Plants synthesize numerous unique small molecules for signaling, defense, growth regulation and other functions. For some of these small molecules, plant cells maintain transport proteins that control compound compartmentalization. However for other small molecules, including many terpenoids and sorgoleone, transport proteins have not been identified. For these compounds, it has been hypothesized that terpenoids may cross biological membranes passively, without the need for a specific carrier protein. Terpenoids are general class of compounds, which sometimes act as a phytoalexin to defend against parasites. Sorgoleone is a unique allelochemical compound secreted by sorghum in roots, and is thought to potentially be an energy store. Through molecular simulation, we study membrane transport for terpenoids and sorgoleone to determine their membrane permeability coefficient. To facilitate accurate simulation, terpenoids and sorgoleone were parameterized through the standard CHARMM methodology. To evaluate the effect of membrane composition on permeability, we simulate the terpenoids in realistic sorghum membrane, and compare the permeability behavior with that for a yeast model membrane. We track permeation events within our unbiased simulations, and compare implied permeation coefficients with those calculated from Replica Exchange Umbrella Sampling calculations using the inhomogeneous solubility diffusion model. The terpenoids are observed to permeate freely through these membranes, indicating that a transport protein may not be needed to export these small molecules from plant cells. We also find that the permeability is membrane-dependent, with increased permeability for these molecules in plant membranes. This suggests that plants have tailored their membranes to facilitate low-energy transport processes for signaling or defense molecules.

Exploring the permeability of small molecules in sorghum membrane through molecular dynamics simulation.

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