Plant Science

Over thousands of years, humans have "domesticated" wild type plants and animals through selective breeding. Examples from the plant world include the breeding of modern hybrid maize from teosinte, or the development of modern wheat from emmer. As our knowledge of genomics and molecular technologies advances, we have developed much more precise and potentially more versatile ways to modify plants: genetic modification. In these two lectures we have a brief look at what biotechnology actually means, and our challenges in the time of rapid population growth and climate change. Using the example of Bacillus thuringiensis toxin we explore the principles behind genetic modification, and follow that up with a brief description of the introduction of herbicide resistance into broad acre crops. To conclude Plant Science in 2012, we move away from those input traits and take a look at Golden Rice, a genetically modified rice that produces beta-carotin in the grain. Golden Rice has the potential to save many thousands of children from blindness, and even death, caused by lack of vitamin A in the prevalent staple diet. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

Over thousands of years, humans have "domesticated" wild type plants and animals through selective breeding. Examples from the plant world include the breeding of modern hybrid maize from teosinte, or the development of modern wheat from emmer. As our knowledge of genomics and molecular technologies advances, we have developed much more precise and potentially more versatile ways to modify plants: genetic modification. In these two lectures we have a brief look at what biotechnology actually means, and our challenges in the time of rapid population growth and climate change. Using the example of Bacillus thuringiensis toxin we explore the principles behind genetic modification, and follow that up with a brief description of the introduction of herbicide resistance into broad acre crops. To conclude Plant Science in 2012, we move away from those input traits and take a look at Golden Rice, a genetically modified rice that produces beta-carotin in the grain. Golden Rice has the potential to save many thousands of children from blindness, and even death, caused by lack of vitamin A in the prevalent staple diet. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

The transition from water to land required plants to develop efficient transport pipelines for water and nutrients to the leaves, and for energy-rich carbohydrates (from photosynthetic carbon dioxide fixation) to tissues that require energy (e.g. roots, storage organs etc.). Xylem and phloem are found together in vascular bundles and transport water (unidirectionally) and photosynthates (bidirectionally), respectively. While these vascular bundles, or steeles, are arranged in a circle in dicots, they are scattered throughout the stem in monocots. This means that monocots can not perform secondary, or thickness, growth. Thickness growth in dicots is due to the activity of a secondary, or vascular, cambium that produces xylem towards the inside, and phloem towards the outside. Old phloem eventually dies and contributes to the bark which protects the active phloem. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

A very important part of plant cells is located outside the cells themselves: plant cell walls. Composed of numerous different building blocks (mostly polysaccharides, but also proteins and, particularly in cells that contribute to structural strength, lignins), cell walls determine the shape of the cells and provide a counterforce to the osmotically generated turgor pressure. In this lecture we look at the major polysaccharides that are found in the middle lamella, primary wall, and secondary wall, and a very simplified model of how we think the plant cell wall is organised. The role of lignins as structural and water-proofing material is discussed briefly. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

So how does photosynthesis actually work? In this lecture we explore the structures that capture light energy, photosystems 1 and 2, and how that light energy is harnessed to generate NADPH, and to build up a proton gradient across the thylakoid membrane. Just like in a hydroelectric plant, the proton gradient drives a little "turbine" that generates ATP. In this "light reaction" part of photosynthesis, light energy that is freely available from our sun is converted into chemical energy in the form of NADPH and ATP. The chemical energy is then used in the Calvin-Benson cycle to fix atmospheric carbon dioxide. There are three main phases: actual fixation, catalysed by an enzyme referred to as Rubisco; reduction from an organic acid to an aldehyde which really is the first sugar; and recycling of the acceptor molecule. Some plants employ a "carbon dioxide enrichment" process. The first fixation results in an organic molecule, most often malate, containing 4 carbon atoms - hence "C4 photosynthesis". Thanks to a very specialised leaf anatomy, the pre-fixed carbon dioxide is released in the bundle sheeth cells, resulting in very high carbon dioxide concentrations which enable Rubisco to work very efficiently. The trade-off is a higher energy requirement. A similar mechanism of pre-fixation with a temporal separation is used by CAM plants. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

After finishing our quite extensive foray into leaf structure, function, and modifications, we finally start to look at what is arguably the main purpose of leaves: absorbing light energy, and using that energy to fix carbon dioxide. In this lecture we will discuss the discovery of photosynthesis, and the general principle of the process. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

Leaves have features that prevent uncontrolled water loss such as cuticles and wax layers. Because these impermeable layers also prevent diffusion of carbondioxide into the leaf air space plants have developed structures that act like valves and control the flow of water vapour and carbondioxide: stomata. The central opening, called pore or aperture, is surrounded by two guard cells that contain chloroplasts. The guard cells can swell (increase in turgor) and shrink (decrease in turgor), thereby regulating the size of the stomatal aperture. In this lecture we have a look at the principles that determine the pore size, and the mechanics behind it. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

Leaves are the main "organs" of plants responsible for capturing light energy, and converting it through fixation of carbondioxide into chemical energy in the form of sugars. In this lecture we look at modified leaves that serve as adaptations to lack or excess of water, lack of nutrients, and as protective structures to prevent herbivory. We also take a look at the structural oganisation of a "typical" leaf. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

Leaves are the main "organs" of plants responsible for capturing light energy, and converting it through fixation of carbondioxide into chemical energy in the form of sugars. In this lecture we look at modified leaves that serve as adaptations to lack or excess of water, lack of nutrients, and as protective structures to prevent herbivory. We also take a look at the structural oganisation of a "typical" leaf. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

The third and final lecture on plant hormones is about brewing beer and Prince. No, of course not, but both are used as examples for the effects of gibberellins and abscisic acid. Gibberellins were first discovered in "foolish rice" (or bakanae) where a fungal infection results in increased levels of gibberellins and very long, extended internodes. They are a large group of compounds (more than 120 gibberellins are known) and also stimulate seed germination, an effect that is taken advantage of in the malting process. Plants with lower amounts of gibberellins are referred to as "dwarf" mutants and have played an important role in the agricultural revolution. Despite its name, abscisic acid is generally not involved (with a small number of exceptions) in leaf abscission. Instead, it plays a major role in all plant stress-related processes. Most importantly, closure of stomata as a result of waterstress is caused by abscisic acid. Another important role of absisic acid is the delay of seed germination - together with gibberellins this results in a very tight regulation. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

This lecture is about specific effects of two groups of plant hormones: cytokinins, and ethylene. The main effects of cytokinins are stimulation of cell proliferation, and stimulation of growth of shoots from undifferentiated tissue (callus) in plant tissue culture. Together with auxins, cytokinins regulate apical dominance which determines the growth pattern of plants. Ethylene is a gas and as such diffuses very easily and quickly. In climacteric fruits like banana or tomato ethylene accelerates the ripening process, and promotes leaf fall and fruit fall (abscission). We also have a look at a couple of mutations called SNORKEL in rice that cause very rapid and extreme elongation of the stem internodes. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

As we saw in previous lectures, plants are "rooted" to one spot. As a consequence they can not avoid adverse conditions such as drought, heat, cold etc. yet are able to survive seemingly extreme conditions. Almost all aspects of plant development and physiology are influenced and directed by plant hormones. In today's lecture we look at the discovery of plant hormones in general - watch out for the appearance of an unlikely scientist - and look at specific effects of the phytohormone auxin, such as phototropism in shoots and geotropism in the roots, and apical dominance. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

Plants can't do it all by themselves. In order to get the most out of the soil and, indeed, the air, most plants form symbiotic relationships with bacteria and fungi. We take a look at Anabaena, root nodules, and mycorrhizae. Roots are often modified to form "survival structures" containing sugars or proteins. Carrots, cassava, batata and yam are typical examples that are used as food sources by humans. In the last part of this lecture we briefly touch on global and local soil problems like erosion, lack of nutrients, and excess of minerals that increases soil toxicity. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

Roots anchor plants in soil, are responsible for the uptake of water and nutrients, and control which molecules are ultimately taken up by the plant. In this lecture we take a look at the root morphology of monocots and dicots, how lateral roots aad root hairs are formed, and describe different pathways for the transport of water and solutes in the roots. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.

In this lecture we take an introductory look at the role of soil in plant growth. We will briefly go over how soil is formed, what makes a good soil or a bad soil, what nutrients can typically be found, and how nutrient cycles critically involve soil. Copyright 2012 La Trobe University, all rights reserved. Contact for permissions.