Roots of the Resurrection Plant Tripogon loliiformis Survive Desiccation Without the Activation of Autophagy Pathways by Maintaining Energy Reserves

Abstract

Being sessile, plants must regulate energy balance, potentially via source-sink relations, to compromise growth with survival in stressful conditions. Crops are sensitive, possibly because they allocate their energy resources toward growth and yield rather than stress tolerance. In contrast, resurrection plants tightly regulate sugar metabolism and use a series of physiological adaptations to suppress cell death in their vegetative tissue to regain full metabolic capacity from a desiccated state within 72 h of watering. Previously, we showed that shoots of the resurrection plant Tripogon loliiformis, initiate autophagy upon dehydration as one strategy to reinstate homeostasis and suppress cell death. Here, we describe the relationship between energy status, sugar metabolism, trehalose-mediated activation of autophagy pathways and investigate whether shoots and roots utilize similar desiccation tolerance strategies. We show that despite containing high levels of trehalose, dehydrated Tripogon roots do not display elevated activation of autophagy pathways. Using targeted and non-targeted metabolomics, transmission electron microscopy (TEM) and transcriptomics we show that T. loliiformis engages a strategy similar to the long-term drought responses of sensitive plants and continues to use the roots as a sink even during sustained stress. Dehydrating T. loliiformis roots contained more sucrose and trehalose-6-phosphate compared to shoots at an equivalent water content. The increased resources in the roots provides sufficient energy to cope with stress and thus autophagy is not required. These results were confirmed by the absence of autophagosomes in roots by TEM. Upregulation of sweet genes in both shoots and roots show transcriptional regulation of sucrose translocation from leaves to roots and within roots during dehydration. Differences in the cell's metabolic status caused starkly different cell death responses between shoots and roots. These findings show how shoots and roots utilize different stress response strategies and may provide candidate targets that can be used as tools for the improvement of stress tolerance in crops.

Keywords: T. loliiformis; T6P; energy metabolism; source/sink; sucrose; tolerance.

Figure 1

(A) Autophagosomes in Tripogon loliiformis shoots during dehydration. (A) Hydrated shoots (B) 80 RWC (C) 60 RWC (D) 40 RWC. (B) Few autophagosomes observed in T. loliiformis roots during dehydration. (A) Hydrated shoots (B) 80 RWC (C) 60 RWC (D) 40 RWC. (C) Comparison in autophagosome number between dehydrated T. loliiformis shoots and roots. P < 0.05. Samples denoted with the same letter were not statistically different from each other using a P-value < 0.05.

Figure 2

(A) Sucrose accumulation between shoots and roots of T. loliiformis during dehydration. P < 0.05. (B) Percentage sucrose accumulation between shoots and roots of T. loliiformis during dehydration. Samples denoted with the same letter were not statistically different from each other using a P-value < 0.05.

Figure 3

Trehalose-6-phosphate accumulation between shoots and roots of T. loliifonrtis during dehydration. P < 0.05. Samples denoted with the same letter were not statistically different from each other using a P-value < 0.05.

Figure 4

Model for T. loliiformis shoot and root response to water deficit based on caloric shift. Roots maintain their energy status during dehydration hence are well-protected. Shoots have their energy compromised and therefore employ mechanisms like autophagy to mitigate adverse effects of drought. Cells denoted in red and green indicate increased and decreased transcript accumulation compared to hydrated controls.