The new insight of auxin functions: transition from seed dormancy to germination and floral opening in plants

In angiosperms, plants go through different transitions to complete a life cycle. These different transition stages are crucial for plant development. External and internal stimuli affect these transitions, including environmental condition and phytohormones. Auxin is one of the classical phytohormones and crucial to plant morphogenesis establishment and various physiological processes, like phototropism, cell differentiation, cell expansion, floral opening, organ abscission and seed germination. Abscisic acid (ABA) and gibberellic acid (GA) are widely known to work antagonistically in seed germination. Apart from GA and ABA, recent studies have demonstrated that auxin is also involved in the transition from seed dormancy to germination and floral opening. Auxin promotes seed dormancy and inhibits seed germination. Moreover, functions of auxin during floral opening is promising for complementing hormonal crosstalk during this process. In this review, we focus on auxin–ABA crosstalk in regulation of seed germination and dormancy mainly in model plant Arabidopsis thaliana and summarize the knowledge about auxin-mediated flowering in several different species. The systematical knowledge of hormonal signaling provides a guidance for uncovering largely unknown functions of auxin.