Natural variation in adventitious root formation

Abstract

Adventitious roots are plant roots formed from any nonroot tissue and produced during normal development. To highlight the potential of exploiting adventitious roots for crop improvement, the previous studies have precisely defined and described the different adventitious root types and their physiological responses in three stress conditions that constitute three case studies of adventitious root physiology, focusing on responses to flooding, nutrient deficiency, and wounding. Understanding the functional similarities and differences shared by these adventitious roots is crucial for maximizing efficient and resilient crop production. Adventitious root (AR) formation is a complex developmental process controlled by a plethora of endogenous and environmental factors. Based on fossil evidence and genomic phylogeny, AR formation might be considered the default state of plant roots, which likely evolved several times independently. The application of next-generation sequencing techniques and bioinformatics analyses to nonmodel plants provides novel approaches to identify genes putatively involved in AR formation in multiple species. Recent results uncovered that the regulation of shoot-borne AR formation in monocots is an adaptive response to nutrient and water deficiency that enhances topsoil foraging and improves plant performance. A hierarchy of transcription factors required for AR initiation has been identified from genetic studies, and recent results highlighted the critical involvement of additional regulation through microRNAs. Here, in this chapter, we try to discuss the current understanding of AR formation in response to specific environmental stresses, such as nutrient deficiency, drought or waterlogging, physiology, and genetics of adventitious root formation. The natural variation of adventitious root formation aims to provide evidence for the integration of the hormone cross talk required for the activation of competent root cells within adult tissues from which the ARs develop.