Plant performance in stressful environments: interpreting new and established knowledge of the roles of arbuscular mycorrhizas

Arbuscular mycorrhizal (AM) symbioses are formed by approximately 80% of vascular plant species in all major terrestrial biomes. In consequence an understanding of their functions is critical in any study of sustainable agricultural or natural ecosystems. Here we discuss the implications of recent results and ideas on AM symbioses that are likely to be of particular significance for plants dealing with abiotic stresses such as nutrient deficiency and especially water stress. In order to ensure balanced coverage, we also include brief consideration of the ways in which AM fungi may influence soil structure, carbon deposition in soil and interactions with the soil microbial and animal populations, as well as plant-plant competition. These interlinked outcomes of AM symbioses go well beyond effects in increasing nutrient uptake that are commonly discussed and all require to be taken into consideration in future work designed to understand the complex and multifaceted responses of plants to abiotic and biotic stresses in agricultural and natural environments.     

Arsenic uptake and toxicity in plants: integrating mycorrhizal influences

Arsenic (As) contamination of soil and water is a global problem that impacts on many areas of biology. This review firstly covers aspects of soil chemistry and soil-plant interactions relevant to the ways plants take up As (particularly arsenate (As(V)) from aerobic soils, with especial attention to As-phosphorus (P) interactions. It then assesses the extent to which studies of plant As tolerance based on short-term uptake of As(V) from nutrient solutions can be extrapolated to longer-term growth in contaminated soil. Mycorrhizal symbioses are then highlighted, because they are formed by ~?90% of higher plants, often with increased uptake of phosphate (Pi) compared with non-mycorrhizal (NM) counterparts. It is therefore likely that mycorrhizas influence As(V) uptake. Published work shows that arbuscular mycorrhizal (AM) plants (the most common mycorrhizal type) have higher P/As ratios than NM plants, and this would be expected to affect sensitivity to soil As. We discuss ways in which higher P/As selectivity might result from differential operation of P and As uptake pathways in AM compared with NM plants, taking into account new understanding of P uptake mechanisms. We also give suggestions for future research required to increase understanding of mechanisms of As(V) uptake, and its interactions with plant P.     

RhoG Protein Regulates Platelet Granule Secretion and Thrombus Formation in Mice

Rho GTPases such as Rac, RhoA, and Cdc42 are vital for normal platelet function, but the role of RhoG in platelets has not been studied. In other cells, RhoG orchestrates processes integral to platelet function, including actin cytoskeletal rearrangement and membrane trafficking. We therefore hypothesized that RhoG would play a critical role in platelets. Here, we show that RhoG is expressed in human and mouse platelets and is activated by both collagen-related peptide (CRP) and thrombin stimulation. We used RhoG^−/− mice to study the function of RhoG in platelets. Integrin activation and aggregation were reduced in RhoG^−/− platelets stimulated by CRP, but responses to thrombin were normal. The central defect in RhoG^−/− platelets was reduced secretion from α-granules, dense granules, and lysosomes following CRP stimulation. The integrin activation and aggregation defects could be rescued by ADP co-stimulation, indicating that they are a consequence of diminished dense granule secretion. Defective dense granule secretion in RhoG^−/− platelets limited recruitment of additional platelets to growing thrombi in flowing blood in vitro and translated into reduced thrombus formation in vivo. Interestingly, tail bleeding times were normal in RhoG^−/− mice, suggesting that the functions of RhoG in platelets are particularly relevant to thrombotic disorders.