15 research outputs found
Quantitative predictions on auxin-induced polar distribution of PIN proteins during vein formation in leaves
The dynamic patterning of the plant hormone auxin and its efflux facilitator
the PIN protein are the key regulator for the spatial and temporal organization
of plant development. In particular auxin induces the polar localization of its
own efflux facilitator. Due to this positive feedback auxin flow is directed
and patterns of auxin and PIN arise. During the earliest stage of vein
initiation in leaves auxin accumulates in a single cell in a rim of epidermal
cells from which it flows into the ground meristem tissue of the leaf blade.
There the localized auxin supply yields the successive polarization of PIN
distribution along a strand of cells. We model the auxin and PIN dynamics
within cells with a minimal canalization model. Solving the model analytically
we uncover an excitable polarization front that triggers a polar distribution
of PIN proteins in cells. As polarization fronts may extend to opposing
directions from their initiation site we suggest a possible resolution to the
puzzling occurrence of bipolar cells, such we offer an explanation for the
development of closed, looped veins. Employing non-linear analysis we identify
the role of the contributing microscopic processes during polarization.
Furthermore, we deduce quantitative predictions on polarization fronts
establishing a route to determine the up to now largely unknown kinetic rates
of auxin and PIN dynamics.Comment: 9 pages, 4 figures, supplemental information included, accepted for
publication in Eur. Phys. J.
Multiuser Cognitive Radio Networks: An Information Theoretic Perspective
Achievable rate regions and outer bounds are derived for three-user
interference channels where the transmitters cooperate in a unidirectional
manner via a noncausal message-sharing mechanism. The three-user channel
facilitates different ways of message-sharing between the primary and secondary
(or cognitive) transmitters. Three natural extensions of unidirectional
message-sharing from two users to three users are introduced: (i) Cumulative
message sharing; (ii) primary-only message sharing; and (iii) cognitive-only
message sharing. To emphasize the notion of interference management, channels
are classified based on different rate-splitting strategies at the
transmitters. Standard techniques, superposition coding and Gel'fand-Pinsker's
binning principle, are employed to derive an achievable rate region for each of
the cognitive interference channels. Simulation results for the Gaussian
channel case are presented; they enable visual comparison of the achievable
rate regions for different message-sharing schemes along with the outer bounds.
These results also provide useful insights into the effect of rate-splitting at
the transmitters, which aids in better interference management at the
receivers.Comment: 50 pages, 15 figures, submitted to IEEE Transactions on Information
Theor
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Membrane vesicles: A simplified system for studying auxin transport
Indoleacetic acid (IAA), the auxin responsible for regulation of growth, is transported polarly in plants. Several different models have been suggested to account for IAA transport by cells and its accumulation by membrane vesicles. One model sees diffusion of IAA driven by a pH gradient. The anion of a lipophilic weak acid like IAA or butyrate accumulates in an alkaline compartment in accord with the size of the pH gradient The accumulation of IAA may be diminished by the permeability of its lipophilic anion. This anion leak may be blocked by NPA. With anion efflux blocked, a gradient of two pH units would support an IAA accumulation of less than 50-fold at equilibrium (2) Another model sees diffusion of IAA in parallel with a saturable symport (IAA[sup [minus]] + nH[sup +]), driven by both the pH gradient and membrane voltage. Such a symport should be highly accumulative, however, with a lipophilic weak acid such as IAA, net diffusive efflux of IAAH whenever IAAHI[sub i] > IAAH[sub o] would constitute a leak. (3) A third model sees a pH change driven IAA uptake and saturable symport enhanced by internal binding sites. Following pH gradient-driven accumulation of IAA, the anion may bind to an intravesicular site, permitting further uptake of IAA. NPA, by blocking anion efflux, enhances this binding. We have reported that membrane vesicles isolated from actively growing plant tissues are a good system for studying the mechanisms involved in the transport and accumulation of auxin