1. 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

2. Effect of light on seedling growth DarkLight ‘etiolated’‘de-etiolated’ time (min) stem length (microns) light stems grow rapidly in darkness light inhibits stem elongation leaves expand slowly in darkness light stimulates leaf expansion stem elongation inhibition strong effect of Blue Light acting via photoreceptor Cryptochrome

3. 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.

4. 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.

5. “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

6. 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