Phloembeladung bei krautigen Pflanzen

2001

S. 8-23

26 Lit. Ang.

Unselbständiges Werk

200084547

Sink organs depend on their supply of phtoassimilates produced by source organs being transported via the phloem. Sucrose is the major transported form of fixed carbon, and its concentration is about 1 M in the phloem sap of apoplastic phloem loaders. Generally two mechanism of phloem loading are discussed: firstly the apoplastic path, in which sucrose is exported to the apoplast and is then taken up into the sieve tube-companion cell complex by an energy dependent carrier mediated transport process and secondly the symplastic path via plasmodesmata, connecting mesophyll, bundle sheath, and phloem cells. A large family of sucrose carries (SUT) from a wide variety of species were isolated and characterized. Phloem loading occurs into the companion cells or into the sieve tubes. Amino acids are the transported form of nitrogen. The concentration of amino acids in phloem sap is high (50-500 mM), but the concentration is similar to the concentration in the cytosol of mesophyll cells. Carrier mediated transport systems are also active in phloem loading of amino acids of apoplastic phloem loaders. The symplastic phloem loading process is still not fully understood. Symplastic phloem loaders translocate mainly oligosaccharides (raffinose, stachyose, verbascose) and one theory ("polymerization trap hypothesis") describes that the higher molecular weight of these oligosaccharides compared to sucrose is important for symplatisc phloem loading.