Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/1/2003
Publication Date: 3/1/2003
Citation: PINEROS, M.A., KOCHIAN, L.V. DIFFERENCES IN WHOLE CELL AND SINGLE CHANNEL ION CURRENTS ACROSS THE PLASMA MEMBRANE OF MESOPHYLL CELLS FROM TWO CLOSELY RELATED THLASPI SPECIES. PLANT PHYSIOLOGY. 2003. Interpretive Summary: Heavy metal contamination of soils poses serious problems to our society, and the current technologies used to remediate soils are quite costly and disruptive. There is considerable interest in the use of terrestrial plants to clean up heavy metals from the soil. A small number of metal hyperaccumulating plants have been identified that can grow in highly contaminated soils and accumulate these metals to high shoot concentrations. We have been studying the mechanisms for metal hyperaccumulation in Thlaspi caerulescens, a zinc/cadmium hyperaccumulator. In this study, we used a highly sensitive electrophysiological system known as the patch clamp technique to study possible differences in heavy metal ion transport between leaf cells isolated from Thlaspi caerulescens and a related non-accumulator, Thlaspi arvense. Interestingly, the major differences we saw were not in zinc or cadmium transporters but in potassium (K) channels that normally function in moving this essential macronutrient cation into and out of plant cells. Two different populations of K channels were identified, with a different type of K channel in the outer membrane of leaf cells of the two Thlaspi species. Evidence was obtained for the movement of zinc and cadmium into leaf cells of the hyperaccumulator plant species (T. caerulescens) via channels that normally function to transport K. These findings suggest that heavy metal hyperaccumulation in T. caerulescens involves not only specialized micronutrient transporters (based on previous research findings from our lab), but also alterations in transporters for the essential macronutrient, potassium.
Technical Abstract: The patch clamp technique was used to study the physiology of ion transport in mesophyll cells from Thlaspi caerulescens, a heavy metal (Zn/Cd) hyperaccumulator species that can tolerate and accumulate very high levels of heavy metals in their leaf cells, and Thlaspi arvense, a related non-accumulator species. The membrane conductance of every T. arvense leaf cell was dominated by a slowly developing time-dependent K+ outward rectifier (SKOR). In contrast, only 23% of T. caerulescens cells showed SKOR activity, while the remaining 77% exhibit a rapidly developing instantaneous K+ outward rectifier (RKOR) current. In contrast to RKOR, the channels underlying the SKOR current were sensitive to changes in the extracellular ion activity. Single channel recordings indicated the existence of K+ channel populations with similar unitary conductances, but distinct channel kinetics and regulation. The correlation between these recordings and the whole cell data indicated that although one type of channel kinetics is preferentially activated in each Thlaspi species, both species have the capability to switch between either type of current. Ion substitution in whole cell and single channel experiments indicated that although the SKOR and RKOR channels mediate a net outward K+ current, they can also allow a significant Zn2+ permeation (i.e. influx). Additionally, single channel recordings allowed us to identify an infrequent type of plasma membrane divalent cation channel that also can mediate Zn2+influx. We propose that the different K+ channels types or channel states may result from and are likely to reflect differences in the cytoplasmic and apoplastic ionic environment in each species. Thus, the ability to interchangeably switch between different channel states allows each species to constantly adjust to changes in their apoplastic ionic environment.