Strigolactones as a hormonal hub for the acclimation and priming to environmental stress in plants

Abstract

Strigolactones are phytohormones with many attributed roles in development, and more recently in responses to environmental stress. We will review evidence of the latter in the frame of the classic distinction among the three main stress acclimation strategies (i.e., avoidance, tolerance and escape), by taking osmotic stress in its several facets as a non-exclusive case study. The picture we will sketch is that of a hormonal family playing important roles in each of the mechanisms tested so far, and influencing as well the build-up of environmental memory through priming. Thus, strigolactones appear to be backstage operators rather than frontstage players, setting the tune of acclimation responses by fitting them to the plant individual history of stress experience.

Keywords: abiotic stress; acclimation; avoidance; escape; strigolactones; tolerance.

Figure 1

The strigolactone (SL) biosynthetic (cell on the left) and signalling pathways (on the right). Catabolic steps are not represented, as well as exceptions to the pictured transduction pathway (see text for details). KAR1 is pictured as a representative of the karrikin family. CCD, CAROTENOID CLEAVAGE DIOXYGENASE; CL, carlactone; CLA, carlactonoic acid; CLAMT, CLA methyl transferase; D, DWARF; DDH, didehydro; 4DO, 4‐deoxy‐orobanchol; 5DS, 5‐deoxy‐strigol; HYD, HYDROXYLASE; KAI2, KARRIKIN INSENSITIVE2; KARs, karrikins; KL, KAI2 ligand; GGPP, geranyl geranyl diphosphate; LBO, LATERAL BRANCHING OXIDASE; LGS1, LOW GERMINATION STIMULANT1; MAX, MORE AXILLARY GROWTH; MeCLA, methyl CLA; PDR1, PLEIOTROPIC DRUG RESISTANCE1; SMAX1, SUPPRESSOR OF MAX2 1; SMXLs, SMAX1‐LIKEs.

Figure 2

The influence of strigolactones (SLs) on the main water‐spending and water‐saving mechanisms in drought avoidance. Strigolactones support water savings by promoting ABA‐dependent and independent stomatal closure, and delayed stomata re‐opening during drought recovery (the ‘after‐effect’ of drought, a feature of drought stress memory). They also contribute to drought‐triggered leaf epidermis and cuticle changes, for which the main driver is however the KAI2 ligand/karrikin pathway in arabidopsis. Finally, they can ensure the sustainability of a water‐spending approach by favouring arbuscular mycorrhizal (AM) symbiosis and biomass allocation to the roots, thus allowing better soil exploration and water capture. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

The influence of strigolactones (SLs) on environmental stress tolerance. Strigolactones support the biochemical mitigation of stress effects by promoting enzymatic and non‐enzymatic antioxidants. Given the cross‐cutting occurrence of reactive oxygen species (ROS) in a variety of stressful conditions, this effect has been recorded so far in conditions of drought, high osmolarity/salinity, high/low light, heat, cold, and heavy metal/metalloid soil contamination. A_N, net assimilation; APX, ASCORBATE PEROXIDASE; CAT, CATALASE; H₂O₂, hydrogen peroxide; g_s, stomatal conductance; MDA, malondialdehyde; POD, PEROXIDASE; SOD, SUPEROXIDE DISMUTASE. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

The positioning of strigolactones (SLs) within the main drought stress‐coping strategies: avoidance, tolerance and escape. Several of the pictured concepts apply to other environmental stresses too, as detailed in the text. Their involvement in drought escape is yet to be confirmed. [Color figure can be viewed at wileyonlinelibrary.com]