Phloem

Background: Sieve elements of the phloem constitute the long-distance pathway for assimilates in the plant. They form a symplasmic, low-resistance domain due to their reduced cytoplasm and wide sieve pores.

 

Phloem transport is driven by the loading of assimilates in the source leaves. Prerequisites for the function of phloem elements are a "sugar-tight" plasma membrane, tightly controlled plasmodesmata, linking to surrounding tissues and presence and activity membrane transporters.

 

(Topic 1) We are about to pin-point the pre-phloem pathway of sugars comparatively for plant species that are known to load sugars with different strategies. The pre-phloem pathway connects the mesophyll with the phloem in source leaves.

Enigma: The mechanism of phloem transport in gymnosperms is an enigma. Neither phloem loading, nor the pathway matches that of angiosperm plants.

 

(Topic 2) We try to characterize the decisive steps in the pre-phloem pathway, and to identify how gymnosperms accumulate sugars in their leaves (=needles).

 

(Topic 3) Furthermore, we aim at understanding the function of the ER in gymnosperm phloem: As seen in the micrographs below, endomembrane aggregates are found on sieve areas in vivo. They do not seem to be compatible with a low-resistance pathway as the phloem of angiosperms is forming!  

 

left:  Living phloem visualised with confocal laser scanning microscopy,
right: Same material,but fixed for electron microscopy. From schulz 1992