Location: Crops Pathology and Genetics ResearchTitle: Mechanical failure of fine root cortical cells initiates plant hydraulic decline during drought
|Cuneo, Italo - University Of California|
|Knipfer, Thorsten - University Of California|
|Broderson, Craig - Yale University|
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2016
Publication Date: 9/15/2016
Citation: Cuneo, I., Knipfer, T., Broderson, C., McElrone, A.J. 2016. Mechanical failure of fine root cortical cells initiates plant hydraulic decline during drought. Plant Physiology. doi: 10.1104/pp.16.00923.
Interpretive Summary: Fine roots are the gatekeepers of water entry into plants, and must maintain adequate water absorption capacity to match canopy water loss. Under drought, roots are thought to operate like an electrical fuse, designed to break when carrying excessive current, but the exact site and timing of dysfunction remains elusive. We provide evidence from intact plants that drought-induced mechanical failure in fine root cortical cells is the primary driver of water uptake capacity under mild to moderate drought stress. This mechanical damage precedes the formation of air bubble blockages in root xylem or root shrinkage, and coincides with a precipitous drop in fine root permeability. These findings provide the sequence and location of reduced water uptake capacity in roots, and a possible mechanism explaining fine root turnover under stress.
Technical Abstract: Root systems perform the crucial task of absorbing water from the soil to meet the demands of a transpiring canopy. Roots are thought to operate like electrical fuses, which break when carrying an excessive load under conditions of drought stress. Yet the exact site and sequence of this dysfunction in roots remains elusive. Using in vivo X-ray computed microtomography (microCT), we found that drought-induced mechanical failure (i.e. lacunae formation) in fine root cortical cells is the initial and primary driver of reduced fine root hydraulic conductivity (Lpr) under mild to moderate drought stress. Cortical lacunae started forming under very mild drought stress (-0.6 MPa 'stem), coincided with a dramatic reduction in Lpr, and preceded root shrinkage or significant xylem embolism. Only under increased drought stress was embolism formation observed in the root xylem, and it appeared first in the fine roots (50% loss of hydraulic conductivity, P50, reached at -1.8 MPa) and then in older, coarse roots (P50 = -3.5 MPa). These results show that cortical cells in fine roots act to decouple plants from drying soil to preserve the hydraulic integrity of the plant’s vascular system under early stages of drought stress. Cortical lacunae formation led to permanent structural damage of the root cortex and non-recoverable Lpr, pointing to a role in fine root mortality and turnover under drought stress.