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