Plant Mineral Nutrition: Solute Transport HORT 301 – Plant Physiology September 22, 2010 Taiz and Zeiger - Chapter 6, Appendix 1 Integral membrane transport proteins – responsible for movement of ions across membranes

Molecular diffusion - net movement of mineral nutrients and other molecules down the chemical potential gradient, passive transport K + > in B right and Cl - > in left, electrical gradient K + and Cl - move across membrane, concentration gradient No net ion movement across membrane

Chemical potential gradient (  ) – forces that drive diffusion Mineral ions and charged molecules – concentration and electrical potential gradients Neutral molecules – concentration gradient, unaffected by charge

APOPLAST pH 5.5 CYTOSOL pH 7.2 ΔE=-100 to -200 mV PLASMA MEMBRANE Membrane potential gradient (  E) – electrical potential gradient Differential ion accumulation on sides of the membrane Inside negative membrane potential across the plasma membrane Antiporter

APOPLAST pH 5.5 CYTOSOL pH 7.2 ΔE=-100 to -200 mV PLASMA MEMBRANE pH gradient - primarily responsible for the plasma membrane potential gradient pH gradient requires energy Chemical energy (ATP hydrolysis) is coupled to H + -transport against the electrochemical gradient ADP + P i ATP ADP + P i H+H+ H + pumps H+H+

APOPLAST pH 5.5 Ion and solute transport across the plasma membrane coupled to ∆pH

Translation of a membrane potential gradient into a force for diffusion ∆E can drive diffusion of ions At equilibrium – ion concentration gradient is balanced by the voltage difference ∆E (electrical potential/membrane potential) = 2.3RT/zF log C o /C i At equilibrium 2.3RT/zF = 59 mV, monovalent ion ∆E = 59 mV log C o /C i if C o /C i = 10, log 10 = 1 then ∆E = 59 mV x 1 An inside negative, membrane potential of -59 mV (~60 mV) can translate into a 10-fold concentration difference (monovalent cation)

Plasma membrane potential effects on a monovalent anion (e.g. Cl - ), a membrane potential of -120 mV (inside negative) requires that [Cl - ] apoplast must be >100X relative to [Cl - ] cytosol for passive transport Each ion has its own electrochemical potential Specificity is due to unique concentration activity Divalent (Ca 2+ or SO 4 2- ) ions have 2X the electrical potential

pH 5.5 pH / -200 mV pH mV Passive and active ion transport across the plasma membrane and tonoplast Dependent on concentration and membrane potential gradient Intracellular distribution of essential elements due to passive (dashed, -----) or active (solid line, →) transport

Transport protein categories – channels, carriers and pumps

Channels – diffusion inwards or outwards across the membrane K + channel

Primary active transport – energy production is coupled to ion transport H + electrochemical gradients across the plasma membrane and tonoplast Smith et al. (2010) Plant Biology (pH 5.2) -100 to -200 mV Apoplast Cytosol Vacuolar H + -ATPase ADP + P i ATP H+H+ PM H + -ATPase

Secondary active transporters (carriers) – couple H + transport to ion transport Down the H + electrochemical gradient Symporter – same direction, antiporter – opposite directions

Model of H + -sucrose symporter function

Raven et al. (2005) Biology of Plants H + -ATPase and H + -sucrose symporter coordination

Transport proteins at the plasma membrane Smith et al. (2010) Plant Biology pH to -200 mV pH 5.5

Tonoplast transport proteins Smith et al. (2010) Plant Biology pH 7.2

+30 mV

Radial ion transport from soil solution to the xylem