Structure-Function Analysis Reveals Amino Acid Residues of Arabidopsis Phosphate Transporter AtPHT1;1 Crucial for Its Activity

Serendipita indica promotes P acquisition and growth in tea seedlings under P deficit conditions by increasing cytokinins and indoleacetic acid and phosphate transporter gene expression

The culturable endophytic fungus Serendipita indica has many beneficial effects on plants, but whether and how it affects physiological activities and phosphorus (P) acquisition of tea seedlings at low P levels is unclear. The objective of this study was to analyze the effects of inoculation with S. indica on growth, gas exchange, chlorophyll fluorescence, auxins, cytokinins, P levels, and expressions of two phosphate transporter (PT) genes in leaves of tea (Camellia sinensis L. cv. Fudingdabaicha) seedlings grown at 0.5 μM (P_0.5) and 50 μM (P₅₀) P levels. Sixteen weeks after the inoculation, S. indica colonized roots of tea seedlings, with root fungal colonization rates reaching 62.18% and 81.34% at P_0.5 and P₅₀ levels, respectively. Although plant growth behavior, leaf gas exchange, chlorophyll values, nitrogen balance index, and chlorophyll fluorescence parameters of tea seedlings were suppressed at P_0.5 versus P₅₀ levels, inoculation of S. indica mitigated the negative effects to some extent, along with more prominent promotion at P_0.5 levels. S. indica inoculation significantly increased leaf P and indoleacetic acid concentrations at P_0.5 and P₅₀ levels and leaf isopentenyladenine, dihydrozeatin, and transzeatin concentrations at P_0.5 levels, coupled with the reduction of indolebutyric acid at P₅₀ levels. Inoculation of S. indica up-regulated the relative expression of leaf CsPT1 at P_0.5 and P₅₀ levels and CsPT4 at P_0.5 levels. It is concluded that S. indica promoted P acquisition and growth in tea seedlings under P deficit conditions by increasing cytokinins and indoleacetic acid and CsPT1 and CsPT4 expression.

Selenate and selenite transporters in proso millet: Genome extensive detection and expression studies under salt stress and selenium

Crops are susceptible to a variety of stresses and amongst them salinity of soil is a global agronomic challenge that has a detrimental influence on crop yields, thus posing a severe danger to our food security. Therefore, it becomes imperative to examine how plants respond to salt stress, develop a tolerance that allows them to live through higher salt concentrations and choose species that can endure salt stress. From the perspective of food, security millets can be substituted to avoid hardships because of their efficiency in dealing with salt stress. Besides, this problem can also be tackled by using beneficial exogenous elements. Selenium (Se) which exists as selenate or selenite is one such cardinal element that has been reported to alleviate salt stress. The present study aimed for identification of selenate and selenite transporters in proso millet (Panicum miliaceum L.), their expression under NaCl (salt stress) and Na₂SeO₃ (sodium selenite)treatments. This study identified eight transporters (RLM65282.1, RLN42222.1, RLN18407.1, RLM74477.1, RLN41904.1, RLN17428.1, RLN17268.1, RLM65753.1) that have a potential role in Se uptake in proso millet. We analyzed physicochemical properties, conserved structures, sub-cellular locations, chromosome location, molecular phylogenetic analysis, promoter regions prediction, protein-protein interactions, three-dimensional structure modeling and evaluation of these transporters. The analysis revealed the chromosome location and the number of amino acids present in these transporters as RLM65282.1 (16/646); RLN42222.1 (1/543); RLN18407.1 (2/483); RLM74477.1 (15/474); RLN41904.1 (1/521); RLN17428.1 (2/522); RLN17268.1(2/537);RLM65753.1 (16/539). The sub-cellular locations revealed that all the selenite transporters are located in plasma membrane whereas among selenate transporters RLM65282.1 and RLM74477.1 are located in mitochondria and RLN42222.1 and RLN18407.1 in chloroplast. The transcriptomic studies revealed that NaCl stress decreased the expression of both selenate and selenite transporters in proso millet and the applications of exogenous 1µM Se (Na₂SeO₃) increased the expression of these Se transporter genes. It was also revealed that selenate shows similar behavior as sulfate, while selenite transport resembles phosphate. Thus, it can be concluded that phosphate and sulphate transporters in millets are responsible for Se uptake.

The Poly-Glutamate Motif of GmMATE4 Regulates Its Isoflavone Transport Activity

Multidrug and toxic compound extrusion (MATE) transporters in eukaryotes have been characterized to be antiporters that mediate the transport of substrates in exchange for protons. In plants, alkaloids, phytohormones, ion chelators, and flavonoids have been reported to be the substrates of MATE transporters. Structural analyses have been conducted to dissect the functional significance of various motifs of MATE proteins. However, an understanding of the functions of the N- and C-termini has been inadequate. Here, by performing phylogenetic analyses and protein sequence alignment of 14 representative plant species, we identified a distinctive N-terminal poly-glutamate motif among a cluster of MATE proteins in soybean. Amongst them, GmMATE4 has the most consecutive glutamate residues at the N-terminus. A subcellular localization study showed that GmMATE4 was localized at the vacuolar membrane-like structure. Protein charge prediction showed that the mutation of the glutamate residues to alanine would reduce the negative charge at the N-terminus. Using yeast as the model, we showed that GmMATE4 mediated the transport of daidzein, genistein, glycitein, and glycitin. In addition, the glutamate-to-alanine mutation reduced the isoflavone transport capacity of GmMATE4. Altogether, we demonstrated GmMATE4 as an isoflavone transporter and the functional significance of the N-terminal poly-glutamate motif of GmMATE4 for regulating the isoflavone transport activity.

Direct inhibition of phosphate transport by immune signaling in Arabidopsis

Soil availability of inorganic ortho-phosphate (PO₄^3-, P_i) is a key determinant of plant growth and fitness.¹ Plants regulate the capacity of their roots to take up inorganic phosphate by adapting the abundance of H⁺-coupled phosphate transporters of the PHOSPHATE TRANSPORTER 1 (PHT1) family² at the plasma membrane (PM) through transcriptional and post-translational changes driven by the genetic network of the phosphate starvation response (PSR).^3-8 Increasing evidence also shows that plants integrate immune responses to alleviate phosphate starvation stress through the association with beneficial microbes.^9-11 Whether and how such phosphate transport is regulated upon activation of immune responses is yet uncharacterized. To address this question, we first developed quantitative assays based on changes in the electrical PM potential to measure active P_i transport in roots in real time. By inserting micro-electrodes into bulging root hairs, we were able to determine key characteristics of phosphate transport in intact Arabidopsis thaliana (hereafter Arabidopsis) seedlings. The fast P_i-induced depolarization observed was dependent on the activity of the major phosphate transporter PHT1;4. Notably, we observed that this PHT1;4-mediated phosphate uptake is repressed upon activation of pattern-triggered immunity. This inhibition depended on the receptor-like cytoplasmic kinases BOTRYTIS-INDUCED KINASE 1 (BIK1) and PBS1-LIKE KINASE 1 (PBL1), which both phosphorylated PHT1;4. As a corollary to this negative regulation of phosphate transport by immune signaling, we found that PHT1;4-mediated phosphate uptake normally negatively regulates anti-bacterial immunity in roots. Collectively, our results reveal a mechanism linking plant immunity and phosphate homeostasis, with BIK1/PBL1 providing a molecular integration point between these two important pathways.

Regulation of Cytosolic pH: The Contributions of Plant Plasma Membrane H+-ATPases and Multiple Transporters

Cytosolic pH homeostasis is a precondition for the normal growth and stress responses in plants, and H+ flux across the plasma membrane is essential for cytoplasmic pH control. Hence, this review focuses on seven types of proteins that possess direct H+ transport activity, namely, H+-ATPase, NHX, CHX, AMT, NRT, PHT, and KT/HAK/KUP, to summarize their plasma-membrane-located family members, the effect of corresponding gene knockout and/or overexpression on cytosolic pH, the H+ transport pathway, and their functional regulation by the extracellular/cytosolic pH. In general, H+-ATPases mediate H+ extrusion, whereas most members of other six proteins mediate H+ influx, thus contributing to cytosolic pH homeostasis by directly modulating H+ flux across the plasma membrane. The fact that some AMTs/NRTs mediate H+-coupled substrate influx, whereas other intra-family members facilitate H+-uncoupled substrate transport, demonstrates that not all plasma membrane transporters possess H+-coupled substrate transport mechanisms, and using the transport mechanism of a protein to represent the case of the entire family is not suitable. The transport activity of these proteins is regulated by extracellular and/or cytosolic pH, with different structural bases for H+ transfer among these seven types of proteins. Notably, intra-family members possess distinct pH regulatory characterization and underlying residues for H+ transfer. This review is anticipated to facilitate the understanding of the molecular basis for cytosolic pH homeostasis. Despite this progress, the strategy of their cooperation for cytosolic pH homeostasis needs further investigation.

Plant resilience to phosphate limitation: current knowledge and future challenges

Phosphorus (P) is an essential macronutrient for all living organisms. Importantly, plants require a large amount of P to grow, and P deficiency causes huge losses in plant production. Although this issue can be mitigated by the appropriate use of phosphate (Pi) rock-derived P fertilizers, phosphate rock is a finite natural resource. Moreover, the increased demand for food as a result of our growing global population is another factor contributing to a prospective P crisis. While creating crops that are resilient to Pi deficiency presents great scientific challenge, the current progress in our understanding of how plants regulate Pi homeostasis offers some opportunities for further study. In this review, we present the published research supporting these opportunities, which are based on the molecular mechanisms that plants have evolved to respond to P deficiency. First, we focus on recent advances in P sensing and signaling pathways in the regulation of root system architecture. Next, we describe the mechanisms that regulate Pi transport and accumulation, in a Pi- (or other nutrient) dependent manner. Integrating these data will help to design an innovative strategy for improving Pi nutrition in plants. In addition, this will help with Pi scarcity, one of the challenges facing agriculture in the twenty first century.