Land plants are plants that live in terrestrial habitats. They form vegetation on the earth through their roots. Their performance is dependent on the soil conditions below and the climate above. Photosynthesis is the process by which plants use sunlight to grow. This involves the conversion of light energy into chemical energy in the chloroplasts (the powerhouses of the plant cells). The quality and quantity of light that is perceived by the chloroplasts via light-absorbing pigments such as chlorophyll is crucial for plant growth and health. Photoassimilates (mainly sugars) are chemical compounds that are produced by the conversion of light energy into chemical energy. They are then transferred to the root compartment of the plant and used to support microbial growth. Roots are home to complex microbial communities, including bacteria and filamentous Eukaryotes (fungi and oomycetes), which have a profound impact on plant performance. It is still unclear how plants can harness the power of belowground microbes in order to manage aboveground stress responses. In a new study published by Nature Plants, Stphane Haciquard and his colleagues at the Department of Plant-Microbe Interactions (MPIPZ) in Cologne, Germany shed light on these aboveground-belowground relationships.
Shiji Hou, the first author of this study, conducted experiments in which the aboveground and belowground light conditions could be controlled. The researchers compared the growth of Arabidopsis Thale Cress in low-light conditions to those that were colonized with a complex community including 183 bacteria, 24 fungal species, and 7 oomycetes. They found that microbes helped to reverse the plant growth-deficiency. Research using leaf pathogens in Inoculation experiments revealed that microbe-colonized plants were more resistant to aboveground pathogens than those grown with germ-free controls. This suggests that root microbes may be able to promote plant growth and defense under low lighting conditions.
The scientists compared the growth and defense responses to colonized plants under different light conditions and found that investments in growth were associated with lower defense. This was because microbiota-induced defense reactions were decreased and plants were more vulnerable to leaf pathogens under low lighting. The study's authors hypothesized that plants prefer microbe-induced growth to microbe-induced defense reactions when the light conditions are poor. The researchers tested this hypothesis by screening different A.thaliana mutants. They wanted to find those that did not invest in low-light growth. The researchers found that the mutants they identified were more resistant to leaf pathogens than their original hypothesis. The scientists also found that MYC2, the host transcription factor, was critical to balance the balance in favor microbiota induced growth over microbiota induced defense under low lighting conditions.
The researchers continued to examine whether the composition of the microbial community beneathground could explain the aboveground investment in high-growth at the expense low-light defense. They analyzed the compositions of root microbiota from different A.thaliana mutants to determine if the plants had invested in low-light growth. This experiment resulted in the identification of 67 bacteria strains that could be linked to plant growth rescue under low lighting. The researchers created three types of bacterial communities to test the possibility of a causal link. They had the option of using all 183 strains, the 183 without the 67 strains that were predicted to be crucial for growth rescue, or the 67 alone. Surprisingly, wild-type A. thaliana plants were able to colonize with the 67-member community that invested in low-light growth, while those colonized by the community without these bacterial strains did so, favoring greater resistance to leaf infection by pathogens.
Stphane Hacquard, the study leader, stated that "our results suggest that both plant growth and defense response are engaged in different feedback loops avec the root microbiota based on aboveground lighting conditions." The light-induced changes in root exudation profiles are likely to be an important mechanism that stimulates growth of beneficial bacterial root commensals. This can boost plant growth at the expense of low-light defense responses. It is similar to recent findings in the context microbiota/root-shoot circuits in plants. These results revealed a direct link between gut and brain commensals. These results indicate that gut commensals and bacterial root play important roles in modulating stress responses, not only in local host organs but also in distant ones.
These findings are important for using belowground microbes in order to increase aboveground stress responses in plant. It is now possible to use the knowledge from this study to create synthetic microbial communities that have modular functions. This could help promote plant resistance and health.