moving assimilates

overview structure -what cell types are involved? distribution - where found in plants function - how does phloem work?

functionality The phloem in higher plants forms the conduit through which photoassimilates as well as a host of other matter (organic and inorganic) moves. Phloem transport can occur because these materials are transported in a watery fluid. conduit = system of open tubes

the system Phloem forms a highly specialized tissue, that is always found in close proximity to the xylem. Xylem and phloem together, form vascular bundles (stems and leaves) or strands in roots. Four sieve elements, forming a sieve tube.

in stems xylem and phloem forms vascular bundles phloem

in leaves xylem phloem

in roots The xylem and phloem form alternating strands in roots, shown here in Zea mays root. xylem vessel xylem vessel phloem

10 µm Sieve tube companioncells the sieve plate: stops organelles from moving out of specific cells

high rate of metabolism The sieve tube takes up carbohydrate at the source from the companion cell, transports it, and unloads via companion cells and associated parenchyma, in sinks. The companion cells are also responsible for the maintenance of the sieve tubes. Companion cells have a high metabolic rate. This micrograph shows high mitochondrial and ER activity in the companion cell, but not in the adjacent sieve tube S CC

The hyperactive nurse, mitochondrial activity The image to the left shows high mitochondrial and ER activity associated with a companion cell (left) and much less in the two glowing sieve elements. The image was made using a confocal microscope and a ER-mitochondrion selective dye. Protein and membrane maintenance of the enucleate sieve elements is carried out by the companion cells

how does phloem work? sugar enters sieve tubes, water follows by osmosis – HIGH TURGOR region of HIGH TURGORPRESSURE SINK SOURCE sugar enters sieve tubes, water follows by osmosis LOW TURGOR PRESSURE Sugar flows to regions of LOW TURGOR PRESSURE leaf bud

production collection transport & export SE pm pvm sm BS CC assimilate movement out of a source (leaf) requires three stages:- production, collection, and transport

integration within the whole plant system

sinks -- above and below groundbudsflowers young leaves roots storage organs NB... the SOURCE is a photosynthetically-active organ

redirection Assimilates can be redirected on demand as a result of changes in metabolic requirements, and the flow (which is pressure- driven) changes. Insects can cause massive long-term redirection of assimilate flow. Aphids are a good example.

stealing assimilate large aphid colonies such as this one, can redirect assimilate flow, by forming a significant local sink. The actively growing parts of the plant are thus deprived of energy needed for growth. Aphids penetrate between and through cells, in search of the phloem. The stylet tips are open in this successful feeder

summary: phloem transport is pressure-driven Loading transport unloading