Discovery of E3 Ubiquitin Ligases That Alter Responses to Nitrogen Deficiency Using Rice Full-Length cDNA OvereXpressor (FOX)-Hunting System

Nitrogen is the most critical nutrient for plant growth. To find potential strategies for enhancing both nitrogen use and tolerance to nitrogen deficiency in rice plants, we used the rice Full-length-cDNA OvereXpressor (FOX)-hunting system, a high-throughput phenotyping screen. After screening 3229 rice FOX lines, we identified 82 FOX-hunting lines that responded differently to nitrogen starvation. Among them, 11 FOX-hunting lines overexpressed putative E3 ligases, of which 6 were RING-type and 5 were F-box type E3 ligases. Of these, two lines overexpressed the same F-box type E3 ligase, OsFBL15. In vitro ubiquitination assay confirmed the auto-ubiquitination activity of OsFBL15. The overexpression of these E3 ligases altered the rice response to nitrogen deficiency and suggests a way to develop rice that is tolerant to nitrogen-deficient field conditions.