The Mineral Nutrition Challenge

Plants are unique among multicellular organisms in their ability to build every organic molecule from inorganic parts, namely, CO₂, H₂O and mineral nutrients. Terrestrial plants have to acquire most mineral nutrients from the soil—an extremely complex, diverse and heterogeneous substrate. With the exception of carnivorous plants, uptake of nutrients by the leaves plays only a minor role and will not be described in detail here. Volatile N- and S-containing molecules can enter the plant via stomata and then be utilised. Another source of nutrients can be the rainwater on leaves of plants such as mosses and epiphytes.

Unlike heterotrophic organisms, which ingest biological material with an elemental composition that is already close to physiological requirements and is fairly balanced with respect to the relative quantities of mineral nutrients (e.g. more N than S, more Zn than Mo), plants depend on a soil solution that under most circumstances shows very low and unbalanced concentrations of nutrients. Thus, a fundamental characteristic of plant nutrition is the enrichment of nutrients relative to the environment. For some elements the enrichment factors exceed 1000—that is, the concentration is 1000-fold higher in the plant than in the soil solution (e.g. K⁺; the soil solution concentration can be below 0.1 mM; the cellular concentration is around 100 mM). Furthermore, the nutrients are present in the environment in extremely fluctuating ratios that can be very far from the ratios needed physiologically.

When the relationship between the supply of a particular nutrient and the growth response is plotted, three regions of the curve can be distinguished: the deficiency range, where the growth response to supply is essentially linear; the adequate range, where an additional supply does not result in further growth stimulation; and the toxicity range, where the concentration of a nutrient is too high and causes growth reduction (Fig. 7.2). Both the deficiency and the toxicity ranges represent a stress condition that plants have to cope with. Hence, both are topics discussed in this chapter.

Fig. 7.2. Growth response of plants to the supply of essential elements and non-essential toxic elements. a Zn as an example of an essential microelement. b Cd as an example of a non-essential toxic element. (Modified from Lin and Aarts (2012))

In the following sections we will first describe the challenges arising from the characteristics of the soil substrate. Then the strategies of plants to cope with nutrient scarcity are discussed, including the two major symbioses: mycorrhizae and biological N₂ fixation. Next are responses to toxicity. Molecular mechanisms underlying the adaptations to particular adverse soil conditions are explained along the way.

Because of their charge, ions cannot pass through biological membranes passively. Thus, transport across membranes plays key roles in mineral nutrition. Consequently, basic features of plant ion transport, as well as the sensing of nutrient status and the regulation of ion transport, are major themes throughout this chapter.