Meristic nature of primary plant growth
In other words, plants grow by addition of segments to the end of a
branch or root.
leaf primordium
procambium
node 1
node 2
axillary bud
node 3
encircling leaf base
node 4
Fosket, p. 469
Esau, p. 284
Patterns of gene expression correspond to zonation patterns in the SAM
Fig 1.
Fig. 2.
Bowman and Eshed (2000) TIPS 5: 110
Shoot apical meristem structure
Tobacco meristem
Fosket, p. 461
Pea meristem
Lyndon, p. 25
Cytological zonation within the SAM
Sunflower:
histological staining
Steeves & Sussex, p. 55, 56
The peripheral zone shows active cell division while the central zone is relatively
quiescent.
Sunflower:
Incorporation of 3H Thymidine
indicates sites of active DNA
synthesis (cell division).
Steeves & Sussex, p. 64
KNOX genes promote SAM identity.
Overexpression of a KNOX
gene induces ectopic SAM
formation on leaves.
35S:KNAT1 induces lobed
leaves with ectopic SAMs in
the sinuses of Arabidopsis
leaves.
normal
Fig 3.
Section through a normal leaf (D) and a
35S:KNAT1 ectopic meristem (F).
Chuck et al., 1996, Plant Cell 8: 1277
35S:KNAT1
Fig 5.
SEMs of a vegetative (A), and an inflorescence
(F) SAM that formed on the surfaces of
35S:KNAT1 leaves.
CLAVATA signaling pathway regulates SAM activity
CLV3
Loss of function clavata mutants show
excessive cell proliferation in the SAM. The
inset shows a normal arabidopsis SAM at the
same magnification as the mutant. Therefore,
CLV inhibits stem cell proliferation.
Courtesy of Steve Clark
wus loss of function
mutants are depleted of
SAM cells showing that
WUS promotes
proliferation. CLV3
overexpression causes a
loss of SAM phenotype
similar to wus loss of
function mutants.
Overexpression of
CLV3 results in loss of
WUS expression
showing that CLV3 is
a negative regulator of
WUS.
CLV1
Localization of CLV1 and CLV3 transcripts in the
arabidopsis SAM. CLV3 is a soluble signal ligand and
CLV1 is a receptor kinase. Thus signaling from the tunica
to the corpus regulates the relative rate of cell
proliferation.
Fletcher, 1999 Science 283: 1911
WT
35S::CLV3
wuschel mutant
WUS
in situs
WT
clv3 mutant
35S::CLV3
But in a wus loss of function mutant, there is no CLV3 expression. Thus, the corpus signals back to the
tunica and a feedback loop regulates the relative rates of proliferation and differentiation..
CLV3
Brand et al. (2000) Science 289, 617-9.
CLV1
WUS
proliferation
Phyllotaxy--the pattern of organ initiation
Removing a leaf primordium alters the position
of subsequent organ initiation suggesting
inhibitory fields
Fosket, p. 472
Steeves + Sussex, p. 115
Localized auxin induces organ initiation
NPA is an auxin transport inhibitor.
NPA treated
normal
NPA treated
Then auxin
paste applied
locally
induces leaf
initiation
NPA
normal
LeT6 (a SAM marker) in situ
Reinhardt et al (2000) Plant Cell 12, 507-18.
Organ initiation may be influenced by biophysical properties of the SAM surface.
Patterns of cellulose reinforcement in cell walls of the SAM
correlate with patterns of leaf initiation
Lyndon, p. 74, 75
SAM promoting gene expression
must be repressed to allow
differentiation of initiating organs.
Expansin gene expression is upregulated at the site of
organ initiation. Expansin is a protein that loosens cell
walls, and therefore may create localized regions of
surface weakness.
Reinhardt, et al (1998) Plant Cell 10, 1427-37.
Expression of the KNOX gene KNOTTED1 in
a maize SAM. Leaf primordia lack expression,
indicating they have acquired a new identity
(other than SAM).
Jackson, 1994