Physiological and transcriptome analysis reveal the nitrogen preference and regulatory pathways of nitrogen metabolism in an epiphytic orchid, Cymbidium tracyanum

Epiphytic orchids acquire nutrients elements, including nitrogen, from the atmosphere. However, the nitrogen form preferred by epiphytic orchids Cymbidium tracyanum and adaptive mechanisms that underlie nitrogen metabolism remain unclear. In this study, we determined which nitrogen form the C. tracyanum prefers, and identified adaptive strategies and molecular pathways activated during nitrogen metabolism. For this purpose, we measured the physiological and morphological responses to different ratios of nitrate and ammonium (NO3−/NH4+ = 1:0, 3:1, 1:1, 1:3, 0:1) and used transcriptome analysis to identify genes differentially expressed in response to these nitrogen forms. Physiological and morphological measures of plant growth (i.e., net photosynthetic rate, biomass accumulation, leaf area, root elongation and branching) were highest in plants treated with nitrate. Furthermore, C. tracyanum plants treated with nitrate had a higher root/mass ratio and a lower leaf/mass ratio. Transcriptome analysis identified that several key genes (NRT1.1–2, NRT1.4–4, NRT1.7–2, and CLC-G-2) highly expressed in plants treated with pure nitrate. These genes may be involved in the absorption and transportation of nitrate, and in auxin transportation from leaf to root in C. tracyanum. Our findings suggest that the nitrogen form preferred by C. tracyanum is nitrate and that nitrate alters both root and leaf traits via several molecular pathways, including the plant hormone signal transduction pathway and MAPK signaling pathway. We speculate that the preference of C. tracyanum for nitrate as a nitrogen form may be a result of long-term adaptation to epiphytic habitat via the regulation of leaf and root traits.