Mary A. Bisson
Plant Physiologist/Cell Biologist
Chara
Alga most closely related to higher plants
Large internodal cells
Species with different salt tolerance
Summary of Research Topics
Ion transport and salt toleranceSodium transport
Turgor regulation
Ligand-gated channels
Gravitropic responses
Ion transport and salt tolerance
Two closely related species of algaeSalt sensitive: Chara australis
Salt tolerant: Chara longifolia
Sodium transport
Keep cytoplasmic sodium low
Transport optionsPrevent entry (low
permeability)
Sequester in vacuole
Export from cytoplasm
ATP ADPNa cytoplasm
H+
H+ ATP H+
ADP H+Na+ ATP
Na+
ADP va cuole
Na +
Sodium transport
Keep cytoplasmic sodium low
Transport optionsPrevent entry (low
permeability)
Sequester in vacuole
Export from cytoplasm
ATP ADPNa cytoplasm
H+
H+ ATP H+
ADP H+Na+ ATP
Na+
ADP va cuole
Na +
Sodium transport
Keep cytoplasmic sodium low
Transport optionsPrevent entry (low
permeability)
Sequester in vacuole
Export from cytoplasm
ATP ADPNa cytoplasm
H+
H+ ATP H+
ADP H+Na+ ATP
Na+
ADP va cuole
Na +
Sodium fluxes: comparison between
species
Use radioactive isotope to measure influx, efflux, compartmentation of Na+
ResultsInflux similar in two species
Sequestration in vacuole low in both species
Efflux differs
Cytoplasmic sodium export: comparison
between species
Export higher in C. longifolia (salt-tolerant) than in C. australis (salt-sensitive)
Export higher when C. longifolia adapted to salt water
Possible mechanisms of sodium export
Na+/H+ exportpH sensitivity
Inhibitor studies
Different in salt-adapted and freshwater cells
Other transport systems? ATPase?
ATP ADPNa cytoplasm
H+
H+ ATP H+
ADP H+Na+ ATP
Na+
ADP va cuole
Na +
Possible mechanisms of sodium export
Na+/H+ exportpH sensitivity
Inhibitor studies
Different in salt-adapted and freshwater cells
Other transport systems? ATPase?
ATP ADPNa cytoplasm
H+
H+ ATP H+
ADP H+Na+ ATP
Na+
ADP va cuole
Na +
Research opportunities
Electrophysiology
Ion fluxes
Molecular biology (in collaboration with M. Hollingsworth)
Summary of Research Topics
Ion transport and salt toleranceSodium transport
Turgor regulation
Ligand-gated channels
Gravitropic responses
Turgor regulation
Need for turgor regulationTurgor = hydrostatic pressure difference
between cell and external medium
Provides structure
Driving force for growth
Turgor regulation
Hypo- and hypertonic stressesHypertonic: increase salt, decrease turgor,
“wilt”
Hypotonic: decrease salt, increase turgor, burst
Measure electrical responses, pressure
Model for mechanism of turgor responses
Model for turgor regulation
Turgor
Error signal
Pre-set Turgor
Osmoticpressure
Membranepotential
K+ channelactivity
K+ conc.
Ca2+ channelactivity
CytoplasmicCa2+ activity
Release fromInternal store
Cl- channelactivity
Cl-
conc.
?
?
?
Research opportunities
Electrophysiology and turgor probe
Patch clamp
Summary of Research Topics
Ion transport and salt toleranceSodium transport
Turgor regulation
Ligand-gated channels
Gravitropic responses
Channel activity
Looking for channels to test model
Characterize new channelCl- channel on the vacuolar membrane
Gated by acetylcholine and nicotine
Affects action potential (?)
Research Opportunities
Patch clamp
Physiologic effects--action potential?
Bioinformatic studies--putative channels?
Extend to higher plants
Summary of Research Topics
Ion transport and salt toleranceSodium transport
Turgor regulation
Ligand-gated channels
Gravitropic responses
Why study Chara?
Single colorless cell
0-1 h: Statoliths aggregate, sediment
2-24 h: Asymmetric growth
24 h: Complete re-orientation
Statoliths do not sediment in vertical rhizoids
Vacuole
Nucleus
Statoliths: suspended in actin. In constant, random motion.
Why should statoliths sediment in gravistimulated rhizoids?
•Actin disintegrates?
Not seen in micrographs (Braun and Wasteneys)
•Actin network distorts to move statoliths lower?
Ambiguous in micrographs
•Statoliths detach from actin, fall straight down?
•Natural cycle of releasing and reattaching to actin shifts in favor of release?
Measure statolith movement
Measure statolith movement continuously after a change in orientation
Quantify statolith movement:
What steps precede statolith sedimentation?
Dependent on the number of statoliths
Rate of settling, gravitropism slows with few statoliths
What steps precede statolith sedimentation?
Dependent on the number of statoliths
Dependent on Ca2+
A number of Ca2+ antagonists inhibit statolith movement and/or gravitropism
Altered gravitropism: Ca2+ antagonists
Why? Cytoskeletal involvement?
How?
Research opportunities: Cell biology
Determining role of actin: microscopy, inhibitor, etc.
Looking at cytoplasmic Ca2+: cell imaging
