Post-Sieve Element Transport of Sucrose in Developing Seeds

Abstract

Developing seeds of cereals and grain legumes have proven to be useful experimental models to examine post-sieve element assimilate transport in sink tissues. Morphologically, these seeds offer well-defined sinks in which the processes of sucrose import plus efflux and influx plus metabolism may be examined independently. In all cases, sucrose is delivered through the phloem to the maternal seed tissues. Unloading from the sieve element-companion cell complexes is symplastic. Subsequently, sucrose moves through a symplastic route to cells responsible for sucrose efflux to the seed apoplast. The efflux cells are located at, or near, the maternal/filial interface. Sucrose is retrieved from the seed apoplast by the outermost cell layers of the filial tissues. Subsequent transfer of sucrose to the sites of storage in the filial tissues is confined principally to a symplastic route. Sucrose efflux from the maternal tissues appears to be passive in cereals and energy dependent in grain legumes, possibly through a sucrose/proton antiport system. Sucrose influx across the plasma membranes of the filial cells is energy dependent and, for grain legumes, is energy coupled through a sucrose/proton symporter. Studies on the control of post-sieve element transport of sucrose have focused largely on the membrane transport steps. The role of phytohormones as modulators of sucrose transport is uncertain in grain legumes, efflux from the maternal cells could be regulated by rates of sucrose utilisation in the filial tissues through a turgor homeostat mechanism located in the efflux cells.

High Affinity Amino Acid Transporters Specifically Expressed in Xylem Parenchyma and Developing Seeds of Arabidopsis

“…Expression of AAP8 was detected in the young seed and funinculi. From the sieve elements of the seed coat, amino acids and sucrose are presumably unloaded symplasmically up to the interface between the maternal seed coat and the filial tissues (50). The symplasmic discontinuity between maternal and filial tissues in seeds necessitates membrane efflux from maternal tissues and subsequent uptake by filial tissues such as endosperm and embryo.…”

The control of carbon acquisition by roots

“…With the benefit of hindsight, it is possible to use existing literature and commonplace observations to support the idea of shared control of C flux. First, the transport system probably has a role in control, as the length of the transport pathway affects flux to sinks (Canny, 1973 ;Cook & Evans, 1978 ;Minchin et al, 1993) and short-distance transport within sinks may present further constraints (Bret-Harte & Silk, 1994 ;Patrick & Offler, 1995). Models of phloem transport also show clearly that there is the potential for control by transport itself (Minchin et al, 1993).…”

“…We have no idea what the mechanism might be. Patrick & Offler (1995) have pointed out that it cannot be the low carbohydrate status of bulk tissue which directly stimulates import, as the concentrations of sugars in roots are too low to alter appreciably the turgor of phloem within the tissue (and Table 3 shows that barley roots growing fast due to elevated atmospheric CO # have high sugar contents). It might be a combination of turgor in expanding cells Pritchard, 1998) and gating of the plasmodesmata which connect them to the phloem (A. Schultz, pers.…”

A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

“…Importantly, sucrose creates the driving force for long-distance translocation of all other compounds in the phloem. Sucrose is imported into the developing embryo by plasma membrane SUT sucrose/proton cotransporters (Patrick and Offler, 1995;Baud et al, 2005;Zhang et al, 2007).…”