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 The most innermost layer of secondary phloem is functional. Sclerenchyma cells trefoil Ash tree p. 364 cell wall: primary and secondary

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

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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

(a) ordinary companion cell smooth inner surface, few plasmodesmata 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 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 Epifluorescent microscopic dahlia Anastomosis/ callose (yellow)

Materials translocated in the phloem Water: the most abundant substance Sugar-rich sap Carbohydrate Sucrose (0.3 to 0.9 M) Nitrogen Asx (Asp, Asn) Glx (Glu, Gln)

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)