A novel method creates a window for studying the transport and distribution of elements in plants: Study

Gatersleben [Germany], June 13 : Plant roots are crucial in the uptake, selection, enrichment, and retention of a variety of mineral elements, which supplies distant plant tissues with nutrients and sequesters excess metals. A variety of ion transporters found at roots mediate the absorption, efflux, and intracellular compartmentalization of various mineral elements to carry out such element-specific tasks.

The majority of ion transporters have distinctive tissue- and cell-type-specific localization patterns that can change in response to both internal and external signals. Finding out how many different ion transporters and transport routes contribute to the distribution of elements in cells and tissues will help us better understand the function of these systems in roots.

The best method for simultaneous quantification of multiple elements is inductively couple plasma mass spectrometry (ICP-MS). However, ICP-MS is still largely restricted to the analysis of whole-tissues instead of single tissues or specific cell types. Overcoming this limitation would allow to simultaneously map the distribution of several mineral elements along different root cell layers, a critical step to fully understand how roots protect highly sensitive stem cells from toxic elements but share essential and beneficial elements with aboveground parts.

"With this in mind, we developed a method in which distinct cell types isolated from roots of various reporter lines are separated via fluorescence-activated cell sorting prior to elemental analysis with ICP-MS", says Dr. Ricardo Giehl, first author of the study. "Our new method enabled us to determine the concentration of up to 11 mineral elements in different cell types, and to explore the consequences of perturbed xylem loading or altered nutrient availabilities at high spatial resolution."

The researchers used the new FACS-ICP-MS method to reveal significant cell type-specific element distribution and the existence of a steep concentration gradient between outer and inner cell layers in roots. "Furthermore, the cellular concentration ranges for most macro- and micronutrients estimated with our method can serve as reference for future studies", emphasises Prof. Dr. Nicolaus von Wiren, head of IPK's research department "Physiology and Cell Biology".

The method also helped the researchers to identify a cell type-specific enrichment of manganese in roots of plants exposed to iron-limiting conditions. By installing a manganese sequestration mechanism in specific cell types, the researchers uncovered that root hairs play a critical role in retaining the excess manganese taken up by iron-deficient plants, thereby preventing that toxic concentrations of manganese build up in shoots.

"Our results highlight the importance of the particular "topographical" placement of ion transporters for directing radial movement of ions destined to shoots or for efficient metal sequestration in roots", says Dr. Ricardo Giehl. "The possibility to combine our method with transcriptomics and to develop it further toward single cell ICP-MS offers the possibility to investigate transcriptome-ionome networks at very high spatial resolution. This knowledge is critical to understand and manipulate transport pathways in order to increase nutrient use efficiency while simultaneously preventing accumulation of toxic elements in aboveground tissues."

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