Tien-Shin Yu
Institute of Plant & Microbial Biology
Plant Physiology
March 31, 2008
Translocation in the Phloem
Plant Physiology
Taiz & Zeiger Chapter 10
Phloem:
•Nature of conduit
•Substance being transported
•Mechanism of phloem translocation
•Direction: source to sink
•An information superhighway
Plant Vasculature: Xylem and Phloem
Arrangement of Plant Vascular Tissue
Phloem
Xylem
Shoot
Root
The Compositions of Phloem
Phloem
Parenchyma
cell
Companion
cell
Seive
element
Seive
plate
Architecture of Sieve Element-Companion Cell (SE-CC) Complex
Protection:
1.P-protein
2.Callose
Angiosperms CC-SE Complex
Sieve
pore
SE
Sieve
pore
CC
SE
Gymnosperms CC-SE Complex
No P-proteins
Pores appear
blocked
P-proteins
PP1: phloem protein that forms filaments
PP2: phloem lectin associated with filaments
Types of Companion Cells: Apoplastic Loading
Types of Companion Cells: Symplastic Loading
Types of Companion Cells: Symplastic Loading
Connection of CC-SE by Specialized Plasmodesmata
Network of the phloem
Stain with
Aniline blue
(callose)
SE
SE
Phloem:
•Nature of conduit
•Substance being transported
•Mechanism of phloem translocation
•Direction: source to sink
•An information superhighway
Molecules in the Phloem
Cucurbit phloem exudate
Rate: 10 mL/hr
Aphid stylet
Rate: 100 nL/hr
• Sugar, Ions, Hormone
• Proteins, Peptides
• RNA
Xylem: water, minerals, and
hormones (cytokinin)
Phloem: photosynthate (sugars),ions,
hormones (IAA, GA),
amino acids, peptides, proteins,
and RNA.
Sugars in Phloem Sap
Most abundant
in the phloem
Nitrogenous Compounds in Phloem Sap
Protein Composition in Phloem Sap
mRNA and small RNA in Phloem Sap
At least 2,500 mRNA have been identified from phloem sap
Phloem:
•Nature of conduit
•Substance being transported
•Mechanism of phloem translocation
•Direction: source to sink
•An information superhighway
Bulk Flow Hypothesis
Flow rate:
1 m/hr
Ernst Münch
1930
Loading into SE: Apoplastic and Symplastic Pathways
Sucrose
transporter
Apoplasmic
Step Model
Symplasmic – Polymer-trap Model
BS-IM PD size
exclusion limit
of 0.8 kDa
IC-SE PD size
exclusion limit
of 25 kDa
Phloem Unloading
Apex
Phloem
Phloem Unloading from SE
More common
Type I
Type II
In Developing
Seed
Phloem:
•Nature of conduit
•Substance being transported
•Mechanism of phloem translocation
•Direction: source to sink
•An information superhighway
Direction & rate of
phloem translocation
is controlled by local
Sink Strength
Phloem translocation
is a Non-Circulatory
vascular system
Phloem Translocation Regulates by Proximity
Leaf Blade Sink-Source Transition from Tip to Base
Major Vein: Unloading; Minor Vein: Loading
Allocation and Partition
Allocation: The regulation
of Sugar distributes into
various metabolic pathway
Partitioning: Differential
distribution of sugar within
the plant
Phloem:
•Nature of conduit
•Substance being transported
•Mechanism of phloem translocation
•Direction: source to sink
•An information superhighway
Signal Molecules in the Phloem
•Hormones: Auxin, GA
•Chemical: JA in pathogen defense
•Sugars
•Protein
•mRNA
•etc
Protein Composition in Phloem Sap
GFP Protein Unloading in the sink leaf
Source Leaf
Sink Leaf
mRNA and small RNA in Phloem Sap
At least 2,000 different RNA identified from phloem sap
Florigen is a Phloem Mobile Macromolecule!!
Non induction
Floral induction
Graft union
Non-induced
plant
PCO-GUS
PFT-GUS
Thank you