Strigolactones...more than enough to keep plant scientists occupied

Who could have thought that a whole morning could be spent discussing one class of compounds? Then again, some of my friends outside science expressed surprise that a conference could be organised on parasitic plants... I certainly hadn't appreciated the great complexity contained within these molecules. It seems to me that plants could be seen as more intelligent than animals in being able to regulate their development and physiological processes using a language of remarkably few words, compared to animals. For instance, strigolactones are known to promote root hair elongation, inhibit shoot branching, promote hyphae branching in Arbuscular Mycorrhiza Fungi and to stimulate the germination of parasitic plants (hence their strong presence at WCPP12). It strikes me that plant language is similar to Chinese, where the same word can have a different meaning depending on how it is pronounced. The roles described above could be simply explained by different tissues exhibiting contrasting responses due to contrasting intrinsic signalling pathways. And yet I have learnt today that the mechanisms of strigolactones are much more complex: for one thing, stereochemistry appears to have a significant affect , with certain stereoisomers showing no physiological activity . In the effort to determine the minimal components required for function, besides attempts to develop synthetic germination stimulants*, many strigolactone analogues have been developed, with surprising consequences. Certain synthetic versions can distinguish between strigolactone responses, depending on the position of the functional group on the phenyl ring. Compounds of the '2,4- distributed class' for instance, act as strong inhibitors of shoot branching but lack germination stimulation activity. More excitingly, the '2-6 distributed compounds' act in the opposite manner, effectively stimulating Striga hermonthica seed germination without affecting shoot branching (Fukui et al. University of Tokyo). By dissecting strigolactone roles into functionally specific molecules, this offers the potential to develop new 'suicidal germination agents' that avoid any detrimental development effects on the host. The talks demonstrated, however, that performances of strigolactone mimics are highly dependent on the context in which they are applied. In the presence of low levels of the classical strigolactone compound GR24, the analogue ST362 has an additive effect on Orobanche seed germination, whereas under high GR24 conditions, ST362 exerts a reductive effect (Kapulnik et al, Institute of Plant Sciences, Bet Degan, Israel). This suggests a mechanism where St362 acts as a competitive inhibitor of GR24 by occupying the same receptor. Furthermore, the model organism Arabidopsis thaliana may be totally inappropriate to study strigolactone function in crops. In most species examined, including members of the grasses (Poaceae) and Fabaceae ( legumes, peas and beans),strigolactone production in root exudates is increased under phosphate or nitrogen limiting conditions, presumably as a means to recruit nutrient- providing Arbuscular Mycorrhizae fungi. In Arabidopsis however, strigolactone production actually decreases in root exudates under low nitrogen or phosphate regimes (Nomura et al. Utsunomiya University, Japan). These studies highlight the importance of establishing an accurate context to assess how strigolactones may be manipulated and exploited to their full potential to control agricultural parasites.  Two of the most exciting talks demonstrated how Knowledge of strigolactones can be applied in the field. there is no known example of natural resistance to Orobanche in tomato, hence any genetic form of resistant must be generated by mutagenesis either using ethyl- methyl sulphonamide or fast- neutron irradiation. In this way, the recently developed SI-ORT1 mutant was identified as a strigolactone deficient line which showed excellent resistance against Orobanche. Not surprisingly, strigolactone deficiency also manifested itself in developmental phenotypes, including smaller fruits with delayed ripening. As the number of fruits was increased, however, the overall yield was not affected, making this a highly interesting mutant line (Evgenia Dor, Newe Ya'ar Research Centre, Israel). There is the question, however, that a reduction in strigolactone production would hinder recruitment of Mycorrhizae fungi, which could be especially problematic in nutrient- poor soils in Africa. Here again, the key may be to characterise compounds with functional specificity: in one study, a cultivar of maize specifically deficient in 5-deoxystrigol had less Striga germination activity but colonisation by Mycorrhizae was not significantly affected (Yoneyama et al. Hokkaido University). My favourite talk, however, demonstrated that natural strigolactones produced by Sorghum, ( infected by Striga hermonthica) can inhibit the germination of Striga gens erodes, which infects cowpea but not Sorghum (Sugimoto, Kobe University). Whilst this suggests that innovative use of natural strigolactone compounds could be used to suppress parasitic infections, problems regarding mass- production, distribution and application remain. Nevertheless, this remains a fascinating class of molecules and one to certainly keep an eye on in the world of parasitic plants!

* the idea here is to force all the parasite seeds to germinate prematurely. In the absence of a host, these fail to survive and the seed soil bank is thus depleted before the main crop is planted.