Melatonin enhances nitrogen metabolism and haustorium development in hemiparasite Santalum album Linn.

The facultative hemiparasite Santalum album can rely entirely on its own roots to obtain nutrients when growing without a host or can use its haustoria to partially rely on host plants for water and nutrients, especially nitrogen (N). However, how S. album responds to different N conditions has not been determined and no attempt has been made to improve N use by S. album. Our previous studies showed that melatonin, which is known to promote tolerance to abiotic stresses in plants, enhances S. album growth. Here, we investigated the mechanisms and key pathways by which melatonin affects N uptake and assimilation in S. album. We evaluated changes in the physiology, transcriptional profiles, and chromatin accessibility in S. album seedlings exposed to low N (LN) or high N (HN) with or without 1 μM melatonin addition. Without melatonin addition, LN disturbed the homeostasis of N metabolism, reducing both N uptake and assimilation. Melatonin addition, in contrast, significantly increased NH4+ uptake under LN and increased NO3− uptake under HN. Melatonin also increased the activities of NR and GS enzymes under LN, resulting in accelerated N metabolism and enhanced haustorium development via a mechanism associated with auxin accumulation. Using global RNA-seq and ATAC-seq, we found that many genes were differentially expressed in response to melatonin addition. The upregulated genes were enriched in N metabolism and hormone signal transduction (auxin) pathways, suggesting that they have key roles in improving N use by S. album. These results strongly suggest that the application of melatonin may enhance S. album growth via accelerated N metabolism and haustorium formation.