Network adaptation improves temporal representation of naturalistic stimuli in drosophila eye: II Mechanisms

Retinal networks must adapt constantly to best present the ever changing visual world to the brain. Here we test the hypothesis that adaptation is a result of different mechanisms at several synaptic connections within the network. In a companion paper (Part I), we showed that adaptation in the photoreceptors (R1-R6) and large monopolar cells (LMC) of the Drosophila eye improves sensitivity to under-represented signals in seconds by enhancing both the amplitude and frequency distribution of LMCs' voltage responses to repeated naturalistic contrast series. In this paper, we show that such adaptation needs both the light-mediated conductance and feedback-mediated synaptic conductance. A faulty feedforward pathway in histamine receptor mutant flies speeds up the LMC output, mimicking extreme light adaptation. A faulty feedback pathway from L2 LMCs to photoreceptors slows down the LMC output, mimicking dark adaptation. These results underline the importance of network adaptation for efficient coding, and as a mechanism for selectively regulating the size and speed of signals in neurons. We suggest that concert action of many different mechanisms and neural connections are responsible for adaptation to visual stimuli. Further, our results demonstrate the need for detailed circuit reconstructions like that of the Drosophila lamina, to understand how networks process information

Potential of exogenous L-amino acids in salinity stress alleviation during germination and early post-germinative seedling growth of Lactuca sativa L.

Soil salinity is a common abiotic stress for plants, that is having an increasing impact on international food production. A practical strategy to help mitigate the adverse effects of salinity stress on crop productivity is to increase salt tolerance of crop plants. It has been shown that exogenous application of L-proline and L-glutamate is capable of reducing the severity of salinity stress on seed germination and early seedling growth of brassica and cucumber, respectively. The main aim of the present study was to investigate the potential of all 20 common protein amino acids to alleviate salinity stress in lettuce (Lactuca sativa L., variety ‘Great Lakes’) during and immediately following germination. Sowing lettuce seeds in different concentrations of sodium chloride (NaCl) adversely affected germination and early seedling growth in a dose-dependent manner. After 48 hours of sowing lettuce seeds in 1 mM of any of the 20 exogenous amino acids in the absence of NaCl, it was found that the amino acids also inhibited seedling growth, particularly root elongation. However, in direct treatment experiments involving addition of seven amino acids singly (L-asparagine, L-isoleucine, L-leucine, L-proline, L-phenylalanine, L-tyrosine and L-valine) to an inhibitory concentration (60mM) of NaCl, it was found that lettuce seedling growth was protected from the salt stress. Additionally, seeds pre-treated for 8 hours before germination with L-arginine, L-glycine, L-histidine, L-methionine and L-phenylalanine, showed significant growth recovery after a further 40 hours growth exposed to 60mM NaCl. The measurements of cell size in root maturation zone and mitotic index at the root tip of lettuce seedlings after 48 hours from sowing seeds suggested that it might be possible that some amino acid treatments could affect cell elongation and / or cell division. However, further in-depth investigations are required and warranted to elucidate the mechanism(s) whereby exogenous amino acids could play a role in alleviation of salt stress in lettuce. It is concluded that several L-amino acids have the potential in pre-sowing seed treatment (seed priming technology) to enhance salt tolerance for crop stand establishment in soils with salinity issues