Editors' ChoicePlant Physiology

Surviving the flood

Sci. Signal.  04 Oct 2016:
Vol. 9, Issue 448, pp. ec227
DOI: 10.1126/scisignal.aal1280

Plant roots take up water, which is transported into the shoots for delivery to stems, leaves, and flowers. The water permeability of roots, which is referred to as hydraulic conductivity, decreases under conditions of low oxygen (hypoxia). Understanding the mechanisms by which plants regulate hydraulic conductivity is important for understanding how plants adapt to and survive flooding and drought. By studying the natural variation in Arabidopsis lines derived from wild plants, Shahzad et al. identified, by quantitative trait loci (QTL) mapping, At3g24715 as a gene associated with differences in hydraulic conductivity. The authors renamed this gene HCR1 for Hydraulic Conductivity of Root 1, which encodes a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family, containing a PB1 domain for protein interactions in the N-terminal region and the kinase domain in the C-terminal region. The kinase domain exhibited activity toward a generic substrate (myelin basic protein) in vitro, and a kinase-dead form failed to rescue a hcr1 mutation in the Col-0 background. Analysis of expression using a promoter reporter indicated that HCR1 was preferentially expressed in the root stele, the central part of the root. HCR1-dependent inhibition of root hydraulics occurred in K+-replete, oxygen-deficient conditions, indicating that HCR1 integrated the response to K+ and oxygen availability. The abundance of HCR1 mRNA and the abundance of HCR1 fused to a fluorescent protein increased under hypoxic conditions but only when grown in the presence of K+; HCR1 mRNA abundance decreased in response to K+ deprivation even in the presence of hypoxia. In an assay that measured the effects of low oxygen, waterlogged conditions (only the roots were subjected to the condition) or low oxygen, submerged conditions (the whole plant was subjected to this condition), differences between hcr1 mutant and Col-0 plants confirmed the K+ dependence of the response and suggested that HCR1 limits adaptation to waterlogging but promotes recovery from submersion. Transcriptomic analysis comparing hcr1 mutant and Col-0 plants under K+-replete, hypoxic conditions indicated that HCR1 promoted the anaerobic transcriptional response, and the K+-dependent difference in the induction of some members of the hypoxia-responsive transcription factors of the ERF VII family required HCR1. However, mRNAs encoding the RAP2 family members of the ERF VII family were not affected by hcr1 mutation or oxygen availability. Instead, under K+-replete, hypoxic conditions, the abundance of the RAP2 proteins was less in the hcr1 mutants compared with Col-0. In vitro the kinase domain of HCR1 phosphorylated RAP2.12. Thus, this study identified HCR1 as a posttranslational regulator of adaptive root responses that integrates oxygen and K+ availability to control water transport in roots.

Z. Shahzad, M. Canut, C. Tournaire-Roux, A. Martinière, Y. Boursiac, O. Loudet, C. Maurel, A potassium-dependent oxygen sensing pathway regulates plant root hydraulics. Cell 167, 87–98 (2016). [PubMed]

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