Abscisic acid (ABA) plays essential roles in plant development and responses to environmental stress. ABA induces subcellular translocation and degradation of the guanine nucleotide exchange factor RopGEF1, thus facilitating ABA core signal transduction. However, the underlying mechanisms for ABA-triggered RopGEF1 trafficking/degradation remain unknown. Studies have revealed that RopGEFs associate with receptor-like kinases to convey developmental signals to small ROP GTPases. However, how the activities of RopGEFs are modulated is not well understood. Type 2C protein phosphatases stabilize the RopGEF1 protein, indicating that phosphorylation may trigger RopGEF1 trafficking and degradation. We have screened inhibitors followed by several protein kinase mutants and find that quadruple-mutant plants in the calcium-dependent protein kinases (CPKs) disrupt ABA-induced trafficking and degradation of RopGEF1. Moreover, partially impairs ABA inhibition of cotyledon emergence. Several CPKs interact with RopGEF1. CPK4 binds to and phosphorylates RopGEF1 and promotes the degradation of RopGEF1. CPK-mediated phosphorylation of RopGEF1 at specific N-terminal serine residues causes the degradation of RopGEF1 and mutation of these sites also compromises the RopGEF1 overexpression phenotype in root hair development in Our findings establish the physiological and molecular functions and relevance of CPKs in regulation of RopGEF1 and illuminate physiological roles of a CPK-GEF-ROP module in ABA signaling and plant development. We further discuss that CPK-dependent RopGEF degradation during abiotic stress could provide a mechanism for down-regulation of RopGEF-dependent growth responses.
Urban community gardens have increased in prevalence as a means to generate fresh fruits and vegetables, including in areas lacking access to healthy food options.However, urban soils may have high levels of toxic heavy metals, including lead and cadmium and the metalloid arsenic, which can lead to severe health risks. In this study, fruit and vegetable samples grown at an urban community garden in southeastern San Diego, the Ocean View Growing Grounds, were sampled repeatedly over a four-year time period in order to measure potential contamination of toxic heavy metals and metalloids and to develop solutions for this problem. Metal nutrient, heavy metal, and metalloid concentrations were monitored in the leaf and fruit tissues of fruit trees over the sampling period. Several of the fruit trees showed uptake of lead in the leaf samples, with Black Mission fig measuring 0.843-1.531 mg/kg dry weight and Mexican Lime measuring 1.103-1.522 mg/kg dry weight over the sampling period. Vegetables that were grown directly in the ground at this community garden and surrounding areas showed arsenic, 0.80 + 0.073 mg/kg dry weight for Swiss chard, and lead, 0.84 ± 0.404 mg/kg dry weight for strawberries, in their edible tissues. The subsequent introduction of raised beds with uncontaminated soil is described, which eliminated any detectable heavy metal or metalloid contamination in these crops during the monitoring period. Recommendations for facilitating the monitoring of edible tissues and for reducing risk are discussed, including introduction of raised beds and collaborations with local universities and research groups. K E Y W O R D S arsenic, cadmium, heavy metal, ion transport, ionomics, Pb lead, phytoremediation, tomato/ Solanum lycopersicum S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Cooper AM, Felix D, Alcantara F, et al. Monitoring and mitigation of toxic heavy metals and arsenic accumulation in food crops: A case study of an urban community garden. Plant Direct. 2020;4:1-12. https ://doi.
C alifornia and the rest of the world are experiencing major changes in the availability of water and the concentration of atmospheric carbon dioxide (CO 2). Water and CO 2-along with energy from the sun-are the inputs to photosynthesis, the basis of plant growth and food production. Elevated CO 2 can influence the water use efficiency and yield of crop plants. A clearer understanding of the mechanisms involved in those responses may lead to the development of crop plant varieties better adapted to expected future water and CO 2 conditions. In recent years, the United States has experienced an increased frequency of heat waves and drought and related loss of crops (Lesk et al. 2016). In 2014, drought cost California an estimated $2.2 billion in lost agricultural production (Howitt et al. 2015). In California and elsewhere, the changing climate, population growth and mandates for sustainable management of surface water and groundwater are likely to make water for irrigation more scarce in the future. From 1950 to 2000, global atmospheric CO 2 concentration rose 33%. It continues to climb (fig. 1) and is a major cause of the continuing rise in global temperature (Victor et al. 2014). Average temperatures in California have risen 2.5°F since 1880, and this rate is expected to increase over the next century due to emissions of greenhouse gases (Pathak et al. 2018). Among other impacts, the rising temperature is expected to reduce the winter snowpack, effectively reducing California's water storage capacity and useful water supply (PPIC 2016). CO 2 and stomatal pore apertures Research in our laboratory investigates the mechanisms by which CO 2 elevation and drought cause closing of leaf stomata. Plant stomata are pores on leaf surfaces that enable the intake of CO 2 and the release of water vapor. Over 90% of the water lost by plants through evapotranspiration is released via stomata (Hetherington and Woodward 2003). Each stoma is surrounded by two specialized guard cells that open and close the central gas exchange pore (fig. 2). CO 2 closes stomatal pores, by entering the guard cells, where it is converted to bicarbonate. The increased concentration of bicarbonate is thought OUTLOOK Possible impacts of rising CO 2 on crop water use efficiency and food security Understanding the mechanisms involved in plants' response to rising CO 2 levels may lead to the development of crop plant varieties better adapted to future drought conditions.
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