Plants are multicellular, terrestrial and photosynthetic

1. Plants are multicellular, terrestrial and photosynthetic Photosynthetic: Plants and many other organisms can convert solar energy to chemical energy Multicellular: Different cells can have various functions, but they must integrate their activities Terrestrial: Plant ancestors were aquatic, but terrestrial plants have to cope with very dry air Leaf cross section image from Bouton, J.H., et al., (1986). Photosynthesis, leaf anatomy, and morphology of progeny from hybrids between C3 and C3/C4 Panicum Species. Plant Physiol. 80: 487-492.

2. What are plants? Plants are photosynthetic eukaryotes (Circles not drawn to scale) ALL LIFE PHOTOSYNTHETIC ORGANISMS Green sulfur bacteria CYANOBACTERIA + “DESCENDANTS” Non-photosynthetic bacteria Cyanobacteria Purple sulfur bacteria EUKARYOTES WITH CYANOBACTERIA-DERIVED CHLOROPLASTS Other bacteria Archaea Diatoms Red algae Brown algae GREEN ALGAE AND DESCENDANTS Fungi PLANTS Animals You are here

3. Plants descended from a eukaryotic ancestor + a cyanobacteria Bacteria Eukarya Archaea Photosynthesis evolved in bacteria. All photosynthetic eukaryotes acquired this ability through endosymbiosis of photosynthetic bacteria Therefore, some “plant” genes (those derived from the ancestral bacteria) are more like bacterial genes than the genes of other eukaryotes FUNGI ALGAE PLANTS ANIMALS >0.5 BYA Plastid endo-symbiosis >1.5 BYA Mitochondrial endosymbiosis ORIGIN OF LIFE >3.5 BYA Adapted from Govindjee and Shevela, D. (2011). Adventures with cyanobacteria: a personal perspective. Frontiers in Plant Science. 2: 28.

4. Plants are photosynthetic eukaryotes High energy, reduced carbon • Plants convert light energy to chemical energy • Photosynthesis evolved in bacteria, and takes place in the descendants of endosymbiotic photosynthetic bacteria • Through photosynthesis, plants and algae are responsible for the transfer of most of the energy that enters the biosphere Energy input from sunlight Oxygen is released as a byproduct Low energy, oxidized carbon in carbon dioxide 6 CO2 + 6 H2O  C6H12O6 + 6 O2

5. Photosynthesis can be understood as two sets of connected reactions e− e− 2 NADPH 2 H+ 2 NADP+ Chloroplast ADP ATP 2 H2O O2 + 2 H+ + 2 H+ The LIGHT reactions take place in the thylakoid membranes The CARBON-FIXING reactions take place in the chloroplast stroma Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78.

6. Light harvesting reactions produce O2, ATP and NADPH e− e− 2 NADPH 2 H+ Cytochrome b6f complex 2 NADP+ The reactions require several large multi-protein complexes: two light harvesting photosystems (PSI and PSII), the cytochrome b6fcomplex, and ATP synthase ATP ADP Photosystem I (PSI) 2 H2O O2 + 2 H+ + 2 H+ Photosystem II (PSII) ATP synthase Adapted from Kramer, D.M., and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol.155: 70–78.

7. Chlorophyll captures light energy to initiate the light harvesting reactions First step of photochemistry e- e- Chl* Chlorin ring captures photons Photon 4 H+ 2 H2O Chlorophyll is held in pigment-protein complexes in a highly organized manner Chl Chl O2 Photon capture by chlorophyll excites the chlorophyll (Chl*). Chl* can lose an electron to become oxidized chlorolphyll (Chl+) Chl+ is reduced by stripping an electron from water, releasing oxygen and protons Buchanan, B.B., Gruissem, W. and Jones, R.L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.

8. Through the Calvin-Benson cycle, ATP and NADPH are used to fix CO2 3 x Ribulose-1,5-bisphosphate (RuBP) Each CO2 fixed requires 3 ATP and 2 NADPH 3 x CO2 Rubisco Carboxylation 6 ADP + 6 Pi 6 ATP Energy input Regeneration 6 x 3-phosphoglycerate (3PG) 6 x glyceraldehyde 3-phosphate (GAP) Reduction Energy input 6 ATP 5 x GAP 6 ADP + 6 Pi For every 3 CO2 fixed, one GAP is produced for biosynthesis and energy Reducing power input 6 NADPH 1 x GAP 6 NADP+ + 6 H+ Adapted from: Buchanan, B.B., Gruissem, W. and Jones, R.L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.

9. Plants are assembled from cells NUCLEUS CHLOROPLASTS Plasma membrane ENDOMEMBRANES The plasma membrane is a barrier that lets cells maintain a different internal environment from their surroundings VACUOLE MITOCHONDRIA Vacuolar membrane (tonoplast) Plasma membrane Cell wall A “typical” plant cell

10. Cells are surrounded by a semi-permeable plasma membrane The membrane is permeable to water and gases, but impermeable to ions and larger molecules Na+ Na+ K+ K+ OUT H2O CO2 IN Photo credit: Wenche Eikrem and Jahn Throndsen, University of Oslo

11. Proteins in the plasma and vacuolar membranes move molecules X H+ X These transporters bring in needed ions and other compounds, and export unwanted molecules. The transporters are also needed to regulate the cell’s osmotic potential ADP ATP H+ ATP ADP H+ H+ VACUOLE Vacuolar membrane Plasma membrane Cell wall Adapted from Hedrich, R. (2012). Ion channels in plants. Physiol. Rev. 92: 1777-1811.

12. (Most) plant cells are connected by plasmodesmata Plasmodesmata are plasma-membrane lined, regulated cytoplasmic bridges between plant cells Signals, nutrients, ions and water can move to adjacent cells, or longer distances via the phloem Guard cells are isolated, without functional plasmodesmata Pathogens can spread through the plant through plasmodesmata Reprinted from Lee, J.-Y. and Lu, H. (2011). Plasmodesmata: the battleground against intruders. Trends Plant Sci. 16: 201-210 with permission from Elsevier.

13. Multicellular organisms need to transmit materials and information Information has to be transmitted to integrate activities Raw materials have to be distributed – diffusion is inadequate for larger organisms Metabolic products have to be distributed throughout the body

14. Vascular plants have long-distance transport systems PHLOEM Photosynthetically-produced sugars (and other molecules) move from their source to sinks (non-photosynthetic tissues) through the phloem XYLEM Water moves from the soil to the atmosphere through the hollow dead cells of the xylem

15. Water uptake and movement in vascular plants Water is pulled through the hollow xylem by tension developed at evaporative sites in the leaves In the leaf, water evaporates out of the xylem into the intracellular spaces, and then through the stomata into the atmosphere Stomate Endodermis Water moves from the soil, into the outer layers of the root , then into the vascular cylinder and xylem Vascular cylinder Outer root layer

16. Water movement in the xylem is driven by evaporation Better model: Hollow tube with a selectivity filter in the roots and a flow regulator at the top Guard cells Simple model: Hollow tube that water evaporates through Ψw= -15 to -100 MPa Ψw= -0.2 MPa Casparian strip of the endodermis Adapted from Lucas, W.J. et al. (2013). The plant vascular system: Evolution, development and functions. J. Integr. Plant Biol. 55: 294-388.

17. Na+ The root endodermis acts as a selectivity filter Na+ Na+ Na+ Na+ Na+ Na+ The endodermis produces a water-impermeable layer, the Casparian strip, that provides selectivity Photo credit Michael Clayton

18. Waxy cuticles prevent water loss; regulated pores allow it OPEN Guard cells change their volume to open and close the pore. Guard cells are sensitive to the atmospheric conditions and the plant’s needs for gas exchange and water conservation CLOSED Most plant aerial surfaces are covered by a waxy cuticle. Pores called stomata, usually covered by pairs of guard cells, permit transpiration

19. Transport in the phloem From source to sink 1. Sugars are loaded into the phloem by active transport 3. The fluid moves through the sieve elements under pressure, by bulk flow (like water in a hose) CC SE Sugars 2. Water moves in by osmosis 4. Sugars are released into the sink tissues 5. Water follows by osmosis Adapted from Lucas, W.J. et al. (2013). The plant vascular system: Evolution, development and functions. J. Integr. Plant Biol. 55: 294-388.

20. Vascular tissues are essential conduits for information flow Some signals move in the xylem. Signals from drought-stressed roots cause guard cells to close In animals, signals moving through the nervous and circulatory systems convey information Other xylem-borne signals convey information about nutrient availability and soil-microbes Reprinted from Schachtman, D.P. and Goodger, J.Q.D. (2008). Chemical root to shoot signaling under drought. Trends Plant Sci. 13: 281-287 with permission from Elsevier; see also Christmann, A., Grill, E. and Huang, J. (2013). Hydraulic signals in long-distance signaling. Curr. Opin. Plant Biol. 16: 293-300.

21. Plants can survive across most of the earth Arctic Mountain Antarctic Desert Photo credits: Hannes Grobe, AWI; Gnomefilliere; Liam Quinn; Florence Devouard

22. Obtaining and retaining water is a challenge for terrestrial plants Freshwater green algae easily take up water from their aquatic environment The water potential of air and soil is usually lower than that of the plant cells. How do terrestrial plants survive?

23. Desiccation tolerance or avoidance, drought evasion or tolerance Most bryophytes can tolerate desiccation (drying out extensively) Some desert plants evade drought. They survive the dry season as seeds, sprouting and flowering in a brief period of rain Most tracheophytes cannot tolerate desiccation – they die Some desert plants tolerate dry conditions through adaptations such as deep roots, C4 photosynthesis, and tiny or absent leaves Photo credits: Mary Williams; Amrum; Scott Bauer;James Henderson, Golden Delight Honey, Bugwood.org

24. SUMMARY Like animals, plants need energy, water, and the ability to tolerate environmental challenges Plants have endosymbiotic photosynthetic organelles that let them produce chemical energy from light The two groups of plants, bryophytes and tracheophytes, differ in size, how they move materials, and how they deal with desiccation Image credit: Forest & Kim Starr, Starr Environmental, Bugwood.org