Regulation of plasmodesmata in leaves of Arabidopsis: ATP, NADPH and chlorophyll b levels matter.

In mature leaves, cell-to-cell transport via plasmodesmata (PD) between mesophyll cells links assimilate production in photosynthesis with their export to sink organs. This study addresses the question how signals derived from chloroplasts and photosynthesis influence PD permeability. Cell-to-cell transport was analyzed in leaves of Arabidopsis chlorophyll b-less ch1-3 mutant, the same mutant complemented with a cyanobacterial CAO gene (PhCAO) over-accumulating chlorophyll b, the trxm3 mutant lacking plastidial thioredoxin m3, and the ntrc mutant lacking functional NADPH : thioredoxin reductase C. The regulation of PD permeability in these lines could not be traced back to the reduction state of the thioredoxin system or types and levels of reactive oxygen species produced in chloroplasts; however, it could be related to chloroplast ATP and NADPH production. The results suggest that light enables PD closure via an increase in ATP and NADPH levels produced in photosynthesis, providing a control mechanism for assimilate export based on the rate of photosynthate production in the Calvin-Benson cycle. The level of chlorophyll b influences PD permeability via not yet identified signals. The data also suggest a role of thioredoxin m3 in the regulation of cyclic electron flow around photosystem I.