Involvement of K+ in Leaf Movements During Suntracking

Involvement of K+ in Leaf Movements During Suntracking

Involvement of K+ in Leaf Movements During Suntracking

Introduction
Many plants orient their leaves in response to directional light signals.

Heliotropic movements, or movements that are affected by the sun, are common

among plants belonging to the families Malvaceae, Fabaceae, Nyctaginaceae, and

Oxalidaceae. The leaves of many plants, including Crotalaria pallida, exhibit

diaheliotropic movement. C. pallida is a woody shrub native to South Africa.

Its trifoliate leaves are connected to the petiole by 3-4 mm long pulvinules

(Schmalstig). In diaheliotropic movement, the plant’s leaves are oriented

perpendicular to the sun’s rays, thereby maximizing the interception of

photosynthetically active radiation (PAR). In some plants, but not all, his

response occurs particularly during the morning and late afternoon, when the

light is coming at more of an angle and the water stress is not as severe

(Donahue and Vogelmann). Under these conditions the lamina of the leaf is

within less than 15° from the normal to the sun. Many plants that exhibit

diaheliotropic movements also show paraheliotropic response as well.

Paraheliotropism minimizes water loss by reducing the amount of light absorbed

by the leaves; the leaves orient themselves parallel to the sun’s rays. Plants

that exhibit paraheliotropic behavior usually do so at midday, when the sun’s

rays are perpendicular to the ground. This reorientation takes place only in

leaves of plants that are capable of nastic light-driven movements, such as the

trifoliate leaf of Erythrina spp. (Herbert 1984). However, this phenomenon has

been observed in other legume species that exhibit diaheliotropic leaf movement

as well. Their movement is temporarily transformed from diaheliotropic to

paraheliotropic. In doing so, the interception of solar radiation is maximized

during the morning and late afternoon, and minimized during midday. The leaves

of Crotalaria pallida also exhibit nyctinastic, or sleep, movements, in which

the leaves fold down at night. The solar tracking may also provide a

competitive advantage during early growth, since there is little shading, and

also by intercepting more radiant heat in the early morning, thus raising leaf

temperature nearer the optimum for photosynthesis.

Integral to understanding the heliotropic movements of a plant is

determining how the leaf detects the angle at which the light is incident upon

it, how this perception is transduced to the pulvinus, and finally, how this

signal can effect a physiological response (Donahue and Vogelmann).

In the species Crotalaria pallida, blue light seems to be the wavelength

that stimulates these leaf movements (Scmalstig). It has been implicated in the

photonastic unfolding of leaves and in the diaheliotropic response in

Mactroptilium atropurpureum and Lupinus succulentus (Schwartz, Gilboa, and

Koller 1987). However, the light receptor involved can not be determined from

the data. The site of light perception for Crotalaria pallida is the proximal

portion of the lamina. No leaflet movement occurs when the lamina is shaded and

only the pulvinule is exposed to light. However, in many other plant species,

including Phaseolus vulgaris and Glycine max, the site of light perception is

the pulvinule, although these plants are not true suntracking plants. The

compound lamina of Lupinus succulentus does not respond to a directional light

signal if its pulvini are shaded, but it does respond if only the pulvini was

exposed. That the pulvinus is the site for light perception was the accepted

theory for many years. However, experiments with L. palaestinus showed that the

proximal 3-4 mm of the lamina needed to be exposed for a diaheliotropic response

to occur. If the light is detected by photoreceptors in the laminae, somehow

this light signal must be transmitted to the cells of the pulvinus. There are

three possible ways this may be done. One is that the light is channeled to the

pulvinus from the lamina. However, this is unlikely since an experiment with

oblique light on the lamina and vertical light on the pulvinus resulted in the

lamina responding to the oblique light. Otherwise, the light coming from the

lamina would be drowned out by the light shining on the pulvinus. Another

possibility is that some electrical signal is transmitted from the lamina to the

pulvinus as in Mimosa. It is also possible that some chemical is transported

from the lamina to the pulvinus via the phloem. These chemicals can be defined

as naturally occuring molecules that affect some physiological process of the

plant. They may be active in concentrations as low as 10-5 to 10-7 M solution.

Whatchemical, if any, is used by C. pallida to transmit the light signal from

the lamina of the leaflet to its pulvinule is unknown. Periodic leaf movement

factor 1 (PLMF 1) has been isolated from Acacia karroo, a plant with pinnate

leaves that exhibits nychinastic sleep movements, as well as other species of

Acacia, Oxalis, and Samanea. PLNF 1 has also been isolated from Mimosa pudica,

as has the molecule M-LMF 5 (Schildknecht).

The movement of the leaflets is effected by the swelling and shrinking

of cells on opposite sides of the pulvinus (Kim, et al.) In nyctinastic plants,

cells that take up water when a leaf rises and lose water when the leaf lowers

are called extensor cells. The opposite occurs in the flexor cells (Satter and

Galston). When the extensor cells on one side of the pulvinus take up water and

swell, the flexor cells on the other side release water and shrink. The

opposite of this movement can also occur. However, the terms extensor and

flexor are not rigidly defined. Rather, the regions are defined according to

function, not position. Basically, the pulvini cells that are on the adaxial

(facing the light) side of the pulvinus are the flexor cells, and the cells on

the abaxial side are the extensor cells. Therefore, the terms can mean

different cells in the same pulvinus at varying times of the day. By

coordinating these swellings and shrinkings, the leaves are able to orient

themselves perpendicular to the sunlight in diaheliotropic plants.

Leaf movements are the result of changes in turgor pressure in the

pulvinus. The pulvinus is a small group of cells at the base of the lamina of

each leaflet. The reversible axial expansion and contraction of the extensor

and flexor cells take place by reversible changes in the volume of their motor

cells. These result from massive fluxes of osmotically active solutes across

the cell membrane. K+ is the ion that is usually implicated in this process,

and is balanced by the co-transport of Cl- and other organic and inorganic

anions.

studied extensively, much data exists detailing nyctinastic movements. Several

ions are believed to be involved in leaf movment. These include K+, H+, Cl-,

malate, and other small organic anions. K+ is the most abundant ion in pulvini

cells. Evidence suggests that electrogenic ion secretion is responsible for K+

uptake in nyctinastic plants. The transition from light to darkness activates

the H+/ATPase in the flexor cells of the pulvinus. This leads to the release of

bound K+ from the apoplast and movement of the K+ into the cells by way of an

ion channel. This increase in K+ in the cell decreases the osmotic potential of

the cells, and water than influxes into the flexor cells, increasing their

volume. In Samanea, K+ levels changed four-fold in flexor cells during the

transition from light to darkness. In a similar experiment, during hour four of

a photoperiod, the extensor apoplast of Samanea had 14mM and the flexor apoplast

had 23 mM of K+. After the lights were turned off, inducing nyctinastic

movements, the K+ level in the apoplast rose to 72 mM in the extensor cells and

declined to 10mM in the flexor cells. Therefore, it appears that swelling cells

take up K+ from the apoplast and shrinking cells release K+ into the apoplast.

In the pulvinus of Samanea saman, depolarization of the plasma membrane

opens K+ channels (Kim et al). The driving force for the transport of K+ across

the cell membranes is apparently derived from activity of an electrogenic proton

pump. This creates an electrochemical gradient that allows for K+ movement.

From concentration measurements in pulvini, K+ seems to be the most important

ion involved in the volume changes of these cells. How then, is K+ allowed to

be at higher concentrations inside a cell than out of it? Studies indicate that

the K+ channels are not always open. In protoplasts of Samanea saman, K+

channels were closed when the membrane potential was below -40mV and open when

the membrane potential was depolarized to above -40mV. A voltage-gated K+

channel that is opened upon depolarization has been observed in every patch

clamp study of the plasma membranes of higher plants, including Samanea motor

cells and Mimosa pulviner cells.

It is proposed that electrogenic H+ secretion results in a proton motive

force, a gradient in pH and in membrane potential, that facilitates the uptake

of K+, Cl-, sucrose, and other anions. External sodium acetate promotes closure

and inhibits opening in Albizzia. This effect could be caused by a decrease in

transmembrane pH gradients. The promotion of opening and inhibition of closure

of leaves by fusicoccin and auxin in Cassia, Mimosa, and Albizzia also implicate

H+ in the solute uptake of motor cells, since both chemicals are H+/ATPase

activators, stimulating H+ secretion from the plant cells into the apoplast.

Vanadate, an H+/ATPase inhibitor, inhibits rhythmic leaflet closure in Albizzia.

Although this conflicts with the movement effected by fusicoccin and auxin, it

is believed that vanadate affects different cells, acting upon flexor rather

than extensor cells. The model indicates that there are two possible types of

H+ pumps. One is the electrogenic pump that creates the pmf mentioned above and

opens the K+ channels. The other pump is a H+/K+ exchanger, in which K+ is

pumped into the cell as H+ is pumped out of the cell in a type of antiport. The

presence of this typ of pump is only hypothetical, however, since at present

there is no evidence to support it. Thus there are two possible ways for K+ to

enter the pulvini cells. The buildup of the pH gradient may also promote Cl-

entry into the cell via a H+/Cl- cotransporter as the H+ trickles back into the

cell. Cl- ions may also be driven by the electrochemical gradient for Cl- via

Cl- channels, as with K+. A large Cl- channel was observed in the membrane of

Samanea flexor protoplasts. The channel closed at membrane potentials above

50mV and opened at potentials as low as -100mV.

Light-driven changes in membrane potential may be involved in the

activation of these proton pumps. This may be mediated by effects on

cytoplasmic Ca2+. Ca2+-chelators inhibit the nyctinastic folding as well as the

photonastic unfolding responses in Cassia. Thus Ca2+ may act as a second

messenger in a calmodulin-dependent reaction. The Ca2+ may be what turns on the

electrogenic proton pumps, causing changes in membrane potential. However,

there is no direct evidence to support this hypothesis, although chemicals that

are known to change calcium levels have been shown to alter the leaf movement of

Cassia fasciculata and other nyctinastic plants. One study involving Samanea

postulates that Ca2+ channels are also present in the plasma membrane of pulvini

cells, and inositol triphoshate, a second messenger in the signal transduction

pathway in animals, stimulates the opening of these channels. This insinuates

that some light signal binds to a receptor on the outside of the cell and

stimulates this transduction pathway. However, whether this hypothesis is true

is unclear. It has also been proposed that an outwardly directed Ca2+ pump

functions as a transport mechanism to restore homeostasis after Ca2+ uptake

through channels.

The changes in Cl- levels in the apoplast are less then that for K+.

The Cl- levels are 75% that of K+ in Albizzia, 40-80% in Samanea, and 40% in

Phaseolus. Therefore, other negatively charged ions must be used to compensate

for the positive charges of the K+. Malate concentrations vary, and it is lower

in shrunken cells than in swollen cells. It is believed that malate is

synthesized when there is not enough Cl- present to counteract the charges of

the K+.

An experiment with soybeans (Cronland) examined the role of K+ channels

by treating the pulvini with the K+ channel blocker tetraethylammonium chloride

(TEA), the H+/ATPase activator fusicoccin, and the H+/ATPase inhibitors vanadate

and erythrosin-B. In all cases the leaf movements of the plant were inhibited,

leading to the hypothesis that the directional light results in an influx of K+

into the flexor cells from the apoplast and an efflux of K+ from the extensor

cells into the apoplast, and these movements are driven by H+/ATPase pumps.

This combined reaction results in the elevation of the leaflet towards the light.

In this study, the diheliotropic movements of C. pallida are examined.

The purpose of this experiment is to determine which ions, if any, are used by

pulvini cells of Crotalaria pallida Aiton to control the uptake of water,

thereby affecting diheliotropic movement. As mentioned before, most studies

investigating the mechanisms of leaf movement have been performed on nyctinastic

plants. These plants respond to light and dark changes, not direction or

intensity of a light stimulus. Therefore, it is of interest to learn whether

the same principles can be applied to diheliotropic movement.

Different inhibitors at varying concentrations will be injected

individually into the pulvinus of C. pallida, and the suntracking ability of the

plant will then be measured. Tetraethylammonium (TEA), a K+ channel blocker

will be added to test whether K+ is involved in suntracking. Likewise, , a Cl-

channel blocker will be added to determine if Cl- is used. Vanadate, a

H+/ATPase inhibitor, will determine if hydrogen ions are pumped across the

plasma membrane, causing a hyperpolarization of the membrane. Fusicoccin, a

H+/ATPase activator will also be tested .

Keywords General: