Root density sensing allows pro-active modulation of shoot growth to avoid future resource limitation

Plants use environmental cues to determine their optimal root and shoot growth. It is well known to gardeners and horticulturists alike that soil volume - most commonly in the form of pot size - strongly restricts plant growth, but the mechanisms by which this effect occurs remain unclear. Here, we show that shoot growth scales directly with soil volume, independently of the nutritional content of the soil, and that plants can become volume restricted even in the presence of abundant resources. We show that plants can detect their soil volume as early as 3 weeks post germination, and that shoot growth restriction therefore constitutes a pro-active decision by the plant to avoid resource limitation later in the life cycle. Shoot growth restriction is not directly linked to root growth restriction, and does not occur in response to the mechanical detection of the pot walls. Rather, we show that plants detect their soil volume by detecting the density of roots in the proximity of their root system. As such, volume restriction may be intimately connected with the mechanism by which plants sense and respond to the roots of other plants in the rhizosphere. Our work demonstrates the remarkable ability of plants to make pro-active decisions about their growth to ensure they can complete their cycle, and has important implications for agricultural practise regarding both nutrient use efficiency and yield.