HPCA1 Is Involved in Long-Distance Electrical Signalling and Transcriptional Responses in Arabidopsis thaliana Under Chronic Low-Dose β-Irradiation.

Low-dose chronic ionising radiation can alter plant physiology and gene expression, although specific signalling pathways involved in long-term responses to this stressor remain largely unknown. Considering the ability of ionising radiation to promote the accumulation of reactive oxygen species and to induce redox shifts, we investigated the role of the HO-sensitive receptor kinase HPCA1 in Arabidopsis thaliana in stress responses under chronic β-irradiation. Wild-type (Col-0) and hpca1 knockout plants were irradiated for 7 weeks, and physiological and transcriptional changes were assessed. Irradiation promoted petiole elongation in wild-type plants and had no morphological effect in hpca1 mutants. Photosynthesis parameters remained unchanged except for decreased non-photochemical quenching in irradiated hpca1. By contrast, both lines showed increased transpiration under irradiation, with hpca1 plants displaying further elevation. Irradiation of wild-type plants caused higher amplitude and propagation velocity of electrical signals (variation potential, VP). In hpca1, irradiation failed to boost VP amplitude or velocity further. The ability of hpca1 mutants to generate VP-triggered photosynthetic responses after local heat stress exposure was hampered; however, irradiation restored the ability of hpca1 plants to form such reactions. Transcriptomic profiling revealed that genes related to calcium, ethylene, and auxin signalling pathways were differentially regulated in irradiated wild-type plants. At the same time, irradiated hpca1 plants had minimal transcriptional overlap with irradiated wild-type plants, and no consistent transcriptional response to chronic irradiation was formed. Together, these findings underscore the role of HPCA1 kinase and apoplastic HO reception in plant transcriptional and physiological adaptation to chronic low-dose radiation exposure.

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