Environmental Control of Phosphorus Acquisition: A Piece of the Molecular Framework Underlying Nutritional Homeostasis

Role of GARP family transcription factors in the regulatory network for nitrogen and phosphorus acquisition

The GARP (Golden2, ARR-B, Psr1) family proteins with a conserved DNA-binding domain, called the B-motif, are plant-specific transcription factors involved in the regulation of various physiological processes. The GARP family proteins are divided into members that function as monomeric transcription factors, and members that function as transcription factors in the dimeric form, owing to the presence of a coiled-coil dimerization domain. Recent studies revealed that the dimer-forming GARP family members, which are further divided into the PHR1 and NIGT1 subfamilies, play critical roles in the regulation of phosphorus (P) and nitrogen (N) acquisition. In this review, we present a general overview of the GARP family proteins and discuss how several members of the PHR1 and NIGT1 subfamilies are involved in the coordinated acquisition of P and N in response to changes in environmental nutrient conditions, while mainly focusing on the recent findings that enhance our knowledge of the roles of PHR1 and NIGT1 in phosphate starvation signaling and nitrate signaling.

Towards a Discovery of a Zinc-Dependent Phosphate Transport Road in Plants

Owing to the impending global scarcity of high-quality sources of phosphate (Pi) fertilizers, lowering its use in crop production requires improved insights into factors stimulating Pi uptake from the soil as well as the efficacious use by plants. Following decades of extensive research on plants' adaptation to Pi deficiency with mitigated success in the field, a better understanding of how plants exposed to zinc (Zn) deficiency accumulate much more Pi provides a novel strategy in comparison to when plants are grown in Zn-rich soils. In this context, we review current knowledge and molecular events involved in the Pi and Zn signaling crosstalk in plants that will bear great significance for agronomical and rudimentary research applications.

A crucial role for a node-localized transporter, HvSPDT, in loading phosphorus into barley grains

Grains are the major sink of phosphorus (P) in cereal crops, accounting for 60-85% of total plant P, but the mechanisms underlying P loading into the grains are poorly understood. We functionally characterized a transporter gene required for the distribution of P to the grains in barley (Hordeum vulgare), HvSPDT (SULTR-like phosphorus distribution transporter). HvSPDT encoded a plasma membrane-localized Pi/H⁺ cotransporter. It was mainly expressed in the nodes at both the vegetative and reproductive stages. Furthermore, its expression was induced by inorganic phosphate (Pi) deficiency. In the nodes, HvSPDT was expressed in both the xylem and phloem region of enlarged and diffuse vascular bundles. Knockout of HvSPDT decreased the distribution of P to new leaves, but increased the distribution to old leaves at the vegetative growth stage under low P supply. However, knockout of HvSPDT did not alter the redistribution of P from old to young organs. At the reproductive stage, knockout of HvSPDT significantly decreased P allocation to the grains, resulting in a considerable reduction in grain yield, especially under P-limited conditions. Our results indicate that node-based HvSPDT plays a crucial role in loading P into barley grains through preferentially distributing P from the xylem and further to the phloem.

Insights to improve the plant nutrient transport by CRISPR/Cas system

We need to improve food production to feed the ever growing world population especially in a changing climate. Nutrient deficiency in soils is one of the primary bottlenecks affecting the crop production both in developed and developing countries. Farmers are forced to apply synthetic fertilizers to improve the crop production to meet the demand. Understanding the mechanism of nutrient transport is helpful to improve the nutrient-use efficiency of crops and promote the sustainable agriculture. Many transporters involved in the acquisition, export and redistribution of nutrients in plants are characterized. In these studies, heterologous systems like yeast and Xenopus were most frequently used to study the transport function of plant nutrient transporters. CRIPSR/Cas system introduced recently has taken central stage for efficient genome editing in diverse organisms including plants. In this review, we discuss the key nutrient transporters involved in the acquisition and redistribution of nutrients from soil. We draw insights on the possible application CRISPR/Cas system for improving the nutrient transport in plants by engineering key residues of nutrient transporters, transcriptional regulation of nutrient transport signals, engineering motifs in promoters and transcription factors. CRISPR-based engineering of plant nutrient transport not only helps to study the process in native plants with conserved regulatory system but also aid to develop non-transgenic crops with better nutrient use-efficiency. This will reduce the application of synthetic fertilizers and promote the sustainable agriculture strengthening the food and nutrient security.

Plant adaptation to low phosphorus availability: Core signaling, crosstalks, and applied implications

Phosphorus (P) is an essential nutrient for plant growth and reproduction. Plants preferentially absorb P as orthophosphate (Pi), an ion that displays low solubility and that is readily fixed in the soil, making P limitation a condition common to many soils and Pi fertilization an inefficient practice. To cope with Pi limitation, plants have evolved a series of developmental and physiological responses, collectively known as the Pi starvation rescue system (PSR), aimed to improve Pi acquisition and use efficiency (PUE) and protect from Pi-starvation-induced stress. Intensive research has been carried out during the last 20 years to unravel the mechanisms underlying the control of the PSR in plants. Here we review the results of this research effort that have led to the identification and characterization of several core Pi starvation signaling components, including sensors, transcription factors, microRNAs (miRNAs) and miRNA inhibitors, kinases, phosphatases, and components of the proteostasis machinery. We also refer to recent results revealing the existence of intricate signaling interplays between Pi and other nutrients and antagonists, N, Fe, Zn, and As, that have changed the initial single-nutrient-centric view to a more integrated view of nutrient homeostasis. Finally, we discuss advances toward improving PUE and future research priorities.

Cross-Talks Between Macro- and Micronutrient Uptake and Signaling in Plants

In nature, land plants as sessile organisms are faced with multiple nutrient stresses that often occur simultaneously in soil. Nitrogen (N), phosphorus (P), sulfur (S), zinc (Zn), and iron (Fe) are five of the essential nutrients that affect plant growth and health. Although these minerals are relatively inaccessible to plants due to their low solubility and relative immobilization, plants have adopted coping mechanisms for survival under multiple nutrient stress conditions. The double interactions between N, Pi, S, Zn, and Fe have long been recognized in plants at the physiological level. However, the molecular mechanisms and signaling pathways underlying these cross-talks in plants remain poorly understood. This review preliminarily examined recent progress and current knowledge of the biochemical and physiological interactions between macro- and micro-mineral nutrients in plants and aimed to focus on the cross-talks between N, Pi, S, Zn, and Fe uptake and homeostasis in plants. More importantly, we further reviewed current studies on the molecular mechanisms underlying the cross-talks between N, Pi, S, Zn, and Fe homeostasis to better understand how these nutrient interactions affect the mineral uptake and signaling in plants. This review serves as a basis for further studies on multiple nutrient stress signaling in plants. Overall, the development of an integrative study of multiple nutrient signaling cross-talks in plants will be of important biological significance and crucial to sustainable agriculture.

Arabidopsis Restricts Sugar Loss to a Colonizing Trichoderma harzianum Strain by Downregulating SWEET11 and -12 and Upregulation of SUC1 and SWEET2 in the Roots

Phosphate (Pi) availability has a strong influence on the symbiotic interaction between Arabidopsis and a recently described root-colonizing beneficial Trichoderma harzianum strain. When transferred to media with insoluble Ca₃(PO₄)₂ as a sole Pi source, Arabidopsis seedlings died after 10 days. Trichoderma grew on the medium containing Ca₃(PO₄)₂ and the fungus did colonize in roots, stems, and shoots of the host. The efficiency of the photosynthetic electron transport of the colonized seedlings grown on Ca₃(PO₄)₂ medium was reduced and the seedlings died earlier, indicating that the fungus exerts an additional stress to the plant. Interestingly, the fungus initially alleviated the Pi starvation response and did not activate defense responses against the hyphal propagation. However, in colonized roots, the sucrose transporter genes SWEET11 and -12 were strongly down-regulated, restricting the unloading of sucrose from the phloem parenchyma cells to the apoplast. Simultaneously, up-regulation of SUC1 promoted sucrose uptake from the apoplast into the parenchyma cells and of SWEET2 sequestration of sucrose in the vacuole of the root cells. We propose that the fungus tries to escape from the Ca₃(PO₄)₂ medium and colonizes the entire host. To prevent excessive sugar consumption by the propagating hyphae, the host restricts sugar availability in its apoplastic root space by downregulating sugar transporter genes for phloem unloading, and by upregulating transporter genes which maintain the sugar in the root cells.