Evolution of petal patterning: blooming floral diversity at the microscale

The flowers of angiosperms are extraordinarily diverse. While most floral variation is visible to the naked eye, this diversity goes beyond the macroscale: Floral organs comprise an underappreciated range of cell types that generate a multitude of patterns across their surfaces and give rise to novel structures. Because diverse cell patterns provide adaptations to biotic and abiotic factors, they also contribute to angiosperm evolution and speciation. Yet, how such diversity originates remains to be understood. In this review, we focus on petals, which together form the corolla, to examine the mechanisms patterning floral surfaces at the cellular level. We summarize current research aiming to understand how cell fate specification and controlled cell growth (proliferation and expansion) are achieved with high spatial resolution during petal development. We also examine the adaptive potential for such patterns and how they contribute to plant fitness and diversification. Finally, we discuss promising directions for future research on the evolution of petal patterning at the microscale and identify outstanding questions that technological advances now make it possible to address.