Exploring the synergistic effects of biochar and arbuscular mycorrhizal fungi on phosphorus acquisition in tomato plants by using gene expression analyses.

Arbuscular mycorrhizal (AM) fungi are symbiotic organisms that contribute significantly to plant mineral nutrition, mainly phosphate. However, their benefits are constricted by the availability of phosphate in the soil, and thus they are recalcitrant as amendment in highly fertilized soils. Biochars are by-products of the pyrolysis of biomass in the absence of oxygen. They can improve soil properties and act as a source of nutrients for plants. However, depending on their origin, the final composition of biochars is extremely variable and thus, their efficiency unpredictable. In order to gain mechanistic insights into how the combined application of biochars and AM fungi contribute to plant phosphate nutrition and growth, we used gene expression analyses of key symbiotic marker genes. We compared for this analysis two biochars originated from very different feedstocks (chicken manure and wheat straw) on tomato plants with or without the AM fungus Rhizophagus irregularis. Our results show that the synergy between AM fungi and biochars as P biofertilizers is greatly governed by the origin of the biochar that determines the speed at which phosphate is released to the soil and absorbed by the plant. Thus, chicken manure biochar quickly impacted on plant growth by readily releasing P, but it turned out detrimental for symbiosis formation, decreasing colonization levels and expression of key symbiotic plant marker genes such as SlPT4 or SlFatM. In contrast, wheat straw biochar was inferior at improving plant growth but stimulated the establishment of the symbiosis, producing plants with the same concentration of phosphate as those with the chicken manure. Taken together, slow P releasing biochars from plant residues appears to be a more promising amendment for long terms experiments in which biofertilizers such as AM fungi are considered. Furthermore, our results indicate that implementing plant transcriptomic analyses might help to mechanistically dissect and better understand the effects of biochars on plant growth in different scenarios.

Overexpression of the Potato Monosaccharide Transporter StSWEET7a Promotes Root Colonization by Symbiotic and Pathogenic Fungi by Increasing Root Sink Strength.

Root colonization by filamentous fungi modifies sugar partitioning in plants by increasing the sink strength. As a result, a transcriptional reprogramming of sugar transporters takes place. Here we have further advanced in the characterization of the potato SWEET sugar transporters and their regulation in response to the colonization by symbiotic and pathogenic fungi. We previously showed that root colonization by the AM fungus Rhizophagus irregularis induces a major transcriptional reprogramming of the 35 potato SWEETs, with 12 genes induced and 10 repressed. In contrast, here we show that during the early colonization phase, the necrotrophic fungus Fusarium solani only induces one SWEET transporter, StSWEET7a, while represses most of the others (25). StSWEET7a was also induced during root colonization by the hemi-biotrophic fungus Fusarium oxysporum f. sp. tuberosi. StSWEET7a which belongs to the clade II of SWEET transporters localized to the plasma membrane and transports glucose, fructose and mannose. Overexpression of StSWEET7a in potato roots increased the strength of this sink as evidenced by an increase in the expression of the cell wall-bound invertase. Concomitantly, plants expressing StSWEET7a were faster colonized by R. irregularis and by F. oxysporum f. sp. tuberosi. The increase in sink strength induced by ectopic expression of StSWEET7a in roots could be abolished by shoot excision which reverted also the increased colonization levels by the symbiotic fungus. Altogether, these results suggest that AM fungi and Fusarium spp. might induce StSWEET7a to increase the sink strength and thus this gene might represent a common susceptibility target for root colonizing fungi.

Virus infection induces resistance to Pseudomonas syringae and to drought in both compatible and incompatible bacteria-host interactions, which are compromised under conditions of elevated temperature and CO2 levels.

Plants are simultaneously exposed to a variety of biotic and abiotic stresses, such as infections by viruses and bacteria, or drought. This study aimed to improve our understanding of interactions between viral and bacterial pathogens and the environment in the incompatible host Nicotiana benthamiana and the susceptible host Arabidopsis thaliana, and the contribution of viral virulence proteins to these responses. Infection by the Potato virus X (PVX)/Plum pox virus (PPV) pathosystem induced resistance to Pseudomonas syringae (Pst) and to drought in both compatible and incompatible bacteria-host interactions, once a threshold level of defence responses was triggered by the virulence proteins P25 of PVX and the helper component proteinase of PPV. Virus-induced resistance to Pst was compromised in salicylic acid and jasmonic acid signalling-deficient Arabidopsis but not in N. benthamiana lines. Elevated temperature and CO2 levels, parameters associated with climate change, negatively affected resistance to Pst and to drought induced by virus infection, and this correlated with diminished H2O2 production, decreased expression of defence genes and a drop in virus titres. Thus, diminished virulence should be considered as a potential factor limiting the outcome of beneficial trade-offs in the response of virus-infected plants to drought or bacterial pathogens under a climate change scenario.