Activation of MAPK signaling in response to nitrogen deficiency in Ulva prolifera (Chlorophyta)

Ulva prolifera (Ulvophyceae, Chlorophyta), one of the main seaweeds species that cause “green tides”, suffers changes in nitrogen levels during drifting. However, the mechanisms of how this alga resist to nitrogen deficiency remains unclear. Transcripts encoding nitrate reductase, nitrite reductase, and glutamine synthase involved in nitrogen assimilation were amplified by RT-PCR, suggesting that their levels were increased in response to nitrogen deficiency. Additionally, Fluorescent Brightener 28 and Evans Blue staining demonstrated that nitrogen deficiency had no influence on the cell-wall regeneration ratio but increased the apoptosis rates of protoplasts. Furthermore, western blot assays indicate that nitrogen limitation triggered the activation of mitogen-activated protein kinase (MAPK) signaling. An inhibitor of MAPK signaling enhanced the effects of nitrogen deficiency on the mortality and decrease the capacity for cell-wall regeneration of protoplasts. All these results provided evidence that MAPK signaling had functional roles in helping U. prolifera acclimate fluctuations in nitrogen availability within a short time.