Phytochrome: regulation of Skotomorphogenesis Seed germination Genes, enzymes and signal transduction Embryo and Seed development Photomorphogenesis Photoreceptors Phytochrome Cells and cell growth Phytochrome: regulation of light responses Photosynthesis: light reaction Photosynthesis: carbon fixation Photorespiration Nitrogen fixation Plant nutrition Respiration Phloem translocation Sucrose, starch, cellulose

Carbohydrates: where are they coming from, where are they going???

Phloem Translocation, and Photosynthate Allocation and Partitioning HORT 301 – Plant Physiology October 29, 2010 Taiz and Zeiger, Chapter 10 paul.m.hasegawa.1@purdue.edu Phloem translocation - bidirectional transport of photosynthate, assimilated nutrients, signaling molecules and redistribution of water and molecules Sources to sinks

Phloem loading and unloading Photosynthate allocation and partitioning Lecture topics: Phloem translocation Phloem loading and unloading Photosynthate allocation and partitioning Photosynthate moves in the phloem Taiz & Zeiger (2010) Web Topic 10.1.A

Phloem tissue is adjacent to the xylem Proximity of the xylem and phloem is critical for translocation Stem (perennial) Leaf

Phloem - sieve (tube) elements and companion cells Sieve elements – protoplasmic w/modified plastids and ER Companion cells – sieve element metabolic functioning, phloem loading and unloading

Sieve plates – pores predominantly at the ends of sieve elements Sieve elements are interconnected cells Callose plugs seal sieve pores

Companion cells facilitate phloem loading and unloading Interconnected with sieve elements Ordinary companion cell Transfer cell OCC and TC – minimal interconnection w/adjacent cells – apoplastic to symplastic uptake IC – interconnection to adjacent cells, symplastic uptake- Intermediary cell

Source to sink translocation Source – photosynthate production (availability) is greater than metabolic need, export Sink – requires photosynthate, import

Change in photosynthate distribution in sink leaves by removal of sources Taiz & Zeiger (2010) Web Topic 10.1.C

Phloem sap sampling for analysis Taiz & Zeiger (2010) Web Topic 10.3.A

Sugars are a primary component of sap

Sugars are translocated in non-reducing form – sucrose is the predominant photosynthate

Pressure flow model for phloem translocation from source to sink Source – phloem loading increases ψp Sink – phloem unloading decreases ψp Ψp gradient between source and sink drives solution movement Ψw gradient between sieve elements and xylem vessel elements recirculates the solution

Phloem translocation requires minimal energy – passive transport

Phloem loading via symplastic or apoplastic pathways Symplastic loading is passive transport Apoplastic loading is active transport

Sucrose-H+ symporter involved in phloem loading via an apoplastic pathway – active transport Symporter is driven by the H+ gradient generated by the plasma membrane H+-ATPase

Polymer trapping model for symplastic phloem loading – passive uptake Sucrose diffuses from the bundle sheath/mesophyll cells into intermediary cells Sucrose is synthesized into raffinose, concentration decreases in intermediary cells More sucrose enters the intermediary cells Raffinose cannot move to bundle sheath/mesophyll cells because plasmodesmata are too small

Phloem unloading by symplastic and apoplastic pathways into sinks Developing leaves, roots, tubers and reproductive organs Unloading, short-distance transport and sink loading

Leaf transition from sink to source 14C import (dark) from a source leaf

Sucrose-H+ symporter gene expression in source and sink leaves AtSUC2 promoter::GUS reporter fusion – phloem loading

Photosynthate allocation and partitioning Allocation – regulation of carbon assimilate distribution into various metabolic pathways or sink cells Partitioning – distribution of photosynthates, assimilation products, ions, hormones, signaling molecules (proteins and perhaps mRNAs)

Allocation is regulated by sinks and sources Partitioning of triose-phosphates Sink strength