Wheat plants sense substrate volume and root density to proactively modulate shoot growth

Abstract

Plants must carefully coordinate their growth and development with respect to prevailing environmental conditions. To do this, plants can use a range of nutritional and non-nutritional information that allows them to proactively modulate their growth to avoid resource limitations. As is well-known to gardeners and horticulturists alike, substrate volume strongly influences plant growth, and maybe a key source of non-nutritional information for plants. However, the mechanisms by which these substrate volume effects occur remain unclear. Here, we show that wheat plants proactively modulate their shoot growth with respect to substrate volume, independent of nutrient availability. We show that these effects occur in two phases; in the first phase, the dilution of a mobile 'substrate volume-sensing signal' (SVS) allows plants to match their shoot (but not root) growth to the total size of the substrate, irrespective of how much of this they can occupy with their roots. In the second phase, the dilution of a less mobile 'root density-sensing signal' (RDS) allows plants to match root growth to actual rooting volume, with corresponding effects on shoot growth. We show that the effects of soil volume and plant density are largely interchangeable and that plants may use both SVS and RDS to detect their neighbours and to integrate growth responses to both volume and the presence of neighbours. Our work demonstrates the remarkable ability of plants to make proactive decisions about their growth, and has implications for mitigating the effects of dense sowing of crops in agricultural practice.

Root Foraging Precision: Do Experimental Conditions Matter?

“…The SVS are used to detect the substrate volume so that plant adapts its future growth accordingly. The third type of substances are ´root density-sensing signals´ (RDS) (Semchenko et al 2007;Wheeldon et al 2020). The RDS affect the plant in the later stage and suppress its growth accordingly to the density of roots.…”

“…The RDS affect the plant in the later stage and suppress its growth accordingly to the density of roots. The RDS affect the root system size and thus indirectly also the shoot growth, while the SVS are thought to affect only shoot growth (Wheeldon et al 2020; but see Semchenko et al 2007).…”

“…But, the RDS are not very mobile throughout the substrate (Wheeldon et al 2020), thus their removal by trickling water was not very likely. There are several possible explanations why we did not detect any RDS effect upon root foraging of the examined plants.…”

“…First, roots produce allelopathic exudates in the vicinity of obstacles (i.e., a pot wall; Falik et al (2005) ), causing root growth suppression. Second, in the early stage of plant growth, plants produce ´substrate volume-sensing signals´ (SVS) (Wheeldon et al 2020). The SVS are used to detect the substrate volume so that plant adapts its future growth accordingly.…”

“…While the experimenter may wish to use the minimal pot size in which the plant can t, pot size affects its biomass, growth, and morphology (Semchenko et al 2007;Poorter et al 2012;Fuellner et al 2012;Wheeldon et al 2020) as well as its effectiveness of root nutrient uptake (Kume et al 2006). Roots reaching the wall and bottom of the pot start to follow their contours, forming a pattern, which may distort root foraging-precision assessment.…”