Redox homeostasis regulates plasmodesmal communication in Arabidopsis meristems

Abstract

Cell-to-cell communication is crucial for multicellular development, and in plants occurs through specialized channels called plasmodesmata (PD). In our recent manuscript we reported the characterization of a PD trafficking mutant, 'gfp arrested trafficking 1' (gat1), which carries a mutation in the thioredoxin-m3 (TRX-m3) gene. gat1 mutants showed restricted GFP transport from the phloem to the root meristem that appears to result from structural modifications in the PD channel. We found accumulation of reactive oxygen species (ROS) and callose, as well as a reduction in starch granules in the gat1 root meristem. Application of oxidants to wildtype plants and expression of our GFP reporter in the mutant root meristemless 1 (rml1) mimic the gat1 phenotype. Our results suggest that mutations in GAT1 cause ROS accumulation and induce the biosynthesis of callose, which in turn block PD transport. Therefore, we propose a model whereby GAT1/TRX-m3 is a component of a redox-regulated pathway that maintains PD permeability in Arabidopsis meristems.

Figure 1

GFP transport, staining of ROS and callose in wildtype, gat and rml1 mutants. (A–F) Confocal images of root apices expressing cytoplasmic GFP driven by the SUC2 promoter from wildtype (wt) and mutants (gat1, gat2, gat4, gat5 and rml1) seedlings. Note that GFP diffuses out of the phloem (P) into the root meristem in wildtype seedlings but not in the mutants. (G–L) show diamino benzidine staining of ROS in the roots of wildtype and the mutant seedlings. (M–R) show aniline blue staining of callose in the roots of wildtype and the mutant seedlings. Note that the mutants accumulate higher levels of ROS and callose. Scale bar = 20 µm.

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.