Strigolactones and abiotic stress in plants: modulation of abscisic acid transport

Shoot strigolactones (SL) are positive regulators of stomatal closure acting in an ABA-dependent as well as independent manner: the focus of this work is on the ABA-dependent one. Root SL have negative effects on both shoot SL and root ABA levels. Thus, osmotic and drought stress repress root SL biosynthesis, increasing both shoot SL and root ABA levels and coordinating whole-plant acclimatization to stress. Although there were demonstrations of osmotic stress-induced SL biosynthesis repression in other dicots roots, data on the model plant At were lacking. We showed here that osmotic stress induces down-regulation of SL biosynthetic genes also in At roots, even after only 30 min of stress. This suggests that the root SL decrease could be an early signal of stress, locally inducing ABA biosynthesis while inducing SL biosynthesis at the shoot level. In order to investigate whether SL could also affect ABA transport, we first treated At plants expressing sGFP:ABCG25, an ABA exporter, with SL, and detected the GFP signal through confocal microscopy. We used ABA treatment as a positive control because it has been previously reported that in At root tips cells, it induces accumulation of sGFP:ABCG25 at the plasma membrane (PM). We demonstrated that also SL are able to localize ABCG25 at the PM. The sGFP:ABCG25 construct was then inserted in the SL-insensitive mutant d14-1 to better understand the role of endogenous SL in this process. Comparisons between the wt and SL-insensitive mutant revealed that the lack of SL perception causes higher cytosolic ABCG25 levels, thus confirming the role of endogenous SL in the accumulation of sGFP:ABCG25 at the PM. Being ABCG25 an ABA exporter, this could negatively affect intracellular ABA levels. At this point we investigated the effect of osmotic stress on the subcellular localization of ABCG25 in the SL-insensitive mutant. We first confirmed that osmotic stress induces an endocytosis of sGFP:ABCG25 in the wt, probably in order to prevent ABA export from root cells and support an intracellular ABA increase. Moreover, we demonstrated that in SL-insensitive mutants, endocytosis of ABCG25 is faster than in the wt. This also corroborates the role of endogenous SL in the accumulation of ABCG25 at the PM. Notably, the internalization in wt increases during the stress time course, reaching the values of the SL-insensitive mutant by the end of the treatment: this could be explained by the formerly highlighted osmotic stress-induced decrease in SL levels in roots. In conclusion, SL may negatively affect intracellular ABA levels in roots by inducing ABCG25 accumulation at the PM, so that, as a consequence, the repression of SL biosynthesis in root triggered by osmotic and drought stress could increase intracellular ABA levels. The rise in intracellular ABA levels could lead to a subsequent increase of ABA export, in order to accumulate ABA in guard cells and facilitate stomatal closure. If in the shoot SL act on ABCG25 as in the root, the increase of SL in the shoot during osmotic and drought stress could mediate stomatal closure also by enhancing ABA transport to guard cells, thus unveiling a mechanism for ABA-dependent SL-mediated stomatal closure.