The trans-Golgi-localized protein BICAT3 regulates manganese allocation and matrix polysaccharide biosynthesis

Transit of NEAT1 and MTP11 to the plasma membrane and co-localization to vesicles support a role for exocytosis-mediated export in metal homeostasis

Understanding the role and mode of action of nutrient transporters requires information about their dynamic associations with plant membranes. Historically, apoplastic nutrient export has been associated with proteins localized at the plasma membrane (PM), while the role of endomembrane localization has been less explored. However, recent work on the PHOSPHATE 1 (PHO1) inorganic phosphate (Pi) exporter demonstrated that, although primarily localized at the Golgi and trans-Golgi network (TGN) vesicles, PHO1 does associate with the PM when clathrin-mediated endocytosis (CME) was inhibited, supporting a mechanism for Pi homeostasis involving exocytosis. We explored whether CME inhibition can identify other transporters that, although primarily localized at Golgi/TGN at steady-state level, also transit via the PM and are potentially involved in export via exocytosis. We found that, similar to PHO1, Golgi-localized transporters NA EFFLUX TRANSPORTER1 (NAET1) and METAL TOLERANCE PROTEIN11 (MTP11) relocate to the PM when CME is inhibited, both transiently in Nicotiana benthamiana and conditionally in Arabidopsis thaliana. Such PM re-localization of transporters upon CME inhibition is specific, since it does not occur with several other Golgi-associated transporters, including MTP5 and BIVALENT CATION TRANSPORTER 3 (BICAT3), as well as resident Golgi/TGN membrane proteins, such as α-1,2-MANNOSIDASE I (Man1) and VESICLE TRANSPORT V-SNARE 12 (VTI12). Additionally, we observed that NAET1, MTP11 and PHO1 all partially co-localize to vesicles. Overall, our study supports a role for synaptic-like vesicle-mediated exocytosis for both NEAT1 and MTP11 in nutrient transport in plants and highlights the importance of assessing the transient localization of Golgi/TGN proteins to the PM.

Mn-CDF family genes enhance the manganese tolerance of the tea plants (Camellia sinensis) under acidic condition

The tea plants cultivated in acidic soils are vulnerable to excessive manganese (Mn), which increases the risk of Mn2+ toxicity to physiology and development. Mn-cation diffusion facilitator (CDF) family genes have been implicated in regulating Mn homeostasis and tolerance. However, the mechanism of Mn tolerance of tea plants in acidic environments is still unknown. In this study, we initially examined the phenotypic characteristics and Mn contents variability in different tissues of tea plants under various Mn concentration at pH 5 and 4. We observed that tea plants exhibited remarkably high Mn tolerance at pH 4, with Mn accumulation notably elevated in the aboveground tissues under pH 4 condition after 28-day treatment. We found the expression levels of Mn-CDF genes, had different subcellular localization, were tissue-specific and significantly induced by high Mn concentrations at pH 4 condition. Furthermore, the yeast complementation assays indicated that the heterologous expression of Mn-CDF genes restored the growth of a Mn2+ sensitive yeast strain, Δpmr1. Taken together, these results suggest that Mn-CDF family genes function as Mn transporters to participate in Mn tolerance in acidic environments. This study provides reference for further study on the mechanism of maintaining Mn homeostasis in tea plants under soil acidification.

The Golgi-localized transporter OsPML4 contributes to manganese homeostasis in rice

Manganese (Mn), an indispensable plant micronutrient, functions as a vital enzyme co-factor in numerous biochemical reactions. In rice, the Golgi-localized PHOTOSYNTHESIS-AFFECTED MUTANT 71-LIKE 3 (OsPML3), a member of the UNCHARACTERIZED PROTEIN FAMILY (UPF0016), plays a pivotal role in Mn homeostasis, particularly in rapidly developing tissues. This study focused on the functional characterization of another UPF0016 family member in rice, OsPML4, to elucidate its involvement in Mn homeostasis. OsPML4 had a 73% sequence identity with OsPML3 and exhibited expression in both shoots and roots, albeit at a lower transcriptional level than OsPML3. Furthermore, subcellular localization studies confirmed that OsPML4 localizes in the Golgi apparatus. Notably, heterologous expression of OsPML4 restored growth in the Mn uptake-deficient yeast strain Δsmf1 under Mn-limited conditions. Under Mn-deficient conditions, OsPML4 knockout exacerbated the decline in shoot dry weight and intensified necrosis in young leaves of OsPML3 knockout lines, which displayed stunted growth. The Mn concentration in OsPML3PML4 double knockout lines was lower than in wild-type (WT) and OsPML3 knockout lines. At the reproductive phase, OsPML3PML4 double knockout lines exhibited reduced fertility and grain yield compared to WT and OsPML3 knockout lines. Notably, reductions were observed in the deposition of cell wall polysaccharides and the content of Lea (Lewis A structure)-containing N-glycans in the young leaves of OsPML3PML4 double knockout lines, surpassing the reductions in WT and OsPML3 knockout lines. These findings underscore the significance of OsPML4 in Mn homeostasis in the Golgi apparatus, where it co-functions with OsPML3 to regulate cell wall polysaccharide deposition and late-stage Golgi N-glycosylation.

Golgi in and out: multifaceted role and journey of manganese

Manganese (Mn) is pivotal for plant growth and development but little is known about the processes that control its homeostasis in the cell. A spotlight on the pools of intracellular manganese and their cellular function has recently been gained through the characterization of new Mn transporters. In particular, transporters catalyzing the ins and outs of Mn at the various Golgi membranes have revealed the central role of the Golgi pool of Mn in the synthesis of the cell wall and as a reservoir for the numerous cellular Mn-dependent pathways whose calibration relies on a set of Golgi-resident transporters of the BICAT and NRAMP families.

Cation diffusion facilitator proteins of Beta vulgaris reveal diversity of metal handling in dicotyledons

Manganese (Mn), iron (Fe), and zinc (Zn) are essential for diverse processes in plants, but their availability is often limiting or excessive. Cation diffusion facilitator (CDF) proteins have been implicated in the allocation of those metals in plants, whereby most of our mechanistic understanding has been obtained in Arabidopsis. It is unclear to what extent this can be generalized to other dicots. We characterized all CDFs/metal tolerance proteins of sugar beet (Beta vulgaris spp. vulgaris), which is phylogenetically distant from Arabidopsis. Analysis of subcellular localization, substrate selectivities, and transcriptional regulation upon exposure to metal deficiencies and toxicities revealed unexpected deviations from their Arabidopsis counterparts. Localization and selectivity of some members were modulated by alternative splicing. Notably, unlike in Arabidopsis, Mn- and Zn-sequestrating members were not induced in Fe-deficient roots, pointing to differences in the Fe acquisition machinery. This was supported by low Zn and Mn accumulation under Fe deficiency and a strikingly increased Fe accumulation under Mn and Zn excess, coinciding with an induction of BvIRT1. High Zn load caused a massive upregulation of Zn-BvMTPs. The results suggest that the employment of the CDF toolbox is highly diverse amongst dicots, which questions the general applicability of metal homeostasis models derived from Arabidopsis.

Open questions in plant cell wall synthesis

Plant cells are surrounded by strong yet flexible polysaccharide-based cell walls that support the cell while also allowing growth by cell expansion. Plant cell wall research has advanced tremendously in recent years. Sequenced genomes of many model and crop plants have facilitated cataloging and characterization of many enzymes involved in cell wall synthesis. Structural information has been generated for several important cell wall synthesizing enzymes. Important tools have been developed including antibodies raised against a variety of cell wall polysaccharides and glycoproteins, collections of enzyme clones and synthetic glycan arrays for characterizing enzymes, herbicides that specifically affect cell wall synthesis, live-cell imaging probes to track cell wall synthesis, and an inducible secondary cell wall synthesis system. Despite these advances, and often because of the new information they provide, many open questions about plant cell wall polysaccharide synthesis persist. This article highlights some of the key questions that remain open, reviews the data supporting different hypotheses that address these questions, and discusses technological developments that may answer these questions in the future.