Search

Manganese (Mn2+) is essential for a diversity of processes, including photosynthetic water splitting and the transfer of glycosyl moieties. Various Golgi-localized glycosyltransferases that mediate cell wall matrix polysaccharide biosynthesis are Mn2+ dependent, but the supply of these enzymes with Mn2+ is not well understood. Here, we show that the BIVALENT CATION TRANSPORTER 3 (BICAT3) localizes specifically to trans-cisternae of the Golgi. In agreement with a role in Mn2+ and Ca2+ homeostasis, BICAT3 rescued yeast (Saccharomyces cerevisiae) mutants defective in their translocation. Arabidopsis (Arabidopsis thaliana) knockout mutants of BICAT3 were sensitive to low Mn2+ and high Ca2+ availability and showed altered accumulation of these cations. Despite reduced cell expansion and leaf size in Mn2+-deficient bicat3 mutants, their photosynthesis was improved, accompanied by an increased Mn content of chloroplasts. Growth defects of bicat3 corresponded with an impaired glycosidic composition of matrix polysaccharides synthesized in the trans-Golgi. In addition to the vegetative growth defects, pollen tube growth of bicat3 was heterogeneously aberrant. This was associated with a severely reduced and similarly heterogeneous pectin deposition and caused diminished seed set and silique length. Double mutant analyses demonstrated that the physiological relevance of BICAT3 is distinct from that of ER-TYPE CA2+-ATPASE 3, a Golgi-localized Mn2+/Ca2+-ATPase. Collectively, BICAT3 is a principal Mn2+ transporter in the trans-Golgi whose activity is critical for specific glycosylation reactions in this organelle and for the allocation of Mn2+ between Golgi apparatus and chloroplasts.

Calcium (Ca2+) and manganese (Mn2+) are essential elements for plants and have similar ionic radii and binding coordination. They are assigned specific functions within organelles, but share many transport mechanisms to cross organellar membranes. Despite their points of interaction, those elements are usually investigated and reviewed separately. This review takes them out of this isolation. It highlights our current mechanistic understanding and points to open questions of their functions, their transport, and their interplay in the endoplasmic reticulum (ER), vesicular compartments (Golgi apparatus, trans-Golgi network, pre-vacuolar compartment), vacuoles, chloroplasts, mitochondria, and peroxisomes. Complex processes demanding these cations, such as Mn2+-dependent glycosylation or systemic Ca2+ signaling, are covered in some detail if they have not been reviewed recently or if recent findings add to current models. The function of Ca2+ as signaling agent released from organelles into the cytosol and within the organelles themselves is a recurrent theme of this review, again keeping the interference by Mn2+ in mind. The involvement of organellar channels [e.g. glutamate receptor-likes (GLR), cyclic nucleotide-gated channels (CNGC), mitochondrial conductivity units (MCU), and two-pore channel1 (TPC1)], transporters (e.g. natural resistance-associated macrophage proteins (NRAMP), Ca2+ exchangers (CAX), metal tolerance proteins (MTP), and bivalent cation transporters (BICAT)], and pumps [autoinhibited Ca2+-ATPases (ACA) and ER Ca2+-ATPases (ECA)] in the import and export of organellar Ca2+ and Mn2+ is scrutinized, whereby current controversial issues are pointed out. Mechanisms in animals and yeast are taken into account where they may provide a blueprint for processes in plants, in particular, with respect to tunable molecular mechanisms of Ca2+ versus Mn2+ selectivity.

High-temperature (HT) stress is a major environmental stress that limits plant growth and development. MAPK cascades play key roles in plant growth and stress signaling, but their involvement in the HT stress response is poorly understood. Here, we describe a 47-kD MBP-phosphorylated protein (p47-MBPK) activated in tomato (Solanum lycopersicum) leaves under HT and identify it as SlMPK1 by tandem mass spectrometry analysis. Silencing of SlMPK1 in transgenic tomato plants resulted in enhanced tolerance to HT, while overexpression resulted in reduced tolerance. Proteomic analysis identified a set of proteins involved in antioxidant defense that are significantly more abundant in RNA interference-SlMPK1 plants than nontransgenic plants under HT stress. RNA interference-SlMPK1 plants also showed changes in membrane lipid peroxidation and antioxidant enzyme activities. Furthermore, using yeast two-hybrid screening, we identified a serine-proline-rich protein homolog, SlSPRH1, which interacts with SlMPK1 in yeast, in plant cells, and in vitro. We demonstrate that SlMPK1 can directly phosphorylate SlSPRH1. Furthermore, the serine residue serine-44 of SlSPRH1 is a crucial phosphorylation site in the SlMPK1-mediated antioxidant defense mechanism activated during HT stress. We also demonstrate that heterologous expression of SlSPRH1 in Arabidopsis (Arabidopsis thaliana) led to a decrease in thermotolerance and lower antioxidant capacity. Taken together, our results suggest that SlMPK1 is a negative regulator of thermotolerance in tomato plants. SlMPK1 acts by regulating antioxidant defense, and its substrate SlSPRH1 is involved in this pathway.

Metal accumulation in seeds is a prerequisite for germination and establishment of plants but also for micronutrient delivery to humans. To investigate metal transport processes and their interactions in seeds, we focused on METAL TOLERANCE PROTEIN8 (MTP8), a tonoplast transporter of the manganese (Mn) subclade of cation diffusion facilitators, which in Arabidopsis (Arabidopsis thaliana) is expressed in embryos of seeds. The x-ray fluorescence imaging showed that expression of MTP8 was responsible for Mn localization in subepidermal cells on the abaxial side of the cotyledons and in cortical cells of the hypocotyl. Accordingly, under low Mn availability, MTP8 increased seed stores of Mn, required for efficient seed germination. In mutant embryos lacking expression of VACUOLAR IRON TRANSPORTER1 (VIT1), MTP8 built up iron (Fe) hotspots in MTP8-expressing cells types, suggesting that MTP8 transports Fe in addition to Mn. In mtp8 vit1 double mutant seeds, Mn and Fe were distributed in all cell types of the embryo. An Fe transport function of MTP8 was confirmed by its ability to complement Fe hypersensitivity of a yeast mutant defective in vacuolar Fe transport. Imbibing mtp8-1 mutant seeds in the presence of Mn or subjecting seeds to wet-dry cycles showed that MTP8 conferred Mn tolerance. During germination, MTP8 promoted reallocation of Fe from the vasculature. These results indicate that cell type-specific accumulation of Mn and Fe in seeds depends on MTP8 and that this transporter plays an important role in the generation of seed metal stores as well as for metal homeostasis and germination efficiency under challenging environmental conditions.

In addition to establishing symbiotic relationships with arbuscular mycorrhizal fungi, legumes also enter into a nitrogen-fixing symbiosis with rhizobial bacteria that results in the formation of root nodules. Several genes involved in the development of both arbuscular mycorrhiza and legume nodulation have been cloned in model legumes. Among them, Medicago truncatula DMI1 (DOESN'T MAKE INFECTIONS1) is required for the generation of nucleus-associated calcium spikes in response to the rhizobial signaling molecule Nod factor. DMI1 encodes a membrane protein with striking similarities to the Methanobacterium thermoautotrophicum potassium channel (MthK). The cytosolic C terminus of DMI1 contains a RCK (regulator of the conductance of K+) domain that in MthK acts as a calcium-regulated gating ring controlling the activity of the channel. Here we show that a dmi1 mutant lacking the entire C terminus acts as a dominant-negative allele interfering with the formation of nitrogen-fixing nodules and abolishing the induction of calcium spikes by the G-protein agonist Mastoparan. Using both the full-length DMI1 and this dominant-negative mutant protein we show that DMI1 increases the sensitivity of a sodium- and lithium-hypersensitive yeast (Saccharomyces cerevisiae) mutant toward those ions and that the C-terminal domain plays a central role in regulating this response. We also show that DMI1 greatly reduces the release of calcium from internal stores in yeast, while the dominant-negative allele appears to have the opposite effect. This work suggests that DMI1 is not directly responsible for Nod factor-induced calcium changes, but does have the capacity to regulate calcium channels in both yeast and plants.

It has long been known that plants facing a localized stress or attack may launch responses in unaffected distant tissues, hence preparing the entire plant for the potential spread of the challenge. Traditionally, this plant-wide “systemic” coordination of responses has been assigned largely to