Transcript No Slide Title

AGR2451 Lecture 13
•No reading this week
•Reminder that all overheads are on Reserve and all text (incl. Lectures
1 + 2) are online
Review of last lecture
Nitrogen - Key Points:
Slide 13.1
Lecture 13 - "Introduction to Environmental Stress: Water"
1. Plants are immobile.
2. What environmental signals does a plant need to integrate? demo
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.931
ASPP, Rockville MD, 2000
3. In agriculture, the losses due to abiotic stresses are enormous as
much as 50-80% compared to maximum yields, due to drought, high
soil salt, flooding, low and high temperature
4. Plants can adapt to their environment using post-embryonic
development from meristems:
How do plants respond to:
too little light?
too much light?
too little water or organic Nitrogen?
Slide 13.2
How do plants respond to:
too much water (flooding). Why is this a problem?
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1180
ASPP, Rockville MD, 2000
5A. Water use
How do land plants/agriculture lose water?
1.
2.
•In the United States, 80% of all consumptive (non-reusible) water
goes to crop irrigation. Water will likely be the most limiting crop
nutrient in this century and the cause of famines and wars unless
solutions are found.
5B. Effect of Crops utilize water at different efficiencies = "water
use efficiency".
Plant
Kg water used per Kg Dry Matter
Alfalfa
850
Soybeans
650
Plants, Genes and Agriculture,p.190
Oats, potatoes
580
M. Chrispeels and D.Sadava
Jones and Bartlett Publishers, Boston, 1994
Wheat
550
Corn
350
Sorghum
300
Why do corn and sorghum use water >2-fold more efficiently than
Slide 13.3
alfalfa or soybeans??
5C. How does soil type influence the amount of water available to a
plant and why?
Water-holding capacities of soils:
Soil Type
Relative Water Availability
Course sand
1.25
Very fine sand
3.00
Silt loam
5.25
Plants, Genes and Agriculture,p.198
M. Chrispeels and D.Sadava
Silty clay
6.50
Jones and Bartlett Publishers, Boston, 1994
Clay
7.00
5D. Since clay holds water the best, why aren't soils that are 100% clay
the best for plants?
6. Three very important plant stresses are drought, freezing and high soil
salt. Salt is left behind by solutes dissolved in irrigation water and left
behind by evaporation and as part of applied fertilizers.
How does each of these stresses cause problems for plants?
a) drought?
b) freezing? demo with sugar
c) soil salinity? demo on board
Remarkably, therefore, each of these apparently separate plant stresses
create the same end-effect for a plant cell cytoplasm --- this is
cytoplasmic dehydration.
Slide 13.4
Effects of Water Stress
A high water content is crucial to a cell. When a plant cell becomes
dehydrated (water content down to ~23%) or even worse, dessicated
(below 23%), proteins and intracellular lipid membranes and the plasma
membrane become severely damaged. Why?
a) Proteins?
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.437
ASPP, Rockville MD, 2000
b) Membranes? demo with oil and water
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.4
ASPP, Rockville MD, 2000
Therefore, water is crucial to permitting macromolecular structures
such as membranes to form, proteins to fold correctly or protein-protein
interactions to occur, because all of these rely on the hydrophobic effect
and/or correct surface interactions with the polar charge on water
molecules.
Slide 13.5
.
Adaptations to Drought, Freezing and Salinity
•Problem -too high of a solute concentration being formed outside of
the cell (in the apoplast or soil). draw on board
•Causes water to flow from a compartment of low solutes (inside the
cell) to a compartment of a high solutes (outside the cell) until the water
potential is balanced.
•Consequence - concentration of charged solutes inside the cell (Na+,
K+, etc.) increases and these can interact with the surface of proteins
and damage their folding.
•The cytoplasm sap becomes viscous due to an increased concentration
of proteins and organic molecules, causing deleterious interactions.
Plants respond ("tolerate") to these problems in two ways:
a) Produce organic molecules such as sugars, the amino acid proline,
and sugars with hydroxyl OH groups (alcohols).
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1165
ASPP, Rockville MD, 2000
•The key feature shared by these molecules is that they are neutrally
charged, but they are polar molecules that form hydrogen bonds.
Why is this important?
Slide 13.6
•Sugars or proline can satisfy the hydrogen bonding requirement of
polar groups on the surface of a protein and thus take the place of water
(they form a a "replacement water" layer on the surface of proteins)
•These molecules are osmolytes. They increase the solute concentration
of the cytoplasm but do not interfere in normal metabolism (unlike Na+
or K+ solutes).
•"Osmotic adjustment" is therefore a strategy used by plant cells to
protect proteins and membranes. How do they do this??
Protein
folding
is protected
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1166
ASPP, Rockville MD, 2000
Slide 13.7
Sugars and prolines protect lipid bilayer membranes from damage
during cytoplasmic dehydration
F.Hoekstra, E.Golovina, and J. Buitink (2001) Mechanisms of plant dessication tolerance.
Trends in Plant Sci. 6, 431-438. Elsevier Science Publishers, London,UK
Slide 13.8
Second adaptation to cytoplasmic dehydration
b) Since the cytoplasm still becomes dessicated, the concentration of
unwanted charged ions (K+, Na+) might still be disruptive. Therefore,
plant cells actively pump these ions into the vacuole or out of the cell.
•Vacuoles can constitute 90% of the volume of a plant cell and can be
used as the "garbage can" of the cell to store toxic compounds or
solutes. How?
Some plants that have adapted to high salinity use active membrane
transporters in root cells to transport salts into their cytoplasms. Why?
Images from
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1164 and 1166
ASPP, Rockville MD, 2000
Slide 13.9
8. The molecular mechanisms of drought, salt and freezing tolerance.
Sketch = old handout
i) By an unknown mechanism, low temperatures causes an influx of
Ca++ from extracellular stores.
ii) Ca++ inhibits phosphatase, which leads to phosphorylation of one or
more protein secondary messengers.
iii) Within 15 minutes of cold induction, these processes switch on the
transcription of genes encoding transcription factors called CBFs. CBF
transcription factors bind to a short promoter sequence (CCGAC) of
suites of genes.
Biochemistry and Molecular Biology of Plants
W.Gruissem, B. Buchanan and R.Jones p.1177
ASPP, Rockville MD, 2000
iv) Genes which have promoters with the CCGAC sequence are
transcriptionally activated.
What do these genes and their protein products do?
a) These proteins protect intracellular and plasma membrane proteins
from freezing damage (stabilize lipid structures).
b) Encode enzymes that shunt carbon/nitrogen reserves to produce
proline and sugars that prevent cell dehydration during freezing.
Therefore, in advance, if the cells produce extra intracellular solutes,
then water will not flow out and thus prevent dehydration.
***Therefore, if a plant receives a prior cold stimulus (few days),
because they produce these protectants they become
"cold-acclimated" to a future freezing stress. Cold-acclimation is
extremely important to crops grown in Canada.
Slide 13.10
v) Evidence: In the eskimo1 plant mutant of Arabidopsis, plants have
increased freezing tolerance without cold acclimation: these plants have
a 30-fold higher level of proline, and a 2-fold higher level of total
soluble sugars than nonacclimated wild-type plants.
Gilmour et al. (2000) Overexpression of the Arabidopsis CBF3 transcriptional
activator mimics multiple biochemical changes associated with cold acclimation.
Plant Physiology 124, 1854-1865. ASPP Publishing, Rockville, MD.
vi) Evidence: Arabidopsis and Canola plants that overexpress the CBF
transcription factors are more freezing, drought and salt tolerant.
Jaglo-Ottosen et al. (1998) Arabidopsis CBF1 overexpression induces
COR genes and enhances freezing tolerance. Science 280, 105-107.
AAAS Press, Washington, DC
Slide 13.11
vii) In dehydration stress, different parts of the plant must communicate.
For example, the stomates must close. The hormone ABA (abscissic
acid) is known to be involved in dehydration cell-to-cell signalling
(cold, drought, salt-stress), including closing stomates.
9. This mechanism of osmotic adjustment and protectants are used
under severe conditions as a normal part of the life cycle of plants and
during reproduction:
Explain how each of the following is related to cytoplasmic dehydration:
Perennial fruit or Maple trees during winter?
Sorghum under drought conditions?
Pollen?
Seeds?
Pollen grains and seeds are both dessicated (down to 13% water content
in seeds). Pollen germinates a pollen tube upon the addition of water by
stigmatic cells (upon correct pollen-stigma recognition).
Thus, common mechanisms between stress tolerance and evolution
of pollen and seeds to be dormant:
i) during gamete production to allow for free airborne and insect-borne
pollen to develop (from flowers)
ii) after grain-fill to permit the embryo to survive harsh conditions in a
dormant (dehydrated) state, that is, to form a seed.
10. Indirect ways to save water:
-preventing pathogens and pests (>50%) loss of plants
-preventing post-harvest losses (fruit ripening, poor storage) (< 30%)
11.Ultimate problem with water is that land plants cannot use salt water.
i) make crop plants grow with saltwater or even in the seas
ii) create sea/ocean algae which produce seeds and fruits (these were
adaptations to land reproduction)
iii) develop a cheap technology to desalinate saltwater
Slide 13.12