Soil metals, chloroplasts, and secure crop production: a review

Abstract

with unbalanced metal concentrations are used worldwide for crop cultivation. Even though plants are able to develop strategies to cope with metal stress, either metal deficiency or excess, this unbalance affects the whole plant. Chloroplasts are key organelles for organic matter synthesis and biomass production. Under metal stress, chloroplasts suffer severe alterations of their ultrastructure, associated with profound molecular and metabolic damages. These alterations are accompanied by unbalanced metal distribution in plants, in particular in edible crop organs. Toxic metals get either accumulated or there is a deficiency of nutrients, resulting in a weak nutritional value. Nonetheless, there is more and more knowledge on the functioning and regulation of metal transporters in plants. Such knowledge will allow growing crops with well-balanced metal concentrations in edible parts even on metal unbalanced soils. This review shows that almost all vital functions of chloroplasts, such as photosynthesis, CO 2 fixation, nitrogen and sulfur assimilation, and protein and nucleic acid metabolism, require metals. Therefore, the uptake of essential metals is necessary for the proper functioning of chloroplasts and, in turn, for crop productivity. We describe nutrient uptake mechanisms of plants and processes that influence essential and non-essential metal concentrations in different plant organs. We present an overview of metal transporters in chloroplasts. Several questions still need to be elucidated about the uptake and the trafficking of essential and non-essential metals into and within chloroplasts. Similarly to transporters present in other cellular compartments, the carriers are often not metal-specific. Therefore, essential and non-essential metals may compete for carriers. As a result, unbalanced soil metal concentrations can be reflected in the plants and in the chloroplasts. Metal deficiency or excess causes reduced growth and decreased productivity of crops. It can lead to human malnutrition. Several complex physiological processes can be responsible for the reduced biomass observed in plants with unbalanced metal concentration. In this review, we have focused on the structural and functional alterations of chloroplasts under metal deficiency or excess. Interestingly, besides specific differences, our data indicate several similarities in the response of chloroplasts to metal deficiency or excess. Indeed, oxidative stress and several ultrastructural alterations, e.g., increase in the number and size of plastoglobuli, disorganized grana and disturbed thylakoids, and swelling of the intrathylakoidal space, are observed in both cases. This indicates that changes in chloroplast ion homeostasis rather than the specific effect of a metal are responsible for decreased plant productivity. Therefore, sustainable agriculture has to take into consideration solutions that enable undisturbed metal and ion homeostasis in chloroplasts of crop plants grown even in soils with unbalanced metal concentrations.Abstract An incr...

Transplastomic plants for innovations in agriculture. A review

“…It also occurs during aging and strongly impacts the structure and function of plastids (Solymosi and Bertrand 2012). Therefore, fine tuning the ROS-scavengi ng antioxidant defense system of transplastomic plants may have a strong effect on their yield and adaptation to various stress conditions, and also contributes to a better understanding of its role in general plant stress tolerance.…”

“…Below, we only discuss data about transplastomic plants with increased food functionality (Table 4). Most available data on biofortification (reviewed in Hirschi 2008;Solymosi and Bertrand 2012) merely demonstrate the improved nutritional content of the new cultivars, but the bioavailability and pharmacological studies by controlled animal and human experiments are often missing, which are crucial for the public acceptance of genetic engineering and also for decision makers to assess the real significance of this molecular approach (NewellMcGloughlin 2010).…”

“…Therefore, breeding of biofortified crops containing increased levels of essential minerals represent an important goal in agriculture (Palmgren et al 2008;White and Broadley 2009;Gómez-Galera et al 2010;Solymosi and Bertrand 2012). Excess metals are mostly sequestered in the vacuole or the cell wall.…”

“…They include important competition/interactions between different metals, and involve also problems about the bioavailability of metals in the soil, processes that are still not very well understood. However, plastids harbor several metalloenzymes and also play a role for example in iron homeostasis by storing excess iron in the form of ferritin inclusions (reviewed in Solymosi and Bertrand 2012;Solymosi and Keresztes 2012). In addition, they are able to express different proteins at very high levels.…”

“…Exploitation, improper land use, and heavy chemical inputs, e.g., from pesticides and fertilizers, by the modern, intensive agriculture in order to achieve high crop yields in monocultures (Zuo and Zhang 2009) often result in pollution and nutrient imbalance in the soil. These perturbances lead to the accumulation of harmful compounds or nutrient deficiency, respectively, in edible plant parts (Khoshgoftarmanesh et al 2010;Solymosi and Bertrand 2012). However, farmers and companies do not necessarily take into account the long-term consequences of their land use and strive, instead, to maximize their profit on a short term.…”