Strigolactone (SL) are regulators of plant growth and development and may play a role in plant resistance against specific pathogens. SL inside the plant acts as hormone and outside the plant play a role as a rhizosphere signal in interactions with the mycorrhizal plant and parasitic weeds i.e. play a role in host recognition signal for arbuscular mycorrhizal fungi (AMF) (Andreo-Jimenez et al., 2015;Foo et al., 2016) and favours the establishment of beneficial association with mycorrhizal fungi and rhizobia (Akiyama and Hayashi 2006; Andreo-Jimenez et al., 2015). In addition to the above function SL also regulates secondary growth and reproductive development (Torres-Vera et al., 2014). The level of SL in plants are affected by the nutritional status (e.g. Phosphorus) of the plant (Garcia-Garrido et al., 2009). This phytohormone induce several responses to AMF, including spore germination, hyphal branching, mitochondrial metabolism, transcriptional reprogramming and production of chitin (Lanfranco et al., 2018). SL is perceived by population of endobacteria with an increase of bacterial divisions and activation of specific transcriptional responses (Lanfranco et al., 2018).
Torres-Vera et al. (2014) proposed strigolactone play a role in the regulation of plant defense through their interaction with other defense-related hormones especially with jasmonic acid signaling pathway. SLs do not have a universal role in plant protection but plays a part in resistance to specific pathogens (Marzec 2016). Complex cross talk and induced hormonal changes modulate plant resistance/susceptibility to pathogen (Robert-Seilaniantz et al., 2011). Belmondo et al. (2017) studied exposure of Botrytis cinerea and Cryphonectria parasitica to GR24 (synthetic strigolactone) leading to reduction of fungal growth. Reactive oxygen species and mitochondria emerged as mediators of SLs action. Soil borne fungi from genus Trichoderma and Fusarium were able to degrade natural and synthetic SLs (Rozpadek et al., 2018).
Under biotic stress condition SLs provide tolerance against pathogen infection. Plants secrete SL in rhizosphere through the SL exporter Pleiotropic Drug Resistance 1 (PDR1). The asymmetrical localization of PDR1 at the plasma membrane is the reason for the SL export out of the cell. PDR1 as a SL exporter mediates SL transport in the plant as well as into the soil (De Cuyper and Goormachtig 2017). The weeds misuse the SL to recognize their hosts (De Cuyper and Goormachtig 2017).
Foo et al. (2016) investigated the influence of SLs on the hemibiotrophic pathogen Fusarium oxysporum f. sp. pisi. and found ethylene signaling influence pea susceptibility to this pathogen and that SLs do not. Blake et al. (2016) suggest that there is no general role of SLs in defense against necrotrophic plant pathogens. However, ethylene signaling play a key role in specific cell types that reduces pathogen invasion. Plant defensins PDF1.2 and pathogenesis related protein 5 (PR5) expression was significantly higher in the absence of the fungal stimulus providing evidence for a link between SL and plant defense (Rozpadek et al., 2018). Decker et al. (2017) demonstrated the modulation of susceptibility to infection by the pathogenic fungus Sclerotinia sclerotiorum depending on SL availability. The role of SL in pathogen defense of Physcomitrella patens (moss)provides another example of the disease resistance mechanisms in bryophytes. Carvalhais et al. (2018) result indicate that the plant’s ability to produce strigolactones is correlated with changes in the composition of rhizosphere fungal but not bacterial communities. Strigolactone released by roots induce hyphal branching in the proximity of the root (Garcia-Garrido et al., 2009). Increased level of strigolactones in pea and Arabidopsis led to reduced cytokinin level in the xylem sap although the level in the shoot remain unchanged (Foo et al., 2007).
SL may alter ABA levels. Torres-Vera et al. (2014) concluded SL-deficient tomato mutant (Slccd8) is more susceptible to necrotrophic fungal pathogens. The reduced levels of phytohormone JA, SA and ABA in Slccd8 suggest a role of SLs in the regulation of plant defense responses through their interaction with other signaling pathway especially with JA-related pathway, than having a direct effect on fungal development (Torres-Vera et al., 2014). ABA has emerged as an important regulator of biotic defense responses (Ton et al., 2009). Therefore it seems likely that the effect of SLs on ABA content may impact on the plants ability to cope with stresses. Synthetic strigolactone GR24 inhibited the growth of root pathogens Fusarium oxysporum f. sp. melonis, Fusarium solani f. sp. mango, Sclerotinia sclerotiorum and Macrophomina phaseolina and of foliar pathogen Alternaria alternata, Colletotrichum acutatum and Botrytis cinerea (Dor et al., 2011). Strigolactone promotes defense against rice blast fungal pathogen Magnaporthe oryzae (Nasir et al., 2019). SL antagonist are chemical that can bind to the receptor and thus disrupt the interaction between them and SL agonist. SL antagonist can be used for crop protection by inhibiting seed germination of root parasitic plants (Takahashi and Asami 2018).
As the number of interconnections and the complexity grows, it becomes necessary to understand the dynamics of these networks.
See Part VII for further information
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