How do plant cells grow?
• cell division (mitosis)
• cell expansion
Growth rate [flux, water uptake = cell volume increase] depends on [driving force] / resistance
(1) GR = water potential gradient / R (resistance to water movement into cell)
(2) GR = turgor pressure greater than yield threshold X wall extensibility
Acid Growth Hypothesis
Effect of light on seedling growth
stems grow rapidly in darkness
light inhibits stem elongation
leaves expand slowly in darkness
light stimulates leaf expansion
stem
length
(microns)
light
time (min)
Dark
‘etiolated’
Light
‘de-etiolated’
stem elongation inhibition
strong effect of Blue Light
acting via photoreceptor
Cryptochrome
Phototropism and role of auxin:
seedlings bend toward light
seedling apex senses light
tissue below apex causes the bend
so:
a signal must move from apex
to the responding tissue
“hormone” = auxin
Hormone activity depends on
1. synthesis
2. transport
3. perception/receptivity
4. degradation
In addition to the light effect
carried out by Cryptochrome
another BL photoreceptor
Phototropin is involved in
bending.
Phototropic bending is
caused by phototropin
redirecting the auxin flow.
Gravitropism – bending of organs along gravity vector: stems bend up, roots bend down.
As in phototropic bending,
gravitropism is due to a
redistribution of auxin.
In both stems and roots,
auxin moves to the lower
side of the organ.
“Flowering”
developmental shift from
vegetative to reproductive growth
appears first (macroscopically)
at shoot apical meristems
Many plants ‘flower’ when
induced by appropriate
photoperiod:
Long-Day plants
Short-Day plants
Day-Neutral plants
Photoperiodism involves the
circadian rhythm
Grafting experiments tell us that plants detect
photoperiod with their leaves. Yet, the stem apex
is responding. What is the signal that communicates
between leaf and stem?
‘Florigen’ is a peptide (Ft) synthesized in leaves, travels
to apex via phloem.
At the apex, ABC genes are expressed, changing the
production of organs from leaf to sepal/petal/stamen/carpel