Self-DNA early exposure in cultivated and weedy Setaria triggers ROS degradation signaling pathways and root growth inhibition

Abstract

The accumulation of fragmented extracellular DNA in decomposing plant litter reduces conspecific seed germination and plantlet growth in a concentration-dependent manner. An inverse relationship between DNA inhibition magnitude and taxonomic distance between the DNA source and the target species has been reported. This phenomenon bears important implications in plant ecology, as the accumulation or removal of DNA in litter can play a fundamental role in determining biodiversity patterns in different ecosystems and could represent a further explanatory process underlying negative plant–soil feedback. In this context, self-DNA has been suggested to act as a stress signaling molecule, possibly as damage-associated molecular pattern (DAMP), triggering plant resistance and growth inhibition in response to environmental stressors (e.g., herbivory, pathogens attack, intraspecific competition) causing death and subsequent release of conspecific DNA in the soil. The underlying mechanisms at cellular and molecular levels are not yet fully clarified, but previous studies reported the induction, in plants, of early innate immune response, ROS production, MAPK activation, extra-floral nectar production, increased intracellular [Ca2+] and plasma membrane depolarization. Moreover, a recent whole-plant transcriptome profiling and confocal microscopy analyses in Arabidopsis thaliana suggested that cells are capable of discriminating self from non-self DNA by specific sensing and highlighted an association between self-DNA exposure and abiotic stress gene response. In the present work we investigated, for the first time, the species-specificity of self-DNA inhibition in cultivated vs. weed congeneric species (respectively, Setaria italica and S. pumila) and carried out a targeted real-time qPCR analysis, under the hypothesis that self-DNA elicits molecular pathways responsive to abiotic stressors. The results of a first cross-factorial experiment on root elongation of plantlets exposed to self-DNA, congeneric DNA and heterospecific DNA from Brassica napus and Salmon salar questioned the species-specificity of the inhibitory effect, possibly related to the confounding effect of contaminants in the treatment solutions. A repeated test after ultra-purification of the treatment solutions confirmed a significantly higher inhibition by self-DNA as compared to non-self treatments, the latter showing a magnitude of the effect consistent with the phylogenetic distance between the DNA source and the target species. Targeted gene expression analysis highlighted an early activation of genes involved in ROS degradation and management (FSD2, ALDH22A1, CSD3, MAPK17) and deactivation of scaffolding molecules acting as negative regulators of stress signaling pathways (WD40-155). While being the first exploration of early response to self-DNA inhibition at molecular level on C4 model plants, our study highlights the need for further investigation of the relationships between DNA exposure and stress signaling pathways, discussing potential applications for species-specific sustainable weed control in agriculture.