Phosphorus (P) is a non-renewable resource that limits plant productivity due to its low bioavailability in the soil. Large amounts of P fertilizer are required to sustain high yields, which is both inefficient and hazardous to the environment. Plants have evolved various adaptive responses to cope with low external P availability, including mobilizing cellular P through phosphate (Pi) transporters and recycling Pi from P-containing biomolecules to maintain cellular P homeostasis. This mini-review summarizes the current research on intracellular P recycling and mobilization in response to P availability. We introduce the roles of Pi transporters and the P metabolic enzymes and expand on their gene regulation and mechanisms. The relevance of these processes in the search for targets to improve phosphorus use efficiency and some of the current challenges and gaps in our understanding of P starvation responses are discussed.

Calcium ions (Ca2+) play pivotal roles in a host of cellular signalling processes. The requirement to maintain resting cytosolic Ca2+ levels in the 100–200 nM range provides a baseline for dynamic excursions from resting levels that determine the nature of many physiological responses to external stimuli and developmental processes. This review provides an overview of the key components of the Ca2+ homeostatic machinery, including known channel-mediated Ca2+ entry pathways along with transporters that act to shape the cytosolic Ca2+ signature. The relative roles of the vacuole and endoplasmic reticulum as sources or sinks for cytosolic Ca2+ are considered, highlighting significant gaps in our understanding. The components contributing to mitochondrial, chloroplast and nuclear Ca2+ homeostasis and organellar Ca2+ signals are also considered. Taken together, a complex picture of the cellular Ca2+ homeostatic machinery emerges with some clear differences from mechanisms operating in many animal cells.

The micronutrient zinc (Zn) is often poorly available but toxic when present in excess, so a tightly controlled Zn homoeostasis network operates in all organisms. This review summarizes our present understanding of plant Zn homoeostasis. In Arabidopsis, about 1,900 Zn-binding metalloproteins require Zn as a cofactor. Abundant Zn metalloproteins reside in plastids, mitochondria and peroxisomes, emphasizing the need to address how Zn reaches these proteins. Apo–Zn metalloproteins do not acquire Zn2+ from a cytosolic pool of free cations, but instead through associative ligand exchange from Zn-buffering molecules. The importance of cytosolic thiols in Zn buffering suggests that, besides elevated Zn influx, a more oxidized redox state is also predicted to cause elevated labile-bound Zn levels, consistent with the suppression of a Zn deficiency marker under oxidative stress. Therefore, we consider a broadened physiological scope in plants for a possible signalling role of Zn2+, experimentally supported only in animals to date.

Homeostats are important to control homeostatic conditions. Here, we have analyzed the theoretical basis of their dynamic properties by bringing the K homeostat out of steady state (i) by an electrical stimulus, (ii) by an external imbalance in the K+ or H+ gradient or (iii) by a readjustment of transporter activities. The reactions to such changes can be divided into (i) a short-term response (tens of milliseconds), where the membrane voltage changed along with the concentrations of ions that are not very abundant in the cytosol (H+ and Ca2+), and (ii) a long-term response (minutes and longer) caused by the slow changes in K+ concentrations. The mechanistic insights into its dynamics are not limited to the K homeostat but can be generalized, providing a new perspective on electrical, chemical, hydraulic, pH and Ca2+ signaling in plants. The results presented here also provide a theoretical background for optogenetic experiments in plants.

Potassium is an essential macronutrient required for plant growth and development. Over the recent decade, an important signalling role of K+ has emerged. Here, we discuss some aspects of such signalling at the various levels of plant functional organisation. The topic covered include: (1) mechanisms of long-distant K+ transport in the xylem and phloem and the molecular identity and regulation of K+ loading and unloading into plant vasculature; (2) essentiality and physiological roles of K+ cycling between shoots and roots; (3) plant sensing and signalling of low K+; (4) maintenance of K+ homeostasis at the cellular level; (5) stress-induced modulation of cytosolic K+ as a signal in plant adaptive responses to hostile environment; (6) stress-specific K+ “signatures” and mechanisms of their decoding by regulation of purine metabolism and H+-ATPase activity; (7) cytosolic K+ loss as a metabolic switch and a regulator of autophagy; and (8) vacuolar K+ transport and sensing.

Iron (Fe) is an essential element in plants, involved in numerous metabolic processes including photosynthesis. Its cellular concentration must be regulated accurately to avoid toxicity while meeting metabolic demands. This review explores the distribution, dynamics, and regulation of Fe pools in plant cells, focusing on recent advances in imaging and quantification techniques. We discuss the major Fe compartments—chloroplasts, vacuoles, apoplasts—and their interaction to maintain Fe homeostasis, as well as novel methodologies like single-cell ICP-MS that have transformed our understanding of Fe localization. By summarizing the current knowledge on intracellular Fe dynamics and the complex interplay between different Fe pools, we provide insights into the mechanisms that underpin Fe regulation in plants, which is crucial for future breeding programs aimed at improving plant resilience and nutritional quality.

Silicon (Si), the most abundant mineral element in soil, functions as a beneficial element for plant growth. Higher Si accumulation in the shoots is required for high and stable production of rice, a typical Si-accumulating plant species. During the last two decades, great progresses has been made in the identification of Si transporters involved in uptake, xylem loading and unloading as well as preferential distribution and deposition of Si in rice. In addition to these transporters, simulation by mathematical models revealed several other key factors required for efficient uptake and distribution of Si. The expression of Lsi1, Lsi2 and Lsi3 genes is down-regulated by Si deposition in the shoots rather than in the roots, but the exact mechanisms underlying this down-regulation are still unknown. In this short review, we focus on Si transporters identified in rice and discuss how rice optimizes Si accumulation (“homeostasis”) through regulating Si transporters in response to the fluctuations of this element in the soil solution.

The micronutrient chloride (Cl―) plays key roles in plant physiology, from photosystem II and vacuolar ATPase activity to osmoregulation, turgor maintenance and drought resilience, while also posing toxicity risks at high concentrations. This review examines Cl― uptake, transport and homeostasis, focussing on adaptations balancing its dual roles as a nutrient and toxicant. Key transporters, including NPF, SLAH, ALMT, CLC and CCC families, mediate Cl― fluxes to maintain ionic balance and prevent toxicity. Plants employ strategies such as selective uptake and vacuolar compartmentalization to cope with high salinity. Cl― also influences nitrogen-use efficiency and plant productivity. Advances in transporter biology reveal the role of Cl― in water-use efficiency, drought resilience and stress adaptation.