PROJECT1
Root Growth Regulation in Response to Environmental Factors
As sessile organisms, plants adapt to fluctuating environments by flexibly altering the shape and growth of their organs. Unlike animals, plants lack specialized organs for perception or movement and do not have a central nervous system to coordinate these processes. Thus, in order to regulate their growth in response to environmental factors, individual plant cells must perceive environmental signals and adjust their division and elongation in a coordinated manner. How this sophisticated mechanism operates remains largely a mystery.
Using the tip of Arabidopsis roots as a model, we aim to elucidate the genetic and cellular dynamics of organ growth in response to environmental factors. Arabidopsis roots are ideal for microscopic observation and have been used to study plant organ development at various scales, from genes and molecules to cells and organs. However, a major challenge has been the inability to perform long-term observations at high magnification because the root tip—the center of environmental perception and growth control—moves rapidly as the root extends.
Our laboratory has developed a motion-tracking confocal microscope system that automatically tracks the moving root tip. This system allows us to capture gene expression, as well as cell and organelle dynamics at the tip of growing roots. Furthermore, by integrating microdevices that apply precisely controlled environmental stimuli and AI-based image quantification techniques, we are developing technologies that quantitatively analyze the molecular and cellular behaviors that governs root growth dynamics. Our goal is to use these technologies to elucidate how chemical and physical environmental factors, such as nutrients and mechanical contact, are perceived and how these factors modulate cell division and elongation to optimize root growth.
The root cap protects the root tip from physical damage and plays essential roles in plant growth. It senses the direction of gravity, reduces friction, and facilitates interactions between the root and the soil environment. Since the root cap is exposed to various stresses at the tip of the growing root, it has a mechanism that constantly renews its constituent cells in order to maintain its function. The innermost layer of the root cap consists of a single layer of stem cells that divide at regular intervals to generate new cell layers. At the same interval, the outermost cells shed autonomously, thereby maintaining the size and function of the tissue.
During the process of cellular turnover, the function of individual root cap cells changes depending on their position within the tissue. Cells in the inner layers differentiate into gravity-sensing cells, and those in the outer layers transform into secretory cells. Cells that detach from the outermost layer disperse into the soil, along with their accumulated metabolites, to mediate the interaction between roots and the soil environment. We have demonstrated that autophagy, an intracellular digestive process conserved in eukaryotes, plays a crucial role in this sophisticated process of cellular differentiation and transition. This discovery elucidates the long-sought role of autophagy in plant development.
We also utilize SMB/BRN transcription factors, the master regulators of root cap cell differentiation, to identify genes responsible for root cap differentiation and function. By integrating live-imaging techniques with molecular genetics, we aim to elucidate the roles of these genes. For instance, we have demonstrated that the RCPG, coding for a cell wall-degrading enzyme, is periodically activated in the outermost root cap layer. Mutants of this gene are defective in root cap cell shedding. These findings represent a significant achievement in clearly visualizing cellular dynamics at the tip of a growing root and elucidating their cellular and molecular roles.