Differential Mechanisms of Photosynthetic Acclimation to Light and Low Temperature in Arabidopsis and the Extremophile Eutrema salsugineum.

Photosynthetic organisms are able to sense energy imbalances brought about by the overexcitation of photosystem II (PSII) through the redox state of the photosynthetic electron transport chain, estimated as the chlorophyll fluorescence parameter 1-qL, also known as PSII excitation pressure. Plants employ a wide array of photoprotective processes that modulate photosynthesis to correct these energy imbalances. Low temperature and light are well established in their ability to modulate PSII excitation pressure. The acquisition of freezing tolerance requires growth and development a low temperature (cold acclimation) which predisposes the plant to photoinhibition. Thus, photosynthetic acclimation is essential for proper energy balancing during the cold acclimation process. Eutrema salsugineum (Thellungiella salsuginea) is an extremophile, a close relative of Arabidopsis thaliana, but possessing much higher constitutive levels of tolerance to abiotic stress. This comparative study aimed to characterize the photosynthetic properties of Arabidopsis (Columbia accession) and two accessions of Eutrema (Yukon and Shandong) isolated from contrasting geographical locations at cold acclimating and non-acclimating conditions. In addition, three different growth regimes were utilized that varied in temperature, photoperiod and irradiance which resulted in different levels of PSII excitation pressure. This study has shown that these accessions interact differentially to instantaneous (measuring) and long-term (acclimation) changes in PSII excitation pressure with regard to their photosynthetic behaviour. Eutrema accessions contained a higher amount of photosynthetic pigments, showed higher oxidation of P700 and possessed more resilient photoprotective mechanisms than that of Arabidopsis, perhaps through the prevention of PSI acceptor-limitation. Upon comparison of the two Eutrema accessions, Shandong demonstrated the greatest PSII operating efficiency (ΦPSII) and P700 oxidizing capacity, while Yukon showed greater growth plasticity to irradiance. Both of these Eutrema accessions are able to photosynthetically acclimate but do so by different mechanisms. The Shandong accessions demonstrate a stable response, favouring energy partitioning to photochemistry while the Yukon accession shows a more rapid response with partitioning to other (non-photochemical) strategies.

Extracellular pH and neuronal depolarization serve as dynamic switches to rapidly mobilize trkA to the membrane of adult sensory neurons.

Activation of the nerve growth factor (NGF) receptor trkA and tissue acidosis are critically linked to inflammation-associated nociceptor sensitization. This study explored how increased acidity is linked to sensory neuron sensitization to NGF. Adult Wistar rat primary sensory neurons grown at physiological pH 7.4, then either kept at pH 7.4 or challenged for 30 min in pH 6.5 medium, provided a model of acidosis. Nonpermeabilizing trkA immunofluorescence revealed a significant increase in trkA mobilization to the plasma membrane from intracellular stores in response to proton challenge. This was confirmed using a surface protein biotinylation assay and Brefeldin A disruption of the rough endoplasmic reticulum-Golgi-trans-Golgi network. Mobilization of trkA to the membrane at pH 6.5 was abolished in neurons treated with the acid-sensitive ion channel blocker, amiloride. While elevated levels of NGF-independent trkA phosphorylation occurred at pH 6.5 alone, the level of activation was significantly increased in response to NGF challenge. Exposure of sensory neurons to pH 6.5 medium also resulted in strong calcium (Ca(2+)) transients that were reversible upon reintroduction to physiological pH. The pH 6.5-induced mobilization of trkA to the membrane was Ca(2+) dependent, as BAPTA-AM Ca(2+) chelation abrogated the response. Interestingly, KCl-induced depolarization was sufficient to induce mobilization of trkA to the cell surface at pH 7.4, but did not augment the response to pH 6.5. In conclusion, increased mobilization of trkA to neuronal membranes in response to either acidosis or neuronal depolarization provides two novel mechanisms by which sensory neurons can rapidly sensitize to NGF and has important implications for inflammatory pain states.

Aspergillus nidulans hypB encodes a Sec7-domain protein important for hyphal morphogenesis.

Aspergillus nidulans strains containing the hypB5 temperature sensitive allele have a restrictive phenotype of wide, highly-branched hyphae. The hypB locus was cloned by phenotype complementation using a genomic plasmid library. hypB5 is predicted gene AN6709 in the A. nidulans genome database, which encodes a putative Sec7 domain protein, likely to act early in COPI-mediated vesicle formation for retrograde Golgi to ER transport. The A. nidulans hypB5 allele has a single mutation, cytosine to guanine predicted to cause a nonconservative amino acid change, alanine to proline, in a conserved helix adjacent to the Sec7p nucleotide binding site. This would likely reduce the stability of a highly conserved loop important for nucleotide binding, and is consistent with temperature sensitivity of hypB5 strains. Deletion of AN6709 showed that hypB(Sec7) was not essential. AN6709Delta hyphae resembled the hypB5 restrictive phenotype. As has been shown previously for hypA1 mutants, shifting established hypB5 mutant hyphae from a growth temperature of 28-42 degrees C caused septation in and death of tip cells and growth activation of basal cells.