We would like to know
about the mechanism by which flowers such as daisies open in the morning and
close at night.
This is called Nyctinasty
Sleep movements of many
species eg French bean (Phaseolus vulgaris), clover (Trifolium repens), wood
sorrel (Oxalis acetosella), silk tree (Albizzia julibrissin) and sensitive mimosa
(Mimosa pudica). Leaflets of nyctinastic plants assume a vertical (ie closed)
position in darkness and revert to horizontal orientation upon illumination.
The closing movement ot the "sleep" position can either be upwards (eg clover)
or downwards (eg wood sorrel).
These movements are not
growth movements. For exampel, in Mimosa and Albizzia the movements involve
changes in the turgor pressure in cells of the pulvini at the point of attachment
of the leaflet to the midrib and are evidently under some degree of phytochrome
control, since they are inhibited by exposure to FR at the end of a light
period and this effect can be reversed by R. These effects of FR can be
observed within 10 minutes. The changes in turgour have been shown to be
associated with the movement of potassium ions out of the cell and thereby
affecting osmotic properties. These movements are controlled by a biological
clock with light not only acting on the ion absorbing mechaninsms, but also
indirectly, by resetting the clock.
Redwood sorrel (Oxalis
oregana)
photosynthesises at light levels of only 1/200th of full sunlight. Sunflecks
penetrate the forest canopy and could damage the delicate species. There is
only a 10 second lag after sunlight strikes the leaves before they begin to
fold downwards, the folding being complete in about 6 minutes. When shade returns,
there is a 10 minute lag before the leaves return to the horizontal position.
The sensitive cells in the joints detect blue wavelengths of light.
Workers have isolated and
identified compounds that activate the pulvini of plants such as Mimosa. These
substances are transported in the transpiration stream and have their effect
on the membranes of the pulvini. Several active substances have been isolated
and these have been given the names "Turgorins". They are active at very low
concentrations 10-5 to 10-7 M.
John Hewitson, SAPS
References:-
Salisbury and Ross "Plant
Physiology" 1992 Wadsworth Publishing Co, Belmost USA
Wareing and Phillips "Growth
and differentiation in plants" 1981 Pergamon
Update (2001)
You
explained the mechanism behind the sensitive leaf curl of mimosa in terms of
change in turgor pressure. I though current thinking was that it involved contractile
proteins in the cytoskeleton.
Movement of leaves and leaflets
in the mimosa-family is controlled by a leaf-moving motor organ called the pulvinus,
located at the base of the petiole of the leaf. Changes in osmotic volume and
turgor of the motor cells within the pulvinus result from the movement of ions
(chiefly potassium and chloride) into and out of the cells.Signals causing leaf
unfolding cause cell shrinking in the top (adaxial, flexor) one-half of the
pulvinus and swelling in the bottom (abaxial, extensor) one-half. Signals causing
leaf folding cause the reverse responses. In both cell types, potassium ions
are released passively from the shrinking cell into the apoplast. In both cell
types, the "shrinking signaling" also results in the formation of
1,4,5-inositol trisphosphate, which is a second messenger in the phosphoinositide
(PI) cell signalling cascade. Calcium ions may also have a role in this regulation,
but current evidence is unclear beyond this.
Kameyama et al. in 2000
(Nature 407:37) suggested that the turgor changes still occur but that the molecular
process of bending may be due to decreased actin tyrosine-phosphorylation in
the pulvinus. Inhibitors of the cytoskeleton such as cytochalasin B and phalloidin,
also prevent these movements. Tthe same process may be associated with stomatal
movements.
Actin is a protein that
can form filaments which (in combination with other proteins) have the ability
to contract. As such, it is an important element of the cytoskeleton of both
muscle and nonmuscle cells, as well as playing a crucial role in cell morphology,motility,
and cytokinesis. In plant cells, the actin cytoskeleton is also involved in
various phenomena such as cell growth, mitosis and cytoplasmic streaming. Studies
(Fleurat-Lessard et al., 1988) using inhibitors (cytochalasin B and phalloidin)
which affect actin filament formation, also prevent the bending movement of
Mimosa leaves suggesting that actin filaments are involved (in addition to the
change in turgor pressure). In the most recent report (Yamashiro et al. June
2001) a protein from Mimosa that severs actin filaments has been isolated (interestingly,
this is calcium ion dependent) and this may be involved in actin/cytoskeleton
regulation.
So, with the best evidence
based on inhibitor studies (in 1988), actin involvement in leaf movement, whilst
very possible, is far from proven.
Update (2007)
Mimosa
pudica closes its leaflets in the "upward" direction
but the petiole of the entire leaf will move in the "downward" direction. How
can we explain this?
Scientists do not yet fully understand the mechanism behind this thigmonastic
(movement in response to tactile stimuli) movement or other plant movements
(e,g, nyctynasty: movement response to diurnal light and temperature changes).
Loss of turgor pressure occurs prior to the plant movement; but, this is not
thought to be the cause. On a more detailed level, what is happening? In stomata,
turgor pressure modulates opening and closing via a process which involves
changes in actin and microtubule organisation. The contribution of these contractile
proteins in the cytoplasm to movement has been observed in M.pudica motor cells
by electron microscopy. Following thigmonasty, these actin cables were seen
to loosen destorying the actin cytoskeleton. This is thought to lead to a loss
of turgor pressure. Studies using actin inhibitors (cytochalasin B and phalloidin)
demonstrated that they prevent nyctinasty, whilst similar studies with microtubule
inhibitors (colchicine and vinblastine) suggest they are not involved in M.pudica.
Inhibitors studies indicate
that the stability of actin filaments may be regulated by phosphorylation
of the tyrosine groups on the actin and an actin-modulating protein (a member
of the gelsolin superfamily) isolated from M.pudica has been shown
to sever actin filaments in a Ca2+-dependent manner; Ca2+ concentration
is known to increase during nyctinasty.
This is a brief summary
of what nastic plant movement may involve, but as to the question of explaining
the differences between petiole and leaflets, I guess these same processes
occur in both movements, but the orientation is determined by other factors,
probably similar to, or the same ones that tell a plant which way is 'up'.
John Hewitson, Liz Rylott,
Roger Delpech, SAPS
