Proteomic insight into growth and defense strategies under low ultraviolet-B acclimation in the cyanobacterium Nostoc...

Prioritizing defense over growth often occurs under ultraviolet (UV)-B radiation while several studies showed its growth-promoting effects on photosynthetic organisms, how they overcome the growth-defense trade-off is unclear. This study deciphered the acclimation responses of the cyanobacterium Nostoc sphaeroides to low UV-B radiation (0.08 W m^-2) using quantitative proteomic, physiological and biochemical analyses. We identified 628 significantly altered proteins, among which energy production and conversion related proteins dominated. The UV-B-acclimated cells exhibited a significant increase in the abundance of the phycoerythrin and chlorophyll synthesis related enzymes, along with enhanced linear and cyclic electron transport rates, which further led to a rise in light-induced NADPH generation (27 %) and ATP content (67 %). The enhanced photosynthetic energy supply could fuel both growth and defense in Nostoc sphaeroides. The UV-B-acclimated cells showed enhanced photosynthetic carbon fixation, as evidenced by an increase in extracellular carbonic anhydrase activity (142 %), ribulose-1,5-bisphosphate carboxylase/oxygenase activity (87 %) and the pH compensation point, compared to non-UV-B-acclimated cells. Low UV-B also induced ribosome heterogeneity, as indicated by significant changes in the abundance of core ribosomal proteins, RNA modification related enzymes, and ribosome biogenesis and translation related accessory factors. Additionally, low UV-B activated multiple defense strategies, such as significant upregulation of mycosporine-like amino acid synthesis, RecA-dependent DNA repair pathways and the glutathione redox system. Our findings suggested that growth and defense were balanced by enhancing the photosynthetic energy supply under low UV-B acclimation in the cyanobacterium Nostoc sphaeroides, which provides novel insight into mechanisms for overcoming growth-defense trade-offs.