A of beneficial fungal symbionts and of devastating parasitic

A new and important discovery is the role of SLs as branching factors for arbuscular mycorrhizal (AM) fungi (Akiyama et al. 2005; Parniske 2005, 2008). The structure–activity relationship of SLs as branching factors was extensively studied by Akiyama et al. (2010), see also Besserer et al. (2006). It was found that (?)-orobanchol had the highest activity followed by 5-deoxystrigol. (?)GR24 is very active, almost as active as (?)-strigol, but its mirror image practically is not (10,000 times less active). GR7 which is lacking the A-ring is 1000 times less active than GR24 whilst ent GR7 is almost inactive. This information suggests that for an SL analog to be active as a branching factor for AM fungi all rings of the ABC scaffold need to be there and also that the stereochemistry must be as in the strigol family. This implies that there is not much molecular freedom to design simpler structure for interaction with AM fungi. However, it was found recently that the B-ring is not strictly necessary. The SL analogs as shown in Fig. 7 where a phenyl is connected with the c-carbon of the D-ring or where a benzyl group is attached to the b-position, both are appreciably active as branching factors (Akayama, personal communication, 2015). Both compounds are not difficult to prepare as it had been reported previously (Nefkens et al. 1997). Stimulation of AM fungi fulfils a symbiotic role with parasitic plants. After the first observation, much attention was given to the beneficial mutualistic and symbiotic associations of AM fungi and parasitic plants (Akiyama and Hayashi 2006; Bonfante and Requena 2011). AM fungi facilitate the uptake of phosphates and nitrates, and in a sense these fungi serve as soil fertiliser which may be of agricultural value. Knowledge of this symbiotic relationship could provide a new strategy for the management and control of beneficial fungal symbionts and of devastating parasitic weeds in agriculture and natural ecosystems. Fig. 7 Simplified SL structures with activity as a branching factor for AM fungi SLs as inhibitors for shoot branching and in their role in controlling plant architecture As mentioned in the introduction, SLs are now recognised as new plant hormones. An important newly discovered activity deals with the control of plant architecture. SLs will not operate standing alone, but in concert with other plant hormones. Until 25 years ago there were 5 types of plant hormones known, namely: auxins, cytokinins, ethene (ethylene), gibberellins and abscisic acid (ABA). More recently, brassinosteroids and jasmonates have been added to the list. The role of the various plant hormones in the plant kingdom is under extensive investigation. There is accumulating evidence that SLs interplay in a crosstalk with several of these plant hormones. Which endogenously SLs are operative in the interplay in planta is unknown in most cases. The crosstalk of SLs with other plant hormones may either take place in a fully concerted manner or sequentially in a cascade of events, although in many cases the precise modus operandi is not known in detail. Phenomenologically, the crosstalk interactions are well documented. As it is common for other phytohormones, the SL biosynthesis and activity is regulated by other hormones. For instance, cytokinins act as antagonists to SLs in regulation of axillary bud outgrowth (Dun et al. 2012) and in regulation of mesocotyl elongation in darkness (Hu et al. 2014). Auxins are not only shown as one of the major regulators of SL biosynthesis (Hayward et al. 2009; AlBabili and Bouwmeester 2015, and references therein), but also they act as antagonists because SLs may enhance auxin transport (Cheng et al. 2013, and references therein). Lopez-Raez et al. (2010) showed that abscisic acid, one of the key regulators of plant response to abiotic stress, has a role in SL biosynthesis, but, on the other hand SLs can also impact biosynthesis of abscisic acid (Al-Babili and Bouwmeester 2015). Besides phytohormones, it is well established that phosphate affects SL biosynthesis, meaning that shortage of phosphate increases SL production (Koltai 2015, and references therein). However, all these facts are still on cellular level, and they do not explain on a molecular basis which exact mechanisms play a role. This is a highly complex research area due to the different effects of phytohormones and varying context of their actions.

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