Pion-Nucleon Scattering at Low Energies

We study pion-nucleon scattering at tree level with a chiral lagrangian of pions, nucleons, and $\Delta$-isobars using a K-matrix unitarization procedure. Evaluating the scattering amplitude to order $Q^2$, where $Q$ is a generic small momentum scale, we obtain a good fit to the experimental phase shifts for pion center-of-mass kinetic energies up to $50$MeV. The fit can be extended to 150 MeV when we include the order-$Q^3$ contributions. Our results are independent of the off-shell $\Delta$ parameter.Comment: 13 pages, RevTex, Two tables and two figures, Minor changes, To appear in PR

Role of Leptin in Metabolic Adaptation During Cold Acclimation

Chronic cold exposure stimulates thermogenesis in brown adipose tissue, resulting in fat mobilization and compensatory hyperphagia. Mostly, these physiological events are accompanied by a remarkable reduction in serum leptin levels. However, the physiological roles of hypoleptinemia in cold adaptation are still not fully clear. We hypothesized that leptin is the keystone of the regulatory systems linking energy balance to cold adaptation. Leptin treatment (5μg/day) decreased food intake, body weight, serum ghrelin levels and hypothalamic melanin-concentrating hormone (MCH) gene expression. Food restriction in the pair-fed group mimicked most of the effects induced by leptin treatment. Central coadministration of ghrelin (1.2 μg/day) partially reversed the effect of leptin on hypothalamic MCH mRNA, but it did not block the reducing effects of leptin on food intake, body weight and serum ghrelin levels. In addition, hypothalamic pro-opiomelanocortin gene expression increased significantly in response to the coadministration of leptin and ghrelin. Collectively, we conclude that the regulatory effects of leptin on energy balance in cold-acclimated rats are dependent on feeding, which may involve the reduction of hypothalamic MCH gene expression. We found no evidence for ghrelin involvement in the regulation of leptin on food intake and body weight during cold acclimation

The pollen receptor kinase LePRK2 mediates growth-promoting signals and positively regulates pollen germination and tube growth

In flowering plants, the process of pollen germination and tube growth is required for successful fertilization. A pollen receptor kinase from tomato (Solanum lycopersicum), LePRK2, has been implicated in signaling during pollen germination and tube growth as well as in mediating pollen (tube)-pistil communication. Here we show that reduced expression of LePRK2 affects four aspects of pollen germination and tube growth. First, the percentage of pollen that germinates is reduced, and the time window for competence to germinate is also shorter. Second, the pollen tube growth rate is reduced both in vitro and in the pistil. Third, tip-localized superoxide production by pollen tubes cannot be increased by exogenous calcium ions. Fourth, pollen tubes have defects in responses to style extract component (STIL), an extracellular growth-promoting signal from the pistil. Pollen tubes transiently overexpressing LePRK2-fluorescent protein fusions had slightly wider tips, whereas pollen tubes coexpressing LePRK2 and its cytoplasmic partner protein KPP (a Rop-GEF) had much wider tips. Together these results show that LePRK2 positively regulates pollen germination and tube growth and is involved in transducing responses to extracellular growth-promoting signals.Fil: Zhang, Dong. Chinese Academy of Sciences; República de ChinaFil: Wengier, Diego Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones en Ingeniería Genética y Biología Molecular "Dr. Héctor N. Torres"; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Fisiología, Biología Molecular y Celular. Laboratorio de Fisiología y Biología Molecular; Argentina. University of California at Berkeley; Estados UnidosFil: Shuai, Bin. University of California at Berkeley; Estados UnidosFil: Gui, Cai Ping. Chinese Academy of Sciences; República de ChinaFil: Muschietti, Jorge Prometeo. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones en Ingeniería Genética y Biología Molecular "Dr. Héctor N. Torres"; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Fisiología, Biología Molecular y Celular. Laboratorio de Fisiología y Biología Molecular; ArgentinaFil: McCormick, Sheila. University of California at Berkeley; Estados UnidosFil: Tang, Wei Hua. Chinese Academy of Sciences; República de China. University of California at Berkeley; Estados Unido

Redundance of Δ\Delta-isobar Parameters in Effective Field Theories

It is shown that the off-shell parameters in the interaction Lagrangian of pions, nucleons, and $\Delta$-isobars are redundant in the framework of effective field theories. Our results also suggest the necessity of including the $\Delta$ as an explicit dynamical degree of freedom.Comment: 11 pages, RevTex, no figures, a minor error corrected, to appear in PL

Pion--Nucleon Scattering in a New Approach to Chiral Perturbation Theory

We study pion--nucleon scattering with a chiral lagrangian of pions, nucleons, and $\Delta$-isobars. The scattering amplitude is evaluated to one-loop $Q^3$ order, where $Q$ is a generic small momentum, using a new approach which is equivalent to heavy baryon chiral perturbation theory. We obtain a good fit to the experimental phase shifts for pion center-of-mass kinetic energies up to 100 MeV. A sigma term greater than 45 MeV is favored, but the value is not well determined.Comment: 46 pages, RevTex, 14 figures and 2 tables, minor changes for publicatio