Isolation and in silico functional analysis of MtATP6, a 6-kDa subunit of mitochondrial F1F0-ATP synthase, in response to abiotic stress.

Mitochondrial F(1)F(0)-ATP synthase is a key enzymatic complex of energy metabolism that provides ATP for the cell. Subunits of this enzyme over-express under stress conditions. Little is known about the structure and regulatory mechanism of the F(0) portion of this enzyme. We isolated the full-length coding sequence of the RMtATP6 gene from rice and wheat, and partial sequences from Aegilops crassa and Triticum monococcum (Poaceae). We found that the sequence of rice RMtATP6 is 1965 bp long and contains two exons and one intron in 3'-UTR. Then, we analyzed the 2000-bp upstream region of the initiation codon ATG of the RMtATP6 and AtMtATP6, as promoter. The RMtATP6 coding sequence was found to be much conserved in the different plant species, possibly because of its key role under stress conditions. Promoter analysis demonstrated that RMtATP6 and AtMtATP6 include cis-acting elements such as ABRE, MYC/MYB, GT element in the upstream region, which respond to abscisic acid stress hormone and might show vital its roles in biotic and abiotic tolerance as an early-stress responsive gene. A mitochondrial signal peptide of 30 amino acids in length and an N-terminal cleavage site between amino acids 20 and 21 were discovered in RMtATP6. In addition, we found a transmembrane domain with an alpha helix structure that possibly passed through the mitochondrial inner membrane and established the 6-kDa subunit in the F(0) portion of the enzyme complex. Apparently, under stress conditions, with increasing ATP consumption by the cell, the 6-kDa subunit accumulates; by switching on F(1)F(0)-ATP synthase it provides additional energy needed for cell homeostasis.

Witches'-broom of Bakraee (Citrus reticulata hybrid) in Iran.

Symptoms, resembling those of witches'-broom disease of lime (WBDL), consisting of proliferation and growth of spindly shoots, rosetting, development of stunted, pale green, cupped leaves, and long thin thorns were observed on some bakraee (Citrus reticulata hybrid) trees, the major rootstock species in Ghazi-Ghale (Hormozgan Province) and Kahnouj (Kerman Province) in southern Iran. Vein clearing, mottling, and mild to severe yellowing of leaves was evident on some rosetted flushes. The disease occurred sporadically, and only some branches of the tree displayed the symptoms. Sour lime (Citrus aurantifolia) trees with symptoms of WBDL were also encountered in adjacent citrus groves in the area where an association of "Candidatus Phytoplasma aurantifolia" with diseased trees was previously established. Total nucleic acids were extracted from symptomatic as well as apparently healthy bakraee (1). The nucleic acids were subjected to nested polymerase chain reaction (PCR) amplification with the universal primers R16mF2/R16mR1, followed by R16F2n/R16R2 (2). The PCR program consisted of an initial denaturation at 94°C for 2 min, followed by 40 cycles of 1 min at 94°C, 2 min at 55°C, and 3 min at 72°C. After a final extension of 10 min at 72°C, the samples were electrophoresed on agarose gels, and the DNA fragments were stained with ethidium bromide and visualized under ultraviolet light. An expected 1.2-kb product was amplified in DNA preparations from broomed bakraee and WBDL-affected sour lime trees. No such amplicon was detected in similar DNA extracts of asymptomatic trees. These results demonstrated an association of a phytoplasma with witches'-broom disease of bakraee. However, its relationship with "Candidatus Phytoplasma aurantifolia" remains to be determined. References: (1) S. L. Dellaporta et al. Plant Mol. Biol. Rep.1:19, 1983. (2) I. M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998.