Chapter 10 Translocation in the phloem

* Root: anchor, absorb water and nutrients; absorption

Leave: absorb light and exchange gas; assimilation

efficiently exchange via long-distance transport

* Xylem: transport water and nutrients from roots to aerial portions

Phloem:

mature leaves (sugars) growth and storage portions

redistributes water and various compounds

transmits signaling molecules such as proteins and RNA (06)

The association of Pi translocation and co-expression of miR399s and PHO2 encoded a ubiquitin-conjugating E2 enzyme in vascular tissues suggests their involvement in long-distance signaling. (Apr 29 – May 1, 2008)

* angiosperms: major; gymnosperms: minor

The long-distance transport pathways— xylem and phloem

trefoilSclerenchyma cells

p. 364 cell wall: primary and secondary

The most innermost layer of secondary phloem is functional.

Ash tree

Xylem:

dead at maturity, lack plasma membrane, lignified 2° wall

tracheary elements: tracheids, vessel elements

Phloem:

living cells, nonlignified walls

sieve element: directly involved in translocation

sieve tube element (angiosperms)

sieve cell (gymnosperms): no companion cells

companion cells

parenchyma cells: which store and release food molecules

(fibers and sclereids): protection and strengthening of the tissue

(laticifers): latex-containing cell

Tracheids/ vessel elements

Sieve element

Lack

nuclei

tonoplast

microfilaments

microtubules

Golgi bodies

ribosomes

Retained

mitochondria

plastids

smooth ER

perforation plate

3600

P-protein (slime): rich in phloem

in all dicots and in many monocots, absent in gymnosperms

different forms: tubular, fibrillar, granular, crystalline

along the periphery of the sieve tube element, or evenly distribute in the lumen

P-protein body: a discrete bodies in the cytosol of immature cells

are synthesized in companion cells

two types:

PP1: the phloem filament protein, similar to cysteine proteinase

inhibitor defense against insect PP2: the phloem lectin, serve to anchor PP1 in the sieve element

P-protein (body): The major function is in sealing off damaged sieve elements short-term solution is reversible and controlled by calcium in the legume family, it is not known whether other plants

Callose deposition long-term solution

Callose: a -1,3-glucan, callus

is accumulated between plasma membrane and cell wall

wound callose, mechanical stimulation and Temp. stress

dormancy

disappear when the damage is recovery

Companion cells is associated with sieve tube element and connected by

plamodesmata (branched)

dense cytoplasm and abundant mitochondria

Functions: to supply energy

to transport the photosynthetic products

take over some of the critical metabolic functions, such as protein synthesis

Three types: (a) ordinary companion cell

smooth inner surface, few plasmodesmata

(b) transfer cell

fingerlike wall ingrowth, the scarcity of connections (c) intermediary cell

numerous plasmodesmata connect to surrounding cells

Sugars of mesophyll cells of mature leaf

apoplast symplast

ordinary company cells, intermediary cells

transfer cells

apoplast symplast

Sieve element

The translocation patterns of phloem

¤ is not exclusively either an upward or downward direction

irrespective to gravity

¤ Source: area of supply, an exporting organ

mature leaves, storage root beet (Beta maritima)

Sink: area of metabolism or storage, a receiving organ

nonphotosynthetic organs, root and shoot apices,

young tuber, developing fruits, immature leaves

¤ Not all sources supply all sinks on a plant

certain sources preferentially supply specific sinks

plasticity

The features of source-to-sink pathways

a. proximity

b. development: vegetative or reproductive stage

c. vascular connections: orthostichy

d. modification of translocation pathways: wounding, pruning

dahlia

Epifluorescent microscopic

Anastomosis/ callose (yellow)

Materials translocated in the phloem

Water: the most abundant substance

Carbohydrate

Sucrose (0.3 to 0.9 M)Nitrogen Asx (Asp, Asn) Glx (Glu, Gln)

Sugar-rich sap

RNAs: mRNA, pathogenic RNA, small regulatory RNA

Phloem sap can be collected and analyzed— the stylet of an aphid, a natural syringe

(WebTopic 10.3)