Figure 2.

Model for the role of GAT1/TRX-m3 in the regulation of PD transport in Arabidopsis meristems (based on Schurmann and Buchanan, 2008). Metabolites are transported through the phloem from source to sink tissues and are unloaded symplasmically into companion cells from the sieve elements. Sucrose, the main form in which photoassimilates are transported, is cleaved to UDP-glucose (UDP-Glc), which is thereafter metabolized to glucose 6-phosphate (Glc-6P). TRX reduction is induced by Glc-6P via NADPH-FNR-FTR pathway. Reduced TRX regulates the activity of target proteins (Pox: oxidized protein, Pred: reduced protein) involved in metabolic processes or in ROS detoxification (RED: reductants). The reduction of ROS (generated during metabolism) also takes place in other organelles such as mitochondria and peroxisomes (MIT/PER), which, together with the plastids, maintain cell redox homeostasis. Loss of TRX-m3, or oxidative stress, alters this redox equilibrium and affects metabolic balance, which induces a “defense” response that diverts UDP-Glc to the synthesis of callose by callose synthases (CALS). Alternatively, ROS or other signaling molecules produced in the plastid might modulate the activity of proteins, the synthesis of starting metabolites or the expression of genes that participate in callose metabolism. Callose deposition constricts the PD channel and blocks the unloading of essential molecules into the cell, which adversely affects meristem development.