The plant hormone auxin acts as a signaling molecule to regulate numerous developmental processes throughout all stages of plant growth. Understanding how auxin regulates various physiological and developmental processes has been a hot topic and an intriguing field. Recent studies have unveiled more molecular details into how diverse auxin responses function in every aspect of plant growth and development. In this review, we systematically summarized and classified the molecular mechanisms of diverse auxin responses, and comprehensively elaborated the characteristics and multilevel regulation mechanisms of the canonical transcriptional auxin response. On this basis, we described the characteristics and differences between different auxin responses. We also presented some auxin response genes that have been genetically modified in plant species and how their changes impact various traits of interest. Finally, we summarized some important aspects and unsolved questions of auxin responses that need to be focused on or addressed in future research. This review will help to gain an overall understanding of and some insights into the diverse molecular mechanisms of auxin responses in plant growth and development that are instrumental in harnessing genetic resources in molecular breeding of extant plant species.

Asparagine synthetase (AS), a key enzyme in plant nitrogen metabolism, plays an important role in plant nitrogen assimilation and distribution. Asparagine (Asn), the product of asparagine synthetase, is one of the main compounds responsible for organic nitrogen transport and storage in plants. In this study, we performed complementation experiments using an Asn-deficient Escherichia coli strain to demonstrate that three putative asparagine synthetase family members in poplar (Populus simonii × P. nigra) function in Asn synthesis. Quantitative real-time PCR revealed that the three members had high expression levels in different tissues of poplar and were regulated by exogenous nitrogen. PnAS1 and PnAS2 were also affected by diurnal rhythm. Long-term dark treatment resulted in a significant increase in PnAS1 and PnAS3 expression levels. Under long-term light conditions, however, PnAS2 expression decreased significantly in the intermediate region of leaves. Exogenous application of ammonium nitrogen, glutamine, and a glutamine synthetase inhibitor revealed that PnAS3 was more sensitive to exogenous glutamine, while PnAS1 and PnAS2 were more susceptible to exogenous ammonium nitrogen. Our results suggest that the various members of the PnAS gene family have distinct roles in different tissues and are regulated in different ways.