AtSUC2 has a role for sucrose retrieval along the phloem pathway: evidence from carbon-11 tracer studies.

The location of the phloem within a plant, and its vulnerability to disruption, make it a difficult tissue to study and therefore non-invasive studies of phloem functionality are important. Here we compare, phloem transport, measured non-invasively, in wild type Arabidopsis thaliana, and transposon-insertion mutants for AtSUC1 or AtSUC2, giving in vivo information on the importance of these sucrose transporters for phloem transport. The suc2 mutant showed an increase in both phloem leakage and transport time, consistent with reduced sucrose uptake into both transport and collection phloem. The results are consistent with the AtSUC2 transporter being important for retrieval of leaked sucrose in the transport phloem of Arabidopsis. There was no difference in phloem transport properties between the wild type and the suc1 mutants, implying that the AtSUC1 transporter does not play a significant role within the transport phloem of Arabidopsis under the conditions of our study.

Source-sink coupling in young barley plants and control of phloem loading.

Phloem loading of carbohydrate within a mature exporting leaf of a barley seedling is shown to respond quickly to a change in the temperature of the root and the shoot meristem. This is interpreted as a close coupling between source supply and sink demand for carbohydrate, through the hydrostatic pressure gradient linking source and sink generated by the solute concentration within the sieve tubes. This interpretation was tested by using anoxia to alter solute concentration within the sieve tubes of one region of a leaf while observing phloem loading in an adjacent region. Responses to anoxia could not be explained by the above model, suggesting that either this model is incorrect or other signalling pathways are involved. There is evidence in the literature for coarse control of phloem loading but no evidence was found of fine control by solute content of the loaded sieve elements.

Identification of a new glucosinolate-rich cell type in Arabidopsis flower stalk.

Distribution of K, Ca, Cl, S, and P in freeze-dried sections of Arabidopsis flower stalk was analyzed by energy dispersive x-ray imaging. Concentrations of these elements in different cell types were quantified by microanalysis of single-cell samples and phloem exudates. Results showed a differential pattern of distribution for all five elements. K concentration was found to be highest in the parenchymatous tissue around vascular bundles. Ca and Cl were present mainly in the central part of the flower stalk. P was largely located in the bundles and in the parenchyma surrounding them. S signal was extraordinary high in groups of cells (S-cells) situated between the phloem of every vascular bundle and the endodermis. Enzymatic hydrolysis by thioglucosidase of cell sap collected from S-cells using a glass microcapillary resulted in the release of glucose, indicating that these cells contain glucosinolates at high (> 100 mM) concentration, which is consistent with the concentration of S (> 200 mM) estimated by x-ray analysis of cell sap samples. Since their position outside of the phloem is ideally suited for protecting the long-distance transport system from feeding insects, the possible roles of these cells as components of a plant defense system are discussed.

What determines carbon partitioning between competing sinks?

Carbohydrate sinks have been described by their ability to attract photosynthate, denoted by sink strength, and by their priority rank ordering for supply in the presence of a reduced availability of photosynthate. Sink strength has been defined as the rate of carbohydrate flow into a sink, but this flow rate is also dependent upon supply, other sinks, and resistance to flow of the transport pathway, so it is not a property of the sink alone. It is a property of the entire system. Hence sink strength defined as a flow rate is not a valid descriptor of a sink. However, a simple model of phloem flow based upon Münch's ideas and with saturable unloading has many properties similar to a plant's carbohydrate source-sink relations, including priorities of sinks, and leads to a set of sink descriptors. This model's ability to mimic observed source-sink relations is reviewed here.