Plasma membrane H+-ATPases sustain pollen tube growth and fertilization

Pollen tubes are highly polarized tip-growing cells that depend on cytosolic pH gradients for signaling and growth. Autoinhibited plasma membrane proton (H+) ATPases (AHAs) have been proposed to energize pollen tube growth and underlie cell polarity, however, mechanistic evidence for this is lacking. Here we report that the combined loss of AHA6, AHA8, and AHA9 in Arabidopsis thaliana delays pollen germination and causes pollen tube growth defects, leading to drastically reduced fertility. Pollen tubes of aha mutants had reduced extracellular proton (H+) and anion fluxes, reduced cytosolic pH, reduced tip-to-shank proton gradients, and defects in actin organization. Furthermore, mutant pollen tubes had less negative membrane potentials, substantiating a mechanistic role for AHAs in pollen tube growth through plasma membrane hyperpolarization. Our findings define AHAs as energy transducers that sustain the ionic circuit defining the spatial and temporal profiles of cytosolic pH, thereby controlling downstream pH-dependent mechanisms essential for pollen tube elongation, and thus plant fertility.

CORNICHON sorting and regulation of GLR channels underlie pollen tube Ca2+ homeostasis

Compared to animals, evolution of plant calcium (Ca²⁺) physiology has led to a loss of proteins for influx and small ligand-operated control of cytosolic Ca²⁺, leaving many Ca²⁺ mechanisms unaccounted for. Here, we show a mechanism for sorting and activation of glutamate receptor-like channels (GLRs) by CORNICHON HOMOLOG (CNIH) proteins. Single mutants of pollen-expressed Arabidopsis thaliana GLRs (AtGLRs) showed growth and Ca²⁺ flux phenotypes expected for plasma membrane Ca²⁺ channels. However, higher-order mutants of AtGLR3.3 revealed phenotypes contradicting this assumption. These discrepancies could be explained by subcellular AtGLR localization, and we explored the implication of AtCNIHs in this sorting. We found that AtGLRs interact with AtCNIH pairs, yielding specific intracellular localizations. AtCNIHs further trigger AtGLR activity in mammalian cells without any ligand. These results reveal a regulatory mechanism underlying Ca²⁺ homeostasis by sorting and activation of AtGLRs by AtCNIHs.

Glutamate receptor-like genes form Ca2+ channels in pollen tubes and are regulated by pistil D-serine

Elevations in cytosolic free calcium concentration ([Ca(2+)](cyt)) constitute a fundamental signal transduction mechanism in eukaryotic cells, but the molecular identity of Ca(2+) channels initiating this signal in plants is still under debate. Here, we show by pharmacology and loss-of-function mutants that in tobacco and Arabidopsis, glutamate receptor-like channels (GLRs) facilitate Ca(2+) influx across the plasma membrane, modulate apical [Ca(2+)](cyt) gradient, and consequently affect pollen tube growth and morphogenesis. Additionally, wild-type pollen tubes grown in pistils of knock-out mutants for serine-racemase (SR1) displayed growth defects consistent with a decrease in GLR activity. Our findings reveal a novel plant signaling mechanism between male gametophyte and pistil tissue similar to amino acid-mediated communication commonly observed in animal nervous systems.

A pH signaling mechanism involved in the spatial distribution of calcium and anion fluxes in ectomycorrhizal roots

Mycorrhization is a typical example of a host-pathogen symbiotic interaction where the pathogen cell biology and the host immune response coevolved several functional links. Here, the role played by ion fluxes across the root concerning nutrient uptake, osmoregulation, growth and signaling events is addressed. An ion-selective vibrating probe system was used to determine the net fluxes of protons (H(+)), calcium (Ca(2+)) and anions (A(-)) along nonmycorrhizal and ectomycorrhizal (ECM) roots of Eucalyptus globulus colonized by Pisolithus sp. These data show that, from five root zones analyzed, the main effect of fungal colonization was localized to the elongation zone. Here, strong changes in ion dynamics and rhizosphere acidification capacity were observed. Additionally, ion fluxes exhibited periodic fluctuations. To verify whether these fluctuations corresponded to sustained oscillations, continuous wavelet time spectrum analysis was applied and it was determined that H(+) and A(-) fluxes from ECM roots had longer periods than nonmycorrhizal roots. By contrast, Ca(2+) oscillations were completely abolished following fungal interaction. These results are interpreted in the light of a working model in which nutrient uptake and stimulation of growth are mediated by ECM fungi and may be pH-dependent. Furthermore, the variations detected in ECM roots for H(+) and A(-) fluxes suggest a main contribution from the plant, while the results obtained for Ca(2+) point to a significant involvement of the fungus.

pH signature for the responses of arbuscular mycorrhizal fungi to external stimuli

Environmental and developmental signals can elicit differential activation of membrane proton (H(+)) fluxes as one of the primary responses of plant and fungal cells. In recent work,1 we could determine that during the presymbiotic growth of arbuscular mycorrhizal (AM) fungi specific domains of H(+) flux are activated by clover root factors, namely host root exudates or whole root system. Consequently, activation on hyphal growth and branching were observed and the role of plasma membrane H(+)-ATPase was investigated. The specific inhibitors differentially abolished most of hyphal H(+) effluxes and fungal growth. As this enzyme can act in signal transduction pathways, we believe that spatial and temporal oscillations of the hyphal H(+) fluxes could represent a pH signature for both early events of the AM symbiosis and fungal ontogeny.

Genetic fingerprinting of Brevibacterium linens by pulsed-field gel electrophoresis and ribotyping

Members of Brevibacterium linens display physiological features that are relevant for cheese production. The genomes of five B. linens strains deposited on culture collections were compared by examining large restriction fragments on pulsed-field gel electrophoresis and detection of polymorphism at the level of 16S rRNA genes. Pulsed-field analysis with the endonucleases DraI and AsnI showed a characteristic restriction profile for each strain and allowed the calculation of genome sizes ranging between 3.2 and 3.9 Mbp. No linear genomic elements were detected. Polymorphisms at the level of 16S rRNA genes were revealed by hybridization with an oligonucleotide probe complementary to a universal domain of the 16S genes. An EcoRI fragment of 1.4 kb was identified as common to all strains under study. According to the number of positive bands detected by the probe, at least four rRNA operons must be present on the genome of the B. linens strains here studied.