Search

The effects of biotic and abiotic stresses on changes in amino acids and polyamine levels in Satsuma orange (Citrus unshiu; cultivar Owari) leaves were investigated. Asian citrus psyllids Diaphorina citri (Kuwayama) (ACP) infestation was used to induce biotic stress while a water deficit was imposed to induce abiotic stress. Potted trees were infested by placing 50 psyllids on 3 citrus leaves enclosed in nylon mesh bags for 5 d. A parallel set of plants were kept water stressed by maintaining the soil at 20% water holding capacity for 5 d. Levels of total free amino acids were higher in water stressed and ACP infested leaves. Polyamine putrescine increased in infested leaves but not in water stressed leaves. Proline was the most abundant amino acid and its levels significantly increased by both biotic and abiotic stresses. Proline levels in infested leaves were significantly higher than the water stressed leaves. Histidine, methionine, asparagine, arginine, serine, and leucine levels also increased significantly in infested leaves, but in water stressed leaves only leucine, methionine, and threonine increased. Levels of amino acids, such as tyrosine, isoleucine, phenylalanine, glutamic acid, and alanine, declined in infested leaves. Under water stress asparagine, phenylalanine, serine, and histidine also declined compared to controls. This indicates that while proteolysis occurred under both stresses, metabolic conversion of amino acids was different under the two stresses. In ACP infested leaves some amino acids may be used as feeding material and/or converted into secondary metabolites for defense.

Elsinoe fawcettii causes citrus scab and E. australis causes sweet orange scab and there are different pathotypes of each species. Citrus scab is widely distributed, whereas sweet orange scab is limited mostly to southern South America. In 2010, E. australis was detected by PCR in Texas for the first time in the USA and subsequently found in commercial areas of Texas. In this study, cultures were obtained in Texas mostly from diseased fruit with symptoms similar to "late-season windscar" and identified as the Natsudaidai pathotype of E. australis by sequencing of the PCR amplification products and sequencing of the translation elongation factor and ITS regions. Inoculations of detached leaves and fruit of grapefruit, sweet orange, and tangerine produced scab-like symptoms on all organs and species. The fungus was re-isolated from the inoculated organs and PCR tests on re-isolated fungal colonies and lesions confirmed the presence of E. australis. The disease represents sweet orange scab of citrus with a new scab disease symptomatology, but the host range, ecology and epidemiology of this pathotype are still not completely understood. Thus far, Natsudaidai pathotype has been reported only from South Korea.

Mycorrhizal fungus spores and propagules were collected from the soils in the vicinity of roots of five different olive cultivars. These mycorrhizal fungus communities were amplified in trap cultures and then their effect on the growth and polyphenol levels of leek plants was determined. All mycorrhizal fungus communities increased plant growth in leeks when compared to controls. In addition, communities from the roots of Frantoio and Manzanillo significantly increased plant growth, in terms of plant height and dry weights, as compared to plants that were given mycorrhizal fungus collected from cultivar Mission. Plants inoculated with mycorrhizal fungus from Frantoio also had an increase in 14 polyphenols compared to uninoculated plants. A majority of polyphenol peaks were also higher in leek plants inoculated with mycorrhizal fungi from Frantoio roots when compared to plants inoculated with mycorrhizal fungi from other olive cultivars. The affected polyphenols were identified by mass spectrometry and were mostly found to be derivatives (e.g., pentose, hexose, malonyl, feruyl, and coumaroyl) of quercetin, kaempferol, and apigenin; four remained unidentified. Molecular fingerprinting of mycorrhizal fungus communities from different olive cultivars indicated that fungi of the family Gigasporaceae were a major component of inocula obtained from Frantoio and Manzanillo roots, which were better performers in terms of plant growth and polyphenol content. Mycorrhizal fungi from cv Mission roots were relatively poor performers and were dominated by the mycorrhizae of the family Glomeraceae, specicifally the genus Rhizophagus.

The Ca(2+)-dependent protein phosphatase calcineurin is an important regulator of ion transporters from many organisms, including the Saccharomyces cerevisiae vacuolar Ca(2+)/H(+) exchanger Vcx1p. In yeast and plants, cation/H(+) exchangers are important in shaping cytosolic Ca(2+) levels involved in signal transduction and providing tolerance to potentially toxic concentrations of cations such as Ca(2+), Mn(2+) and Cd(2+). Previous genetic evidence suggested Vcx1p is negatively regulated by calcineurin. By utilizing direct transport measurements into vacuolar membrane vesicles, we demonstrate that Vcx1p is a low-affinity Ca(2+) transporter and may also function in Cd(2+) transport, but cannot transport Mn(2+). Furthermore, direct Ca(2+) transport by Vcx1p is calcineurin sensitive. Using a yeast growth assay, a mutant allele of VCX1 (VCX1-S204A/L208P), termed VCX1-M1, was previously found to confer strong Mn(2+) tolerance. Here we demonstrate that this Mn(2+) tolerance is independent of the Ca(2+)/Mn(2+)-ATPase Pmr1p and results from Mn(2+)-specific vacuolar transport activity of Vcx1-M1p. This Mn(2+) transport by Vcx1-M1p is calcineurin dependent, although the localization of Vcx1-M1p to the vacuole appears to be calcineurin independent. Additionally, we demonstrate that mutation of L208P alone is enough to confer calcineurin-dependent Mn(2+) tolerance. This study demonstrates that calcineurin can positively regulate the transport of cations by VCX1-M1p.

Reactive oxygen species (ROS) are continuously produced during the normal aerobic metabolism and also under environmental stress conditions. They are the major damaging factors to the photosynthetic machinery under stress conditions and need to be scavenged for the normal growth of the plant. Ascorbate peroxidase (APX) is the key enzyme in detoxifying H2O2, one of ROS from chloroplast and cytosol. A cDNA encoding a putative APXcit was isolated from mature `Dancy' tangerine (Citrus reticulata Blanco) juice vesicles using differential display reverse transcription-polymerase chain reaction (RT-PCR). Subsequently, full-length APXcit cDNA clone and genomic clone were obtained and sequenced. The full-length APXcit sequence is composed by 1082-bp nucleotides, including an open reading frame (ORF) of 753 bp, encoding a protein of 250 amino acids (27 kDa). The 5' un-translated region (UTR) of the APXcit gene consisted of 91 nucleotides and the 3' UTR consisted of 238 nucleotides. Homology search for APXcit at GenBank database showed high similarity to APX from several plant species.

Ascorbate Peroxidase (APX) is a heme-containing, non-glycosylated enzyme that destroys harmful hydrogen peroxide via the ascorbate glutathione pathway in plants. This enzyme is considered to be an indispensable part of the electron-scavenging pathway and is involved in preventing oxidative damage in plants. Using differential display RT-PCR and 5' RACE a full length c-DNA clone was isolated, from citrus, with very high similarity at the nucleotide and amino acid level, to ascorbate peroxidases from several plant species. It is well known that APXs have highly conserved motifs like the Arg-38, Ars-71, Glu-65 and Asp-208 residues around the distal Hist-42 and proximal His-163. These residues are essential for binding the ligand heme. Additionally, Trp-179 is conserved in most APXs and is the third participant in hydrogen bonding network, together with His-163 and Asp-208. All these conserved motifs were present in the putative APX from citrus in addition of the presence of the peroxidase active site motif residues (APITLRLAWHSA) and the peroxidase heme-ligand motif (DIVVLSGGHTL). Expression analysis in E. Coli reviewed a recombinant protein of 27 Kda.