Pincushion Hakea Cluster-Root Secrets stimulate the absorption of phosphorus in the soils poor in nutrients

Plants need phosphorus to develop and survive. In environments with low levels of available soil phosphorus, plants must adapt to stay alive.

The Pinceushion Hakea is a large woody shrub and persistent leaves native to southwest Australia, an area that has incredibly low levels of soil phosphorus. This plant has adapted to these conditions by forming cluster roots – a large number of smaller royalties extending from the root axis which resemble a bottle brush – to extract the small amount of phosphorus in the ground.

The cluster roots help plants in soils with low nutrients by increasing the amount of root surface in contact with the soil, improving their ability to extract limited resources. In addition, cluster roots secrete chemicals and enzymes to improve the bioavailability of nutrients, mainly phosphorus, in the soil.

Acid phosphatase, for example, is an enzyme secreted by cluster roots that converts organic phosphorus into a form that plants can easily absorb. A better understanding of these survival mechanisms could ultimately help researchers develop food crops that can prosper in soils deficient in nutrients.

Although researchers have successfully identified many chemicals secreted by cluster roots to improve the availability of phosphorus, the genes and molecular pathways responsible for the secretion of roots and absorption in the Protecaea plants, including Pincushion Hakea, had not been identified.

In order to better understand how the cluster roots work at the molecular level, researchers from the University of Hiroshima, the University of Australia-Western, the University of Okayama, the Hokkaido University, the University of Yamagata and other institutions have carried out an RNA-SEQ experience on Pincushion Hakea to identify the genes expressed in its cluster roots.

The team published their research in the journal New phytologist.

“Our main question was: how is the Pincushion Hakea, Hakea Laurina, survived in its environment extremely limited to phosphorus? Our hypothesis was that Hakea Laurina has distinct strategies to maximize the release of root exudates, such as carboxylates and acid phosphatases, from its cluster roots, which are important to assist Soft School of Integrated Sciences for Life at the University of Hiroshima and first author of the research document.

The researchers compared the genes expressed in the roots of mature clusters with those of the adjacent side roots as a witness. This comparison has identified 4,210 genes that have been expressed at higher levels in the cluster roots, providing a large number of potential genes associated with increased secretion and absorption.

These included phosphate transporters involved in the absorption of phosphate in root and acid phosphatases. In addition, the analysis of the Kyoto Encyclopedia of Genes and Genomes (KEGG) track indicated that these cluster roots have improved the metabolism of carboxylates, which would support an increase in the supply of Malatian carboxylates and Citrate for secretion in soils with low phosphorus.

One of the highly expressed genes in the Pincushion Hakea cluster roots was a Malatian carrier protein activated by aluminum (ALMT) that researchers identified as halalmt1.

This halalmt1 shares 51% of its sequence of amino acids deducted with Laalmt1, a carrier of Malatian Lupine Albus (Lupine Blanc) which secretes Malatis in the soil and thus improves the availability of phosphorus.

Electrophysiological tests and overexpression in Arabidopsis Thaliana have established that Malatis mediated by Halalmt1 is released in the ground. Its activity has been further improved in the presence of aluminum, which can be toxic to plants in acidic soil. These results suggest that Halalmt1 help both mobilize phosphorus and reduce the toxicity of aluminum to plants by secretion of Malatis.

The researchers also found a unique Halalmt1 model of expression in the cluster roots which still contributes to survival in the soil deficient in phosphorus.

“Our results show that the cortex cells in the Hakea Laurina cluster rootlets are carboxylate and phosphatase acid secretion sites, potentially facilitating a rapid release of root exudates. Integrated sciences for life at the University of Hiroshima and the main research document.

Although the discovery of a new secretion path in the bunches roots has considerably contributed to the understanding of the field of plant survival mechanisms, additional questions remain.

“It is essential to acquire a complete understanding of the training and physiological functions of the cluster roots, as well as to identify the key factors which regulate them and to apply the exquisite strategies of phosphorus-acquisition of cluster roots to cultures.