Root-shoot communications play important roles in plant stress responses. Here, we examined the roles of root-sourced signals in the shoot response to heat in cucumber plants. Cucumber plants grafted onto their own roots and luffa roots were exposed to aerial and root-zone heat to examine their tolerance by assessing the levels of oxidative stress, PSII photoinhibition, accumulation of abscisic acid (ABA), H2 O2 and heat shock protein (HSP) 70 using immunoblotting, chlorophyll fluorescence, immunoassay, CeCl3 staining and Western blotting, respectively. Grafting onto the luffa rootstock enhanced the shoot tolerance to the heat. This enhanced tolerance was associated with increased accumulation of ABA and apoplastic H2 O2 , RBOH transcripts and HSP70 expression and a decrease in oxidative stress in the shoots. The increases in the ABA and H2 O2 concentrations in the shoots were attributed to an increase in ABA transport from roots and an increase in ABA biosynthesis in the shoots when the root-zone and shoots were heat stressed, respectively. Inhibition of H2 O2 accumulation compromised luffa rootstock-induced HSP70 expression and heat tolerance. These results suggest that, under heat stress, ABA triggers the expression of HSP70 in an apoplastic H2 O2 -dependent manner, implicating the role of an ABA-dependent H2 O2 -driven mechanism in a systemic response involving root-shoot communication.

HighlightHigh CO2 concentrations can counteract the negative impact of salt stress in an apoplastic H2O2-dependent manner by regulating stomatal movement and Na+ delivery from the xylem to leaf cells.

Phosphorus is an essential element for plant growth often limiting agroecosystems. To identify genetic determinants of performance under variable phosphate supply, we conducted genome-wide-association studies on five highly predictive phosphate starvation response traits in 200 Arabidopsis (Arabidopsis thaliana) accessions. Phosphate concentration in phosphate-limited organs had the strongest, and primary root length had the weakest genetic component. Of 70 trait-associated candidate genes, 17 responded to phosphate withdrawal. The PHOSPHATE TRANSPORTER1 gene cluster on chromosome 5 comprised PHT1;1, PHT1;2 and PHT1;3 with known impact on phosphorus status. A second locus featured uncharacterized endomembrane-associated auxin efflux carrier encoding PIN-LIKES7 (PILS7) which was more strongly suppressed in phosphate-limited roots of phosphate-starvation sensitive accessions. In the Col-0 background, phosphate uptake and organ growth were impaired in both phosphate-limited pht1;1 and two pils7 T-DNA insertion mutants, while phosphate-limited pht1;2 had higher biomass and pht1;3 was indistinguishable from wild type. Copy number variation at the PHT1 locus with loss of the PHT1;3 gene and smaller scale deletions in PHT1;1 and PHT1;2 predicted to alter both protein structure and function suggest diversification of PHT1 is a key driver for adaptation to phosphorus limitation. Haplogroup analysis revealed a phosphorylation site in the protein encoded by the PILS7 allele from stress-sensitive accessions as well as additional auxin-responsive elements in the promoter of the ‘stress tolerant’ allele. The former allele’s inability to complement the pils7-1 mutant in the Col-0 background implies the presence of a kinase signaling loop controlling PILS7 activity in accessions from phosphorus-rich environments, while survival in phosphorus-poor environments requires fine-tuning of stress-responsive root auxin signaling.

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