Translocation in the Phloem

Plant PhysiologyTaiz & Zeiger Chapter 10

Tien-Shin YuInstitute of Plant & Microbial Biology

Plant PhysiologyMarch 31, 2008

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

RootShoot

SeiveSeiveelementelement

SeiveSeiveplateplate

Phloem Phloem Parenchyma Parenchyma

cellcell

CompanionCompanioncellcell

The Compositions of Phloem

Architecture of Sieve Element-Companion Cell (SE-CC) Complex

Protection:1.P-protein2.Callose

Angiosperms CC-SE Complex

Sievepore

Sievepore

SE

SECC

Gymnosperms CC-SE Complex

No P-proteins

Pores appear blocked

P-proteins

PP1: phloem protein that forms filamentsPP2: 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

SE

Stain withAniline blue(callose)

SE

Phloem:

•Nature of conduit•Substance being transported•Mechanism of phloem translocation•Direction: source to sink•An information superhighway

• Sugar, Ions, Hormone• Proteins, Peptides• RNA

Molecules in the PhloemAphid styletCucurbit phloem exudate

Rate: 10 mL/hr Rate: 100 nL/hr

Xylem: water, minerals, andhormones (cytokinin)

Phloem: photosynthate (sugars),ions,hormones (IAA, GA),amino acids, peptides, proteins, and RNA.

Sugars in Phloem Sap

Most abundantin 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 Münch1930

Loading into SE: Apoplastic and Symplastic Pathways

Sucrosetransporter

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

Apex

Phloem

Phloem Unloading

Phloem Unloading from SE

More common

In DevelopingSeed

Type I

Type II

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 translocationis 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

Source Leaf Sink Leaf

GFP Protein Unloading in the 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!!

Graft union

Non-inducedplant

Floral induction Non induction

PCO-GUS PFT-GUS

Thank you