Petunia PLEIOTROPIC DRUG RESISTANCE 1 Is a Strigolactone Short-Distance Transporter with Long-Distance Outcomes

Abstract

Phytohormones of the strigolactone (SL) family have been characterized as negative regulators of lateral bud outgrowth and triggers of symbioses between plants and mycorrhizal fungi. SLs and their precursors are synthesized in root tips as well as along shoot and root vasculature; they either move shoot-wards and regulate plant architecture or are exuded from roots into the soil to establish mycorrhizal symbiosis. Owing to the difficulty in quantification of SL in shoot tissues because of low abundance, it is not yet clear how SL distribution in plants is regulated at short- and long-distances from SL biosynthetic and target tissues. To address this question, we grafted wild-type scions and rootstocks from different petunia mutants for SL biosynthesis/transport and investigated SL activity by quantifying lateral bud outgrowth in the main shoot. Based on these results, we show that (i) the previously reported petunia SL transporter PLEIOTROPIC DRUG RESISTANCE 1 (PDR1) directly accounts for short-distance SL transport and (ii) long-distance transport of SLs seems to be partially and not directly dependent on PDR1. These data suggest that the root-to-shoot transport of SLs occurs either via the vasculature bundle through transporters other than PDR1 or involves SL precursors that are not substrates of PDR1.

Recent progress in the chemistry and biochemistry of strigolactones

“…Recently, PLEIOTROPIC DRUG RESISTANCE 1 (PDR1), an ABC-type transporter in petunia, which participates in the cell to cell transport of SLs, 96) was shown to be responsible for short-distance transport but only partially for long-distance transport of SLs. 97) These results together suggest that BIHs are substrates of PDR1 and that PDR1 is involved in short-distance transport of BIHs and regulating shoot branching. The possibility that BIHs are not canonical SLs is further supported by the observation that targeted mutation in SICYP722C, a gene responsible for the conversion of CLA to orobanchol in tomato, resulted in a significant reduction in germination stimulation activity of root exudates due to blockage of orobanchol biosynthesis but did not show the apparent phenotypes of the SL-deficient mutant Slccd8.…”

Efficiency and bioavailability of new synthetic strigolactone mimics with potential for sustainable agronomical applications

“…A molecular quantification of SL mimics in buffer based on plant extracts might be considered in the future as an additional approach to investigate SL half-life in plant tissue equivalents. It was recently published that the role of PDR1 in SL transport seems to be direct for SL root exudation but not for SL root-to-shoot translocation (Shiratake et al 2019). Therefore, the exogenous function of PDR1 in Arabidopsis is likely exclusively associated to root exudation of GR-24 and SL mimic compounds, underlying the significance of the toxicity assay results here reported.…”

“…Me-CLA SL2 and SL1 were here characterized as having the highest affinity to PDR1 together with rac-GR24. It was previously hypothesized that substrates to PDR1 might be not only orobanchol but also SL precursors, since CCD8/DAD1 and PhMAX1 are both expressed in petunia roots, the latter albeit within a narrow pattern (Shiratake et al 2019). The presence of PDR1 in the root tip (Kretzschmar et al 2012) is proposed to contribute to root tip unloading of SL and SL-related compounds, thus releasing a negative feedback loop on SL biosynthesis.…”

“…This soil nutrient / root exudation feedback-regulation increases the chance of high mycorrhization levels and likely enhances plant nutrient uptake, thus optimizing plant growth and biomass production also on not optimal soils and/or growth conditions (Liu et al 2018a(Liu et al , 2018b. In the Solanacea Petunia hybrida, SL root exudation is actively regulated by the ATP-BINDING-CASSETTE (ABC) class G protein PLEIOTROPIC DRUG RESISTANCE1 (PhPDR1) (Kretzschmar et al 2012;Sasse et al 2015;Shiratake et al 2019;Liu et al 2019). The ectopic expression of PhPDR1 in Petunia hybrida showed that the enhanced exudation of endogenous SL also induced SL biosynthesis (Sasse et al 2015), increased mycorrhization levels and plant biomass production on soils with low nutrient conditions (Liu et al 2018a(Liu et al , 2018bLiu et al 2019).…”

On the outside looking in: roles of endogenous and exogenous strigolactones

“…Based on grafting experiments using dad1 (an SL biosynthetic mutant), pdr1 and WT scions and rootstocks from petunia, Shiratake et al . (2019) proposed that PDR1 is involved in short‐distance but not in long‐distance SL transport. These data and also the fact that plants exuded a blend of SLs strongly suggest the presence of other transporters for SLs or their precursors in long‐distance transport (Yoneyama et al ., 2018).…”