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Apocarotenoids as signaling molecules in the arbuscular mycorrhizal symbiosis

Final Report Summary - APOMYC (Apocarotenoids as signaling molecules in the arbuscular mycorrhizal symbiosis)

Plants are exposed to ever changing and often unfavourable environmental conditions. Agrochemicals are normally used to prevent stress conditions. However, the environmental pollution caused by excessive use and misuse of agrochemicals has led to public concerns about the use of these chemicals in agriculture. Therefore, there is an urgent need to find alternatives more environmentally friendly. Plants have evolved sophisticated mechanisms to adapt themselves to overcome abiotic and biotic stresses. One of these strategies is the establishment of beneficial associations with soil microorganisms. Arbuscular mycorrhizal (AM) symbiosis is an association established between AM fungi and the vast majority of land plants. It improves the nutritional status and enables the host plant to perform better under stressful conditions. AM symbiosis establishment requires a high degree of coordination between the two partners based on a molecular dialogue in which apocarotenoids play a key role (López-Ráez et al., 2010c). Apocarotenoids form part of the large group of carotenoid cleavage products found in nature, including the phytohormones strigolactones (SLs) and abscisic acid (ABA). The aim of APOMYC was to elucidate the signalling mechanisms involved in the regulation of the AM symbiosis, studying specifically the role of SLs. The final goal is to reduce the agrochemical inputs by the rational exploitation of natural resources such as beneficial organisms.

By analysing the SL and ABA content in different tomato ABA-deficient mutants and using specific inhibitors, we have found that ABA is one of the regulators of SL biosynthesis affecting the expression of the genes SlCCD7 and SlCCD8 (López-Ráez et al., 2010b). To get further insights in SL biosynthesis, the tomato gene SlCCD8 has been cloned and characterised. By generating transgenic tomato plants containing a SlCCD8 ribonucleic acid interference (RNAi) construct, the key role of SlCCD8 in SL biosynthesis and regulation of plant architecture has been confirmed (Kohlen et al., submitted). Moreover, new functions for SLs in reproductive development have been envisaged. SLs are also host detection cues for root parasitic plants of the Orobanchaceae. Their dual role in the rhizosphere opens the possibility of potential interactions between both biosystems and develop new biocontrol strategies against parasitic weeds (López-Ráez et al., 2011a,c). We have recently shown that AM symbiosis in tomato leads to a reduction in the germination of seeds of the parasite Phelipanche ramosa, and have analytically demonstrated that this reduction is caused by a reduction in SLs (López-Ráez et al., 2011b). Therefore, AM symbiosis a suitable and promising tool for the biological control of parasitic weeds, where crops would also profit from all the other well-known benefits of the AM symbiosis (López-Ráez et al., 2011c). On the other hand, in transgenic tomato plants containing a SlCCD8 RNAi construct, a mild reduction in SL biosynthesis is sufficient to reduce root parasitism by P. ramosa without excessively compromising AM symbiosis (Kohlen et al., submitted).

In addition to gain a deeper insight of the role of apocarotenoids in AM symbiosis, an integrative analysis of the host plant response to different AM fungi combining multiple hormone determination and transcriptional profiling have been performed during APOMYC. Both analyses evidenced common and divergent responses of tomato roots to different AM fungi (López-Ráez et al., 2010c). The study revealed the regulation of the oxylipin pathway during the AM symbiosis, pointing to a key regulatory role of jasmonates. Changes in other important defence-related compounds as phenolic acids were also observed during the symbiosis (López-Ráez et al., 2010a). Altogether, the results suggest that specific responses to particular fungi underlie the differential impact of individual AM fungi on plant physiology, and particularly on its ability to cope with biotic stresses.