Bio 236: Plant Biology

Lab Two - Why Is The lazy-2 Tomato Mutant Lazy?



Introduction


Tropisms are one type of plant movement that lead to a change in a plant's morphology. Tropisms are growth responses that result in curvatures of whole plant organs toward (positive) or away (negative) from environmental stimuli, such as light (phototropism), gravity (geotropism or gravitropism) and touch (thigmatropism). Differential rates of cell elongation on opposing sides of an organ, such as a stem or root, lead to the curvature. Phototropism and gravitropism function to differentially adjust the position of the plant's major organ systems so that, regardless of how the seed is orientated in the soil, the shoot grows up into the sunlight and the roots grow down into the soil for water and mineral absorption.

To date, we lack a full mechanistic understanding of plant growth responses to light and gravity. However, there is strong evidence that both light and gravity induce the asymmetric redistribution of auxin, a growth regulator that modulates cell elongation, leading to differential growth and curvature. Perception of the environmental stimuli may involve various light receptors and statoliths (starch granules). Differential sensitivies of the root and shoot account for their opposite responses to increased auxin.

Investigations into the regulation of plant tropic responses has been approached from many directions, using descriptive, experimental, biochemical and genetic techniques. Mutant analysis has been a particularly powerful approach towards identifying relevant genes that not only cause developmental abnormalities but also the genetic basis of normal ontogeny. For example, the role of auxin was implicated by the study of the tomato (Lycopersicon esculentum Mill.) diageotropica mutant. Hicks, Rayle, and Lomax (1989) found that this tomato mutant lacked certain auxin binding sites.

In this lab study you will investigate the growth of wildtype and mutant tomato seedling stems in response to gravity and various light frequencies. You will be working with the lazy-2 tomato mutant which also has a defective gravitropic response and its wild type cultivar, Ailsa Craig to address the hypotheses and questions raised in the purpose section above.

References Cited

Gaiser, JC, TL Lomax (1993) The altered gravitropic response of the lazy-2 mutant of tomato is phytochrome regulated. Plant Physiol. 102:339-344.
Hicks, GR, DL Rayle, TL Lomax (1989) The diageotropica mutant of tomato lacks high specific activity auxin binding sites. Science 245:52-54.

Purpose




Investigation


We will have mature lazy-2 and wild-type plants for you to look at in lab. You will also be provided with imbibed and dry mutant and wild-type seed. Your task is to design and carry out experiments to demonstrate the role of light and auxin in the gravitropic response in tomato. Use the Gaiser and Lomax paper as a guide for the phytochrome studies. You can test for the role of auxin using endogenous appications of auxin and/or the auxin transport inhibitor, 2,3,5- triiodobenzoic acid (TIBA). Light chambers with different wavelengths of light available. It will be easiest to grow your seedlings in petri plates sealed with parafilm as described in the Gaiser and Lomax paper.

Order of Events

  1. You will be given some background information on phytochrome, fluence, growth regulators (including phytohormones), etc.
  2. Next we will work on the Gaiser and Lomax paper. Be sure to come with your questions.
  3. In groups of 2 or 3 you will design experiments in consultation with me. Use the Experimental Design Worksheet, which you can find in Fabio, Bio 236, Misc folder. I will show you how to use the light chambers, set up seeds, and scan images of your hypocotyls into the computer so that you will have a permanent record of your results. You may find it easier to measure angles with NIH image software than with a protractor. If you would like to stain for starch granules (statoliths), there will be KI2 stain available along with slides, microscopes, etc.
  4. Set up your experiments and determine whether you will need to stop by lab to check on things during the week.
  5. Next week in lab you will collect your data and share it with your classmates. We will discuss collectively and individually how you can best present your data graphically.
  6. Each of you will prepare a laboratory report with an introduction and discussion supported with information from the primary literature. This will be due the week after you complete the lab (Thurs., January 30).

    If you would like more general background for starters, read the Scientific American article by Evans, Moore, and Hasenstein (How roots respond to gravity) that will be available in lab. Also, check out pp. 547-575 in your text.

    Return to Bio 236 Home Page.