physiology

1. Aquaporins and plant growth

2. Definition
• Aquaporins:
• These are also called water channels, are integral
membrane proteins from a larger family of major int
rinsic proteins that form pores in the membrane of b
iological cells, mainly facilitating transport of water
between cells. .

3. Introduction
• The 2003 Nobel Prize in Chemistry was awarded joi
ntly to Peter Agre for the discovery of aquaporins, a
nd Roderick MacKinnon for his work on the structu
re and mechanism of potassium channels
• Solomon and his co-workers performed pioneering
work on water permeability across the cell membran
e in the late 1950s.

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• In the mid-1960s an alternative hypothesis (the "part
ition–diffusion model") sought to establish that the
water molecules partitioned between the water phas
e and the lipid phase and then diffused through the
membrane, crossing it until the next interphase wher
e they left the lipid and returned to an aqueous phas
e.
• The cell membranes of a variety of different bacteri
a, fungi, animal and plant cells contain aquaporins t
hrough which water can flow more rapidly into and
out of the cell than by diffusing through the phospho
lipid bilayer.

5. Function
• Aquaporins are "the plumbing system for cells". Wa
ter moves through cells in an organized way, most r
apidly in tissues that have aquaporin water channels.
• For many years, scientists assumed that water leake
d through the cell membrane, and some water does.
However, this did not explain how water could mov
e so quickly through some cells

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• Aquaporins selectively conduct water molecule
s in and out of the cell, while preventing the pas
sage of ions and other solutes.
• Also known as water channels, aquaporins are i
ntegral membrane pore proteins. Some of them,
known as aquaglyceroporins, also transport oth
er small uncharged dissolved molecules includi
ng ammonia, CO2, glycerol, and urea.

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• The presence of water channels increases me
mbrane permeability to water. These are also e
ssential for the water transport system in plant
s and tolerance to drought and salt stresses.

8. Structure
• Aquaporin proteins are composed of a bundle
of six transmembrane α-helices. They are em
bedded in the cell membrane.
• The amino and carboxyl ends face the inside o
f the cell. The amino and carboxyl halves rese
mble each other, apparently repeating a patte
rn of nucleotides. Some researchers believe t
hat this was created by the doubling of a form
erly half-sized gene.

10. What about aquaporins in plants and lo
wer organisms?
• Aquaporins are expressed in various plants and micr
obes, including bacteria and yeast.
• A large number of plant aquaporins have been identi
fied, called PIPs (plasma membrane intrinsic protein
s) and TIPs (tonoplast intrinsic proteins) for their cel
lular expression patterns.

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• Deletion studies have elucidated a variety of interest
ing roles for aquaporins in plants,
• including transpiration, metabolism and reproductio
n, which are important in plant adaptation to various
environmental stresses.

12. Plant Growth
• Most plants continue to grow throughout their lives.
Like other multicellular organisms, plants grow thro
ugh a combination of cell growth and cell division.
• Cell growth increases cell size, while cell division (
mitosis) increases the number of cells. As plant cells
grow, they also become specialized into different cel
l types through cellular differentiation. Once cells di
fferentiate, they can no longer divide.

13. Role of Meristem
• Meristem is a type of plant tissue consisting of undiffer
entiated cells that can continue to divide and differentia
te.
• Apical meristems:
• are found at the apex, or tip, of roots and buds, allo
wing roots and stems to grow in length and leaves a
nd flowers to differentiate.
• Roots and stems grow in length because the meriste
m adds tissue “behind” it, constantly propelling itsel
f further into the ground (for roots) or air (for stems)

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• Apical meristems differentiate into the three basic t
ypes of meristem tissue which correspond to the thr
ee types of tissue:
• protoderm produces new epidermis
• ground meristem produces ground tissue
• procambium produces new xylem and phloem.
These three types of meristem are considered prima
ry meristem because they allow growth in length or
height, which is known as primary growth.

15. Secondary meristem
• Secondary meristems allow growth in diameter (se
condary growth) in woody plants. Herbaceous plant
s do not have secondary growth.
• The two types of secondary meristem are both nam
ed
• cambium, meaning “exchange” or “change.”
• Vascular cambium produces secondary xylem (to
ward the center of the stem or root) and phloem (to
ward the outside of the stem or root), adding growth
to the diameter of the plant.
•

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• This process produces wood, and builds the sturdy t
runks of trees. Cork cambium lies between the epi
dermis and the phloem, and replaces the epidermis o
f roots and stems with bark, one layer of which is co
rk.
•