene, suggesting a link with stress responses. Indeed, slo2 mutants are hypersensitive to salt and osmotic stress during the germination stage, while adult plants show increased drought and salt tolerance. Moreover, slo2 mutants are more susceptible to Botrytis cinerea infection. An increased expression of nuclear-encoded stress-responsive genes, as well as mitochondrial-encoded NAD genes of complex I and genes of the alternative respiratory pathway, was observed in slo2 mutants, further enhanced by ABA treatment. In addition, H2O2 accumulation and altered amino acid levels were recorded in slo2 mutants. We conclude that SLO2 is required for plant sensitivity to ABA, ethylene, biotic, and abiotic stress. Although two stress-related RNA editing factors were reported very recently, this study demonstrates a unique role of SLO2, and further supports a link between mitochondrial RNA editing events and stress response.

Biosynthesis of auxin by the gram-positive phytopathogen Rhodococcus fascians is controlled by compounds specific to infected plant tissues.

The role and metabolism of indole-3-acetic acid in gram-negative bacteria is well documented, but little is known about indole-3-acetic acid biosynthesis and regulation in gram-positive bacteria. The phytopathogen Rhodococcus fascians, a gram-positive organism, incites diverse developmental alterations, such as leafy galls, on a wide range of plants. Phenotypic analysis of a leafy gall suggests that auxin may play an important role in the development of the symptoms. We show here for the first time that R. fascians produces and secretes the auxin indole-3-acetic acid. Interestingly, whereas noninfected-tobacco extracts have no effect, indole-3-acetic acid synthesis is highly induced in the presence of infected-tobacco extracts when tryptophan is not limiting. Indole-3-acetic acid production by a plasmid-free strain shows that the biosynthetic genes are located on the bacterial chromosome, although plasmid-encoded genes contribute to the kinetics and regulation of indole-3-acetic acid biosynthesis. The indole-3-acetic acid intermediates present in bacterial cells and secreted into the growth media show that the main biosynthetic route used by R. fascians is the indole-3-pyruvic acid pathway with a possible rate-limiting role for indole-3-ethanol. The relationship between indole-3-acetic acid production and the symptoms induced by R. fascians is discussed.