Na+ Tolerance and Na+ Transport in Higher Plants

Elicitor signal transduction leading to production of plant secondary metabolites.

Phospholipids are emerging as novel second messengers in plant cells. They are rapidly formed in response to a variety of stimuli via the activation of lipid kinases or phospholipases. These lipid signals can activate enzymes or recruit proteins to membranes via distinct lipid-binding domains, where the local increase in concentration promotes interactions and downstream signaling. Here, the latest developments in phospholipid-based signaling are discussed, including the lipid kinases and phospholipases that are activated, the signals they produce, the domains that bind them, the downstream targets that contain them and the processes they control.

Phospholipid-based signaling in plants

Alkaloids represent a highly diverse group of compounds that are related only by the occurrence of a nitrogen atom in a heterocyclic ring. Plants are estimated to produce approximately 12,000 different alkaloids, which can be organized into groups according to their carbon skeletal structures. Alkaloid biosynthesis in plants involves many catalytic steps, catalyzed by enzymes that belong to a wide range of protein families. The characterization of novel alkaloid biosynthetic enzymes in terms of structural biochemistry, molecular and cell biology, and biotechnological applications has been the focus of research over the past several years. The application of genomics to the alkaloid field has accelerated the discovery of cDNAs encoding previously elusive biosynthetic enzymes. Other technologies, such as large-scale gene expression analyses and metabolic engineering approaches with transgenic plants, have provided new insights into the regulatory architecture of alkaloid metabolism.

Alkaloid Biosynthesis: Metabolism and Trafficking

Uptake and translocation of cations play essential roles in plant nutrition, signal transduction, growth, and development. Among them, potassium (K+) and sodium (Na+) have been the focus of numerous physiological studies because K+ is an essential macronutrient and the most abundant inorganic cation in plant cells, whereas Na+ toxicity is a principal component of the deleterious effects associated with salinity stress. Although the homeostasis of these two ions was long surmised to be fine tuned and under complex regulation, the myriad of candidate membrane transporters mediating their uptake, intracellular distribution, and long-distance transport is nevertheless perplexing. Recent advances have shown that, in addition to their function in vacuolar accumulation of Na+, proteins of the NHX family are endosomal transporters that also play critical roles in K+ homeostasis, luminal pH control, and vesicle trafficking. The plasma membrane SOS1 protein from Arabidopsis thaliana, a highly specific Na+/H+ exchanger that catalyses Na+ efflux and that regulates its root/shoot distribution, has also revealed surprising interactions with K+ uptake mechanisms by roots. Finally, the function of individual members of the large CHX family remains largely unknown but two CHX isoforms, AtCHX17 and AtCH23, have been shown to affect K+ homeostasis and the control of chloroplast pH, respectively. Recent advances on the understanding of the physiological processes that are governed by these three families of cation exchangers are reviewed and discussed.

Alkali cation exchangers: roles in cellular homeostasis and stress tolerance

The elicitation of phytoalexin biosynthesis in cultured cells of California poppy involves a shift of cytoplasmic pH via the transient efflux of vacuolar protons. Intracellular effectors of vacuolar proton transport were identified by a novel in situ approach based on the selective permeabilization of the plasma membrane for molecules of < or = 10 kD. Subsequent fluorescence imaging of the vacuolar pH correctly reported experimental changes of activity of the tonoplast proton transporters. Lysophosphatidylcholine (LPC) caused a transient increase of the vacuolar pH by increasing the Na(+) sensitivity of a Na(+)-dependent proton efflux that was inhibited by amiloride. In intact cells, yeast elicitor activated phospholipase A(2), as demonstrated by the formation of LPC from fluorescent substrate analogs, and caused a transient increase of endogenous LPC, as determined by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry. It is suggested that LPC generated by phospholipase A(2) at the plasma membrane transduces the elicitor-triggered signal into the activation of a tonoplast H(+)/Na(+) antiporter.

Elicitor-Activated Phospholipase A2 Generates Lysophosphatidylcholines That Mobilize the Vacuolar H+ Pool for pH Signaling via the Activation of Na+-Dependent Proton Fluxes

Intracellular pH signals in the induction of secondary pathways--The case of Eschscholzia californica

The function of a Galpha protein in the elicitation of phytoalexin (benzophenanthridine) biosynthesis was characterized in cultured cells of California poppy (Eschscholzia californica). Both the decrease of Galpha content via antisense transformation and the expression of recombinant anti-Galpha single-chain antibodies strongly impaired the induction of alkaloid biosynthesis by low elicitor concentrations. All transgenic cell types were deficient in two elicitor-triggered early signal events: activation of phospholipase A2 (PLA2) and efflux of vacuolar protons. The lacking H+ efflux could be restored (1) by adding lysophosphatidylcholine (LPC), a product of PLA2 activity, to vacuoles in situ and (2) by exposing intact cells to isotonic, near-neutral HEPES buffers. The latter treatment induced alkaloid biosynthesis in the absence of elicitor and in Galpha-deficient cells. We conclude that Galpha mediates the stimulation of PLA2 by low elicitor concentrations and that the resulting peak of LPC initiates a transient efflux of vacuolar protons. In this way, an acidic peak of the cytoplasmic pH is generated that causes the expression of enzymes of phytoalexin production independent of the hypersensitive response.

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Katrin Viehweger

Papers

Elicitor-Activated Phospholipase A2 Generates Lysophosphatidylcholines That Mobilize the Vacuolar H+ Pool for pH Signaling via the Activation of Na+-Dependent Proton Fluxes

Intracellular pH signals in the induction of secondary pathways--The case of Eschscholzia californica

The Gα Protein Controls a pH-Dependent Signal Path to the Induction of Phytoalexin Biosynthesis in Eschscholzia californica

Immuno-histochemical detection of MRPs in human lung cells in culture.