Shoring up the base: the development and regulation of cortical sclerenchyma in grass nodal roots

Abstract

Plants depend on the combined action of a shoot-root-soil system to maintain their anchorage to the soil. Mechanical failure of any component of this system results in lodging, a permanent and irreversible inability to maintain vertical orientation. Models of anchorage in grass crops identify the compressive strength of roots near the soil surface as the key determinant of resistance to lodging. Indeed, studies of disparate grasses report a ring of thickened sclerenchyma cells surrounding the root cortex, present only at the base of nodal roots. Here, in the investigation of the development and regulation of this agronomically important trait, we used the model species Brachypodium distachyon and show that development of these cells is uncoupled from the maturation of other secondary cell wall-fortified cells and that cortical sclerenchyma wall thickening is stimulated by mechanical forces transduced from the shoot to the root. We also show that exogenous application of gibberellic acid stimulates thickening of lignified cell types in the root, including cortical sclerenchyma, but is not sufficient to establish sclerenchyma identity in cortex cells. Leveraging the ability to manipulate cortex development via mechanical stimulus, we show that cortical sclerenchyma development alters root mechanical properties and improves resistance to lodging. We describe transcriptome changes associated with cortical sclerenchyma development under both ambient and mechanically stimulated conditions and identify SECONDARY WALL NAC7 as a putative regulator of mechanically responsive cortex cell wall development at the root base. Overall, our findings show that grasses use a coordinated system involving mechanoperception, phytohormone signaling, and transcriptional regulation to modulate investment in cortical sclerenchyma, proactively reinforcing anchorage in response to mechanical challenges.