Nitrogen assimilation and gene regulation of two Kentucky bluegrass cultivars differing in response to nitrate supply

• The low-N tolerant Bluemoon had higher photosynthetic capacity, N metabolic activity, and NUE than the sensitive Balin. • Genes related to carbon metabolism were highly expressed in Bluemoon. • Cytoplasmic PpGS1.3 gene plays a key role in differentiating N responses between Bluemoon and Balin. • R2R3-MYB transcription factors, binding the promoter of PpGS1.3 , enhance the efficiency of the GS/GOGAT cycle. Kentucky bluegrass ( Poa pratensis L.) is one of the most popular cool-season turfgrasses worldwide, but mechanisms of this species in response to low nitrogen (N) still remain unclear. In this study, we characterized two cultivars ‘Bluemoon’ and ‘Balin’ differing in morphological, chromosomal, physiological and molecular attributes and in plant response to N supply. Bluemoon was more tolerant to low N than Balin by exhibiting better turf quality (TQ), and higher photosynthetic ability, activities of N reductases and synthetases, and nitrogen use efficiency (NUE). Gene expression profiling showed the existence of 838 and 10,156 differentially expressed genes (DEGs) in Bluemoon and Balin respectively during low N stress, and these DEGs were highly enriched in ‘Nitrogen metabolism’, ‘Pyruvate metabolism’ and ‘Carbon fixation in photosynthetic’ pathways. Those identified genes related to carbon (C) metabolism were highly expressed in Bluemoon, which could generate more NADPH, hence more N reduction than in Balin. Cytoplasmic PpGS1.3 gene is viewed to likely play a key role in leading to different N responses between Balin and Bluemoon. This gene is detected to have similar physicochemical properties to PpGS1.1 , while sharing a similar tertiary structure of protein to PpGS2. Moreover, R2R3-MYB transcription factors were predicted to bind the promoter of PpGS1.3 to enhance the efficiency of the GS/GOGAT cycle. These results suggest the functioning of crucial molecular regulations for improving the varietal tolerance to low N and the NUE in Bluemoon. The findings may help elucidate the low N tolerance mechanisms in Kentucky bluegrass and therefore facilitate genetic improvement of NUE aiming to promote low-input turfgrass management.