Although radiation-induced bystander effects (RIBE) in Arabidopsis thaliana have been well demonstrated in vivo, their underlying mechanisms, particularly with regard to the participating signaling molecules and signaling pathways, still remain unclear.

A study team led by Prof. BIAN Po in Institute of Technical Biology & Agriculture Engineering, Hefei Institutes of Physical Science (CASHIPS) of the Chinese Academy of Sciences, revealed molecular mechanisms underlying RIBE in plants.

In the study, they found that pretreatment of seedlings with Salicylhydroxamic acid, an inhibitor of lipoxigenase (LOX) in JA biosynthesis, significantly suppressed RIBE-mediated expression of the AtRAD54 gene.

After root irradiation, the aerial parts of A. thaliana mutants deficient in JA biosynthesis (aos) and signaling cascades (jar1-1) showed suppressed induction of the AtRAD54 and AtRAD51 genes and TSI and 180-bp repeats, which have been extensively used as endpoints of bystander genetic and epigenetic effects in plants.

These results suggest an involvement of the JA signal pathway in the RIBE of plants. Using the root micro-grafting technique, the JA signal pathway was shown to participate in both the generation of bystander signals in irradiated root cells and radiation responses in the bystander aerial parts of plants.

The over-accumulation of endogenous JA in mutant fatty acid oxygenation up-regulated 2 (fou2), in which mutation of the Two Pore Channel 1 (TPC1) gene up-regulates expression of the LOX and allene oxide synthase (AOS) genes, inhibited RIBE-mediated expression of the AtRAD54 gene, but up-regulated expression of the AtKU70 and AtLIG4 genes in the non-homologous end joining (NHEJ) pathway.

Considering that NHEJ is employed by plants with increased DNA damage, the switch from HR to NHEJ suggests that over-accumulation of endogenous JA might enhance the radio-sensitivity of plants in terms of RIBE.

Their work entitled "A Pivotal Role of the Jasmonic Acid Signal Pathway in Mediating Radiation-induced Bystander Effects in Arabidopsis thaliana" was published in Mutation Research/ Fundamental and molecular mechanisms of mutagenesis. 

Those work were supported by the grants from National Natural Science Foundation of China and Anhui province.

A schematic model for the role of the JA signal pathway in mediating RIBE in plants. (Image by WANG Ting)