Salt-Induced Stomatal Closure Suppresses Parasitism by Phtheirospermum japonicum.

In nature, parasitic plants may grow in marginal and stressful environments. However, the effects of concurrent abiotic stress, such as salinity, on parasitism remain underexplored, particularly in facultative root hemiparasites. Here, we examined the effect of salt on the relationship between the parasite Phtheirospermum japonicum and its host, Arabidopsis thaliana. Split-root systems allowing separate application of salt treatment to hosts and parasites were used, enabling disambiguation of effects and the quantification of host-to-parasite salt transfer and nutrient abstraction rates. Host attachment increased parasite susceptibility to salt stress, due to a combination of enhanced salt uptake by the parasite and significant salt transfer from salt-treated hosts. Salt treatment suppressed parasitic growth benefits, seemingly through a decrease in nutrient transfer rates. Follow-up studies suggested that nutrient abstraction from the host was strongly tied to parasite stomatal conductance, which was attenuated by salt-induced stomatal closure. Summarily, we propose that parasitism-induced stomatal dysregulation led to salt accumulation within parasite leaves, which exacerbates salt effects, compromising its ability to abstract nutrients from its host. Our findings highlight the importance of local growing conditions and the presence of concurrent abiotic stressors in mediating the outcomes of the host-hemiparasite relationship.