Bacterial arsenic efflux genes enabled plants to transport boron efficiently

Stubby roots, brittle branches, poor fertility - these are some of the symptoms which can occur in the case of boron deficiency in plants. Thanks to transport proteins called Nodulin26-like-intrinsic-proteins (NIPs), modern seed plants can efficiently take up and distribute this essential micronutrient. However, this ability was not always inherent to plants. An international collaboration of researchers has now uncovered that the enigmatic functional origin of NIPs lies within genetically encoded arsenic detoxification units of bacteria. After its horizontal gene transfer a protein, which is important for arsenic efflux and detoxification in bacteria, became incorporated into the pool of transport proteins of land plants and evolved to channel proteins which are essential for the transport logistics of the for higher plants important nutrients boron and silicon.

The majority of essential micronutrients for plants - such as copper, iron and zinc to name a few - are metals. The two metalloids boron and silicon are also considered essential (boron) and highly beneficial (silicon) for seed plant development though most other organisms from bacteria to humans, including 'lower' plants, have probably no essential need for these two elements. Both micronutrients contribute to the proper differentiation, structural support and elasticity of cell walls in vascular plants and promote pathogen defence and general stress tolerance. Another shared property of these metalloids is that they are taken up and distributed within seed plants via Nodulin26-like-intrinsic channel proteins (NIPs). NIPs are highly conserved proteins and are unique to plants. NIPs belong to the channel protein superfamily of aquaporins which transport uncharged solutes such as water, hydrogen peroxide, glycerol, ammonia and metalloids in various organisms. However, until recently, the original transport selectivity and functional origin of NIPs were inscrutable.

By combining intense sequence-, phylogenetic and genetic context analyses, an international collaboration of researchers was now able to resolve the functional trans-organismal beginnings of NIPs. Led by Dr. Gerd Patrick Bienert of the Emmy Noether research group "Metalloid Transport" from the Leibniz Institute of Plant Genetics and Crop Plant Research (IPK) in Gatersleben, the scientists revealed that NIPs originated in plants due to the horizontal gene transfer of a probable arsenic resistance operon-localised arsenous acid-transporting bacterial AqpN-aquaporin into the genome of a charophytic algae. Alike boron and silicon, arsenic is also a metalloid. The metalloid species boric acid, silicic acid and arsenous acid sterically resemble each other from a protein channel pathway point of view.

The ancestral bacterial AqpN-aquaporins had been functionally uncharacterised so far. The researchers were able to identify NIPs with characteristics very similar to the ancestral bacterial proteins in archetype plants such as algae, moss and ferns. Interestingly, these ancestral NIPs as well as their bacterial progenitors were near to impermeable to water and silicon but transported arsenic and also boron. Utilising a mutational approach, the researchers showed that during the evolution of terrestrial plants, a shift in the functional selectivity of NIPs had occurred. The transport proteins, which had originally functioned as bacterial arsenic efflux channels, over time turned into the essential nutrient transporters found in our modern seed plants.

Moreover, the results of the study also explain, why seed plants which have a high demand for boron or silicon, such as e.g. rice, often accumulate very high levels of arsenic when growing on arsenic-rich soils: The ancestral bacterial substrate selectivity, which is arsenic permeability, still resides in our present day crop plant NIPs and leads to an 'accidental' arsenic uptake and translocation when NIPs aim for boron or silicon transport regulation in an arsenic-rich environment.

"Without the horizontal gene transfer of bacterial arsenic detoxification channels, our modern-day crop plants would not be able to efficiently regulate the transport logistics of boron or silicon and our crop yields would probably not be nearly as high as they are", describes Dr. Bienert the importance of these findings. Despites this transport-ability, boron deficiency still causes losses in agricultural plant production. This is one of the reasons why the researchers within the "Metalloid Transport" group will continue investigating the mechanisms behind the uptake and distribution of boron in plants - with the aim of breeding crops with an improved boron efficiency and contributing to an optimised boron fertiliser management in the field.

Legend for figure 1: Prokaryotes possess arsenic resistance (ars) operons for arsenic (As) detoxification and export. Some bacteria possess an ars operon-located aqpN aquaporin gene, which facilitates the transmembrane diffusion of As(III) species. In prokaryotes, arsenate [As(V)] enters the cells via phosphate transporters. Transcription of arsR is induced by As(V) and the protein produced, ArsR, regulates the expression of aqpN and arsC. The chemical gradient needed to cause As efflux through AqpNs is maintained by the reduction of As(V) to As(III), catalysed by the arsenate reductase ArsC. An aqpN gene was probably transferred to charophytic algae by a horizontal gene transfer event. During evolution, this ancestral plant AQPN-type channel protein converted into the so-called Nodulin 26-like Intrinsic Proteins (NIPs) which are essential to regulate the uptake and translocation of the important nutrients silicon (Si) and boron (B) in present-day seed plants. When Si and B became beneficial and essential elements for vascular plant groups, respectively, these proteins underwent structural changes which resulted in a neo- and subfunctional-isation enabling them to form efficient B and Si transport regulators.

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