intimate mechanisms involved, as well as its effect on the production of signalling molecules 4 associated to the host plant-AM fungus interaction remains largely unknown. In the present 5 work, the effects of drought on lettuce and tomato plant performance and hormone levels 6 were investigated in non-AM and AM plants. Three different water regimes were applied and 7 their effects analysed over time. AM plants showed an improved growth rate and efficiency of 8 photosystem II than non-AM plants under drought from very early stages of plant 9 colonization. The levels of the phytohormone abscisic acid, as well as the expression of the 10 corresponding marker genes, were influenced by drought stress in non-AM and AM plants. 11The levels of strigolactones and the expression of corresponding marker genes were affected 12 by both AM symbiosis and drought. The results suggest that AM symbiosis alleviates drought 13 stress by altering the hormonal profiles and affecting plant physiology in the host plant. In 14 addition, a correlation between AM root colonization, strigolactone levels and drought 15 severity is shown, suggesting that under these unfavourable conditions plants might increase 16 strigolactone production in order to promote symbiosis establishment to cope with the stress. 17 18 19

Both strigolactones (SLs) and abscisic acid (ABA) biosynthetically originate from carotenoids. Considering their common origin, the interaction of these two hormones at the biosynthetic and/or regulatory level may be anticipated. Here we show that, in rice, drought simultaneously induces SL production in the root, and ABA production and the expression of SL biosynthetic genes in the shoot. Under control conditions, the ABA concentration was higher in shoots of the SL biosynthetic rice mutants dwarf10 (d10) and d17 than in wild-type plants, while a similar trend was observed for the SL perception mutant d3. These differences were enhanced under drought. However, drought did not result in an increase in leaf ABA content in the rice mutant line d27, carrying a mutation in the gene encoding the first committed enzyme in SL biosynthesis, to the same extent as in the other SL mutants and the wild type. Accordingly, d10, d17, and d3 lines were more drought tolerant than wild-type plants, whereas d27 displayed decreased tolerance. Finally, overexpression of OsD27 in rice resulted in increased levels of ABA when compared with wild-type plants. We conclude that the SL and ABA pathways are connected with each other through D27, which plays a crucial role in determining ABA and SL content in rice.

Background Plants are exposed to ever changing and often unfavourable environmental conditions, which cause both abiotic and biotic stresses. They have evolved sophisticated mechanisms to flexibly adapt themselves to these stress conditions. To achieve such adaptation, they need to control and coordinate physiological, developmental and defence responses. These responses are regulated through a complex network of interconnected signalling pathways, in which plant hormones play a key role. Strigolactones (SLs) are multifunctional molecules recently classified as a new class of phytohormones, playing a key role as modulators of the coordinated plant development in response to nutrient deficient conditions, especially phosphorus shortage. Belowground, besides regulating root architecture, they also act as molecular cues that help plants to communicate with their environment. Scope This review discusses current knowledge on the different roles of SLs below-ground, paying special attention to their involvement in phosphorus uptake by the plant by regulating root architecture and the establishment of mutualistic symbiosis with arbuscular mycorrhizal fungi. Their involvement in plant responses to other abiotic stresses such as drought and salinity, as well as in other plant-(micro)organisms interactions such as nodulation and root parasitic plants are also highlighted. Finally, the agronomical implications of SLs below-ground and their potential use in sustainable agriculture are addressed. Conclusions Experimental evidence illustrates the biological and ecological importance of SLs in the rhizosphere. Their multifunctional nature opens up a wide range of possibilities for potential applications in agriculture. However, a more in-depth understanding on the SL functioning/signalling mechanisms is required to allow us to exploit their full potential.

Background Strigolactones represent the most recently described group of plant hormones involved in many aspects of plant growth regulation. Simultaneously, root exuded strigolactones mediate rhizosphere signaling towards beneficial arbuscular mycorrhizal fungi, but also attract parasitic plants. The seed germination of parasitic plants induced by host strigolactones leads to serious agricultural problems worldwide. More insight in these signaling molecules is hampered by their extremely low concentrations in complex soil and plant tissue matrices, as well as their instability. So far, the combination of tailored isolation—that would replace current unspecific, time-consuming and labour-intensive processing of large samples—and a highly sensitive method for the simultaneous profiling of a broad spectrum of strigolactones has not been reported. Results Depending on the sample matrix, two different strategies for the rapid extraction of the seven structurally similar strigolactones and highly efficient single-step pre-concentration on polymeric RP SPE sorbent were developed and validated. Compared to conventional methods, controlled temperature during the extraction and the addition of an organic modifier (acetonitrile, acetone) to the extraction solvent helped to tailor strigolactone isolation from low initial amounts of root tissue (150 mg fresh weight, FW) and root exudate (20 ml), which improved both strigolactone stability and sample purity. We have designed an efficient UHPLC separation with sensitive MS/MS detection for simultaneous analysis of seven natural strigolactones including their biosynthetic precursors—carlactone and carlactonoic acid. In combination with the optimized UHPLC–MS/MS method, attomolar detection limits were achieved. The new method allowed successful profiling of seven strigolactones in small exudate and root tissue samples of four different agriculturally important plant species—sorghum, rice, pea and tomato. Conclusion The established method provides efficient strigolactone extraction with aqueous mixtures of less nucleophilic organic solvents from small root tissue and root exudate samples, in combination with rapid single-step pre-concentration. This method improves strigolactone stability and eliminates the co-extraction and signal of matrix-associated contaminants during the final UHPLC–MS/MS analysis with an electrospray interface, which dramatically increases the overall sensitivity of the analysis. We show that the method can be applied to a variety of plant species.

Background and Aim Water is an increasingly scarce resource while some crops, such as paddy rice, require large amounts of water to maintain grain production. A better understanding of rice drought adaptation and tolerance mechanisms could help to reduce this problem. There is evidence of a possible role of root-associated fungi in drought adaptation. Here, we analyzed the endospheric fungal microbiota composition in rice and its relation to plant genotype and drought. Methods Fifteen rice genotypes (Oryza sativa ssp. indica) were grown in the field, under well-watered conditions or exposed to a drought period during flowering. The effect of genotype and treatment on the root fungal microbiota composition was analyzed by 18S ribosomal DNA high throughput sequencing. Grain yield was determined after plant maturation. Results There was a host genotype effect on the fungal community composition. Drought altered the composition of the root-associated fungal community and increased fungal biodiversity. The majority of OTUs identified belonged to the Pezizomycotina subphylum and 37 of these significantly correlated with a higher plant yield under drought, one of them being assigned to Arthrinium phaeospermum. Conclusion This study shows that both plant genotype and drought affect the root-associated fungal community in rice and that some fungi correlate with improved drought tolerance. This work opens new opportunities for basic research on the understanding of how the host affects microbiota recruitment as well as the possible use of specific fungi to improve drought tolerance in rice.

Enhancing soil suppressiveness against plant pathogens or pests is a promising alternative strategy to chemical pesticides. Organic amendments have been shown to reduce crop diseases and pests, with chitin products the most efficient against fungal pathogens. To study what organic products characteristics are correlated with disease suppression, an experiment was designed where ten types of organic amendments with different physico-chemical properties were tested against the soil borne pathogen Rhizoctonia solani in sugar beet seedlings. Organic amendments rich in keratin or chitin reduced Rhizoctonia solani disease symptoms in sugar beet plants. The bacterial and fungal microbial communities in amended soils were distinct from the microbial communities in non-amended soil, as well as in soils that received other non-suppressive treatments. The Rhizoctonia-suppressive amended soils were rich in saprophytic bacteria and fungi that are known for their keratinolytic and chitinolytic properties (i.e., Oxalobacteraceae and Mortierellaceae). The microbial community in keratin- and chitin-amended soils was associated with higher zinc, and copper and selenium respectively. Importance Our results highlight the importance of soil microorganisms in plant disease suppression and the possibility to steer the soil microbial community composition by applying organic amendments to the soil.

Pathogenic Escherichia coli strains are responsible for food-borne disease outbreaks upon consumption of fresh vegetables and fruits. The aim of this study was to establish the transmission route of E. coli strain 0611, as proxy for human pathogenic E. coli, via manure, soil and plant root zones to the above-soil plant compartments. The ecological behavior of the introduced strain was established by making use of a combination of cultivation-based and molecular targeted and untargeted approaches. Strain 0611 CFUs and specific molecular targets were detected in the root zones of lettuce and leek plants, even up to 272 days after planting in the case of leek plants. However, no strain 0611 colonies were detected in leek leaves, and only in one occasion a single colony was found in lettuce leaves. Therefore, it was concluded that transmission of E. coli via manure is not the principal contamination route to the edible parts of both plant species grown under field conditions in this study. Strain 0611 was shown to accumulate in root zones of both species and metagenomic reads of this strain were retrieved from the lettuce rhizosphere soil metagenome library at a level of Log 4.11 CFU per g dry soil.

Rice and its importance as a cropRice (Oryza sativa L.) is the most produced and consumed crop worldwide, feeding more than 3,000 million people around the globe (FAO). It is the primary food for more than half of the world population, especially in developing countries where water scarcity and drought are threats to food security (Molden, 2009). Rice is a monocot plant that needs well-watered conditions to yield high levels of grain. The majority of rice is cultured in a water layer of about 5-10 cm deep. This flooded condition helps to supress the growth of weeds, to store water during the monsoon season and it allows the growth of the fern Azolla sp. that fixes nitrogen through its symbiosis with a bacterium. Flooded conditions also facilitate the plant to dissolve and take up immobile nutrients like phosphorous. The origin of domesticated rice traces back to between 8,200 and 13,500 years ago, and there is evidence supporting a single origin from the wild variety Oryza rufipogon Griff. (Molina et al., 2011). This wild ancestor is a semiaquatic species that grows in diverse environments, from flooded patches to dry upland fields, and even though its yield is low compared with the domesticated varieties, it is well adapted to survive under drought conditions for long periods (Mohapatra et al., 2011). Domesticated rice, including the subspecies 'indica' and 'japonica', has been bred under water logged conditions during the last five thousand years of crop production, which has resulted in a relatively high sensitivity to drought (Xia et al., 2014). Nowadays, 75% of the world rice production comes from irrigated lowland rice, while the other 25% coming from rainfed lowland and upland rice (IRRI database; Fig. 1). Water deficit is by far the main limitation for rice production in rainfed ecosystems, generating a yield gap between rainfed and irrigated rice of approx. 1 to 10 t ha -1 . Water deficit: a problem on the rise in agricultureThe availability of water is declining dramatically and the consumptive water use (CWU) of rice is relatively high (27%) while its water productivity (CWP) is low (0.60 kg / m 3 ) when compared with other crops like maize (2.19 kg / m 3 ) (Liu et al., 2009). This makes rice a very inefficient crop in terms of water use. Drought stress (water deficit) is by far the most important factor limiting crop productivity in the world, and improving yield under drought conditions is a major goal in plant breeding nowadays. Plants have evolved many forms of adaptations to water stress, and drought resistance mechanisms can be divided into several types. Drought tolerance is the ability to function when partially dehydrated, usually losing between 1-20% of relative water content, but there are species that are able to survive even when they lose 95% 14 them in the defense against pathogens and help plants to cope with abiotic stresses Bacon & White, 2016). Besides AM fungi, there is another class of fungi that has been studied thoroughly during the last years especially because of their role in ...