Hungry Plants Activate Unavailable Iron Using Associated Bacteria

Naturally, the healthy plants contain bacterias and other-microbes in abundant amounts, the soil being the rich source. This microbial community, called the plant-microbiota, is vital for the optimum growth of a plant and also protects the plants from the attack of pathogenic insects and micro-organisms and their harmful effects. It is believed that during nutrient deficiency the plant root microbiota also improves plant productivity. But till now researchers have found only a few-examples of such positive connections among microbiota’s and plants (plant associated bacteria).

Being the most abundant micro-nutrient in soil and important for the growth of plants and their productivity, iron deficiency often limits the growth of plants because of its available form in the soil which plants cannot absorb. In order to achieve adequate yields and fulfill the iron deficiency, it is therefore often necessary to use chemical fertilizers, the overuse of which can be environmentally harmful.

Now, under the direction of Paul Schulze-Lefert, MPIPZ researchers have discovered a new strategy with which plants are tackling this problem. Plants, from roots, secrete special substances that guide plant-associated bacteria to activate soil iron so that the plants can easily absorb it.

When plants are exposed to iron in unavailable-form, to avoid iron deficiency they develop a compensatory hunger response. This response includes wide re-programming of expression of genes to produce and secrete coumarin, an aromatic compound that excretes from plant-roots and can improve iron solubility.

Interestingly, it has recently been revealed that coumarins are a selective-force that affects the composition of plants and microbes. Now it turns out that some coumarin act as “S.O.S signal”, which induce the root-microbiota to support the iron supply of plants.

In order to first evaluate the contribution of the root-microbiota to iron limiting plant productivity, the first author, Christopher-Harbor, and his colleague, used a control system with which they could control the iron availability and the existence of root associated bacteria.

In laboratory, using model plants, they performed an experiment, comparing plants completely free of bacteria and plants with an additional bacterial synthetic community (SynCom) that reflects the diversity of root-bacteria detected in nature. Results showed improved productivity of plants as growth on inaccessible iron because of the addition of bacterial SynCom. But plants grown without bacteria showed no improvement.

By growing plants in conjunction with individual strains of bacteria, they were able to detect that this iron-rescuing ability is wide-spread in several bacterial lines of root-microbiota. Later scientists experimented on plants which compromise coumarins secretion, the provided bacterial community did not show any benefit. They were able to show that coumarins secreted by plants are responsible for maintaining the nutritional value of bacteria and at the same time limiting their iron content.

Source: (Harbor et al 2020).

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