Auxin positively regulates nitrogen remobilization in cucumber leaves

Nitrogen (N) remobilization is an important N metabolic process in plants. The objective of this study was to increase our understanding of the relationship between low N tolerance and N remobilization in cucumber. Two cucumber cultivars, namely D0328 and D0422, were included in the analyses, the former of which displays a stronger capacity for low N tolerance. Source leaf transcriptomes were analyzed, which revealed differentially expressed genes in a number of interesting pathways, including amino acid biosynthesis, tyrosine and tryptophan biosynthesis, plant hormone signal transduction, and E3 ligase activity, implying that these pathways are involved in low N tolerance in cucumber. In agreement with transcriptome data, old D0328 leaves had significantly higher tryptophan and indole-3-acetic acid contents than old D0422 leaves under low N conditions. N remobilization efficiency in old leaves was increased by naphthaleneacetic acid treatment, whereas it was repressed by antiauxins alpha-(p-chlorophenoxy) isobutyric acid treatment. Taken together, the results of this study demonstrate that auxin promotes N remobilization in cucumber leaves during the reproductive stage of plant development and this positive regulation is associated with low N tolerance in cucumber.