Transcriptomic and Biochemical Analysis Reveal Integrative Pathways Between Carbon and Nitrogen Metabolism in Guzmania monostachia (Bromeliaceae) Under Drought

Most epiphytes are found in low-nutrient environments with an intermittent water supply. To deal with water limitation, many bromeliads perform crassulacean acid metabolism (CAM), such as Guzmania monostachia , which shifts from C 3 to CAM and can recycle CO 2 from the respiration while stomata remain closed during daytime and nighttime (CAM-idling mode). Since the absorbing leaf trichomes can be in contact with organic (urea) and inorganic nutrients (NO 3 − , NH 4 + ) and the urea hydrolysis releases NH 4 + and CO 2 , we hypothesized that urea can integrate the N and C metabolism during periods of severe drought. Under this condition, NH 4 + can be assimilated into amino acids through glutamine synthetase (GS), while the CO 2 can be pre-fixated by phosphoenolpyruvate carboxylase (PEPC). In this context, we evaluated the foliar transcriptome of G. monostachia to compare the relative gene expression of some genes involved with CAM and the N metabolism when bromeliads were submitted to 7days of drought. We also conducted a controlled experiment with an extended water deficit period (21days) in which bromeliads were cultivated in different N sources (urea, NH 4 + , and NO 3 − ). Our transcriptome results demonstrated an increment in the expression of genes related to CAM, particularly those involved in the carboxylation metabolism ( PEPC1 , PPCK , and NAD-MDH ), the movement of malate through vacuolar membrane ( ALMT9 ), and the decarboxylation process ( PEPCK ). Urea stimulated the expression of PEPC1 and ALMT9 , while Urease transcripts increased under water deficit. Under this same condition, GS1 gene expression increased, indicating that the NH 4 + from urea hydrolysis can be assimilated in the cytosol. We suggest that the link between C and N metabolism occurred through the supply of carbon skeleton (2-oxoglutarate, 2-OG) by the cytosolic isocitrate dehydrogenase since the number of NADP - ICDH transcripts was also higher under drought conditions. These findings indicate that while urea hydrolysis provides NH 4 + that can be consumed by glutamine synthetase-cytosolic/glutamate synthase (GS1/GOGAT) cycle, the CO 2 can be used by CAM, maintaining photosynthetic efficiency even when most stomata remain closed 24h (CAM-idling) as in the case of a severe water deficit condition. Thus, we suggest that urea could be used by G. monostachia as a strategy to increase its survival under drought, integrating N and C metabolism.