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

Dark Light‘etiolated’ ‘de-etiolated’

time (min)

stemlength(microns)

light

stems grow rapidly in darknesslight inhibits stem elongationleaves expand slowly in darknesslight stimulates leaf expansion

stem elongation inhibitionstrong effect of Blue Light acting via photoreceptor Cryptochrome

Phototropism and role of auxin:

seedlings bend toward lightseedling apex senses lighttissue below apex causes the bendso:a signal must move from apexto the responding tissue“hormone” = auxin

Hormone activity depends on1. synthesis2. transport3. perception/receptivity4. degradation

In addition to the light effectcarried out by Cryptochromeanother BL photoreceptorPhototropin is involved inbending.

Phototropic bending iscaused by phototropinredirecting the auxin flow.

Gravitropism – bending of organs along gravity vector: stems bend up, roots bend down.

As in phototropic bending,gravitropism is due to aredistribution of auxin.

In both stems and roots,auxin moves to the lowerside of the organ.

“Flowering”

developmental shift fromvegetative to reproductive growth

appears first (macroscopically)at shoot apical meristems

Many plants ‘flower’ wheninduced by appropriate photoperiod:

Long-Day plantsShort-Day plantsDay-Neutral plants

Photoperiodism involves thecircadian rhythm

Grafting experiments tell us that plants detectphotoperiod with their leaves. Yet, the stem apexis responding. What is the signal that communicatesbetween leaf and stem?

‘Florigen’ is a peptide (Ft) synthesized in leaves, travelsto apex via phloem.

At the apex, ABC genes are expressed, changing theproduction of organs from leaf to sepal/petal/stamen/carpel