Biology of adventitious root formation

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Biology of adventitious root
formation on cuttings
• The biology of what actually triggers
adventitious root formation is largely
unknown…
• But here’s what we do know…
Adventitious root formation
• Roots
• Arise from tissue other
than existing roots (e.g.
stems, leaves)
• Absolutely necessary
for cutting propagation
to work
Two types of adventitious roots
Preformed (latent)
• Root primordia are preformed but lie dormant
• Emerge in response to
environmental conditions
• Easy to root species
–
–
–
–
Salix (Willow)
Hydrangea
Populus (Poplar)
Ribes (Currant)
Wound induced
• Develop only after the
cutting is taken
• In response to wounding
• De novo = “anew”
• Direct – cells in close
proximity to the vascular
system (easy-to-root taxa)
• Indirect – from callus
(difficult-to-root taxa)
Emerging preformed root initials of
Hedera helix
Preformed adventitious roots on corn
Preformed adventitious roots on
Kalanchoe panamensis
Aerial (preformed) roots on Ficus
Two types of adventitious roots
Preformed (latent)
• Root primordia are preformed but lie dormant
• Emerge in response to
environmental conditions
• Easy to root species
–
–
–
–
Salix (Willow)
Hydrangea
Populus (Poplar)
Ribes (Currant)
Wound induced
• Develop only after the
cutting is taken
• In response to wounding
• De novo = “anew”
• Direct – cells in close
proximity to the vascular
system (easy-to-root taxa)
• Indirect – from callus
(difficult-to-root taxa)
Response to wounding
1. Outer cells die and form a necrotic plate,
wound is sealed with suberin, xylem plugs
2. Living cells behind the plate begin to divide
and form callus (parenchyma cells)
3. Some cells near the vascular cambium and
phloem begin to divide and initiate new
adventitious roots (where exactly is still
unclear!)
Callus
Response to wounding
1. Outer cells die and form a necrotic plate,
wound is sealed with suberin, xylem plugs
2. Living cells behind the plate begin to divide
and form callus (parenchyma cells)
3. Some cells near the vascular cambium and
phloem begin to divide and initiate new
adventitious roots (where exactly is still
unclear!)
Direct formation of wound induced roots
1. Dedifferentiation of parenchyma cells
2. Formation of root initials from cells near
vascular tissue (meristematic)
3. Formation of root primordia (more
organized)
4. Growth and emergence of the new root,
connect to existing vascular tissue
Root initials (slightly organized)
Root
initial
Direct formation of wound induced roots
1. Dedifferentiation of parenchyma cells
2. Formation of root initials from cells near
vascular tissue (meristematic)
3. Formation of root primordia (more
organized)
4. Growth and emergence of the new root,
connect to existing vascular tissue
Root primordia (more organized)
Root cap
Meristematic
cells
Direct formation of wound induced roots
1. Dedifferentiation of parenchyma cells
2. Formation of root initials from cells near
vascular tissue (meristematic)
3. Formation of root primordia (more
organized)
4. Growth and emergence of the new root,
connect to existing vascular tissue
Growth and emergence of new root
Root initial
Indirect formation of wound induced roots
• Callus formation (non-directed cell division;
parenchyma)
• Cell differentiation
• Root initials
• Root primordia
• New roots and vascular connections
An important difference
Leaf and root cuttings
• Have to generate
adventitious roots and
adventitious shoots
Stem and leaf bud cuttings
• Have to generate
adventitious roots only
Adventitious shoot formation
• Arise from any
plant part other
than terminal,
lateral or latent
buds on stems
• Kalanchoe
panamensis
Rubber plant
• Adventitious shoot formation is the limiting
factor!
References
• Principles of Propagation by Cuttings, Dr. Fred
Davies, Department of Horticultural Sciences,
Texas A&M University, College Station, Texas
• Hartman, H.T. and D.E. Kester. 1997. Plant
propagation: principles and practices. Sixth
ed. Prentice Hall, N.Y.
A word about auxin
• Auxins will only speed up and enhance rooting
of cuttings.
• Difficult to root species or cuttings taken from
physiologically mature stock plants may not
respond to auxin.
Proximal and distal
Distal
Proximal
Cone of juvenility
• Adult =
reproductive
Chronologically
“newer” but
physiologically
“older”
Juvenile =
vegetative
Chronologically
“older” but
Physiologically
“young”
Hardwood Cuttings
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