PusALDH1 gene confers high levels of volatile aroma accumulation in both pear and tomato fruits

Aroma is an important commercial trait that determines fruit quality and has an important influence on the overall flavor of fruits. Plant ALDH genes have been implicated in diverse pathways and play crucial roles in physiological activities. In this study, via genome resequencing we identified one gene PusALDH1 (Pbr034873.1) related to aroma biosynthesis that can respond to the induction of methyl jasmonate. Transient transformation of pear fruits and heterologous stable transformation of tomato further confirmed the function of PusALDH1 in aroma accumulation. The content of ALDH precursor substance, benzaldehyde, was reduced in the overexpressing pear and tomato fruits, and the content of ALDH product, benzoic acid and benzoic acid derivatives, was increased in the pear fruits. Meanwhile, transgenic tomato fruits with PusALDH1 overexpression exhibited a greater area of yellow placenta, indicating that the gene may be related to the growth and development of the fruit. Taken together, PusALDH1 could act as a strong candidate gene in aroma synthesis.

Alternative Oxidase Is Positive for Plant Performance

The alternative pathway of mitochondrial electron transport, which terminates in the alternative oxidase (AOX), uncouples oxidation of substrate from mitochondrial ATP production, yet plant performance is improved under adverse growth conditions. AOX is regulated at different levels. Identification of regulatory transcription factors shows that Arabidopsis thaliana AOX1a is under strong transcriptional suppression. At the protein level, the primary structure is not optimised for activity. Maximal activity requires the presence of various metabolites, such as tricarboxylic acid-cycle intermediates that act in an isoform-specific manner. In this opinion article we propose that the regulatory mechanisms that keep AOX activity suppressed, at both the gene and protein level, are positive for plant performance due to the flexible short- and long-term fine-tuning.

Isolation and characterization of a salt stress-responsive betaine aldehyde dehydrogenase in Lycium ruthenicum Murr

As compatible solute, glycine betaine (GB) plays a significant role in salinity tolerance in GB accumulating plants. Solanaceous crops such as tomato (Solanum lycopersicum) and tobacco (Nicotiana tabacum) are salt sensitive and naturally GB non-accumulators. In Solanaceae, only the Lycium genus has been recorded as halophytes in China, and several Lycium species have been reported as GB accumulators. The last biosynthetic step of GB is catalyzed by aminoaldehyde dehydrogenase (AMADH) with betaine aldehyde dehydrogenase (BADH) activities. Failure of GB synthesis in tomato and tobacco was attributed to lack of BADH activity. Here, by comparing the BADH functional residues of AMADHs between the Lycium genus and solanaceous crops, we predict that all studied AMADH1s have low BADH activities while only LbAMADH2 from L. barbarum has high BADH activity. For two AMADHs in L. ruthenicum, results from substrate enzyme assays confirmed low BADH activity of LrAMADH1 and no BADH activity of LrAMADH2. Despite the very low GB contents in L. ruthenicum seedlings (< 0.5 μmol g^-1 fresh weight), GB contents in fruits are up to 150 μmol g^-1 FW, inferring fruits of L. ruthenicum as good GB sources. In NaCl treated seedlings, accompanied by elevated GB accumulation, expression of LrAMADH1 was up-regulated, indicating response of LrAMADH1 to salt stress in L. ruthenicum. Virus-induced silence of LrAMADH1 leads to less GB accumulation than control, revealing that LrAMADH1 participates in GB synthesis in planta. Collectively, our results show that LrAMADH1 is the bona fide BADH, which responds to salt stress in L. ruthenicum.

Mechanisms of protection against irreversible oxidation of the catalytic cysteine of ALDH enzymes: Possible role of vicinal cysteines

The catalytic mechanism of the NAD(P)⁺-dependent aldehyde dehydrogenases (ALDHs) involves the nucleophilic attack of the essential cysteine (Cys302, mature HsALDH2 numbering) on the aldehyde substrate. Although oxidation of Cys302 will inactivate these enzymes, it is not yet well understood how this oxidation is prevented. In this work we explore possible mechanisms of protection by systematically analyzing the reported three-dimensional structures and amino acid sequences of the enzymes of the ALDH superfamily. Specifically, we considered the Cys302 conformational space, the structure and residues conservation of the catalytic loop where Cys302 is located, the observed oxidation states of Cys302, the ability of physiological reductants to revert its oxidation, and the presence of vicinal Cys in the catalytic loop. Our analyses suggested that: 1) In the apo-enzyme, the thiol group of Cys302 is quite resistant to oxidation by ambient O₂ or mild oxidative conditions, because the protein environment promotes its high pK_a. 2) NAD(P)⁺ bound in the "hydride transfer" conformation afforded total protection against Cys302 oxidation by an unknown mechanism. 3) If formed, the Cys302-sulfenic acid is protected against irreversible oxidation. 4) Of the physiological reductant agents, the dithiol lipoic acid could reduce a sulfenic or a disulfide bond in the ALDHs active site; glutathione cannot because its thiol group cannot reach Cys302, and other physiological monothiols may be ineffective in those ALDHs where their active site cannot sterically accommodate two molecules of the monothiols. 5) Formation of the disulfides Cys301-Cys302, Cys302-Cys304, Cys302-Cys305 and Cys-302-Cys306 in those ALDHs that have these Cys residues is not probable, because of the permitted Cys conformers as well as the conserved structure and low flexibility of the catalytic loop. 6) Only in some ALDH2, ALDH9, ALDH16 and ALDH23 enzymes, Cys303, alone or in conjunction with Cys301, allows disulfide formation. Interestingly, several of these enzymes are mitochondrial.