Recent Advances in Onion Genetic Improvement

Onions are one of the most important vegetable crops worldwide. However, their production faces many challenges. Genetic improvement is one mechanism to address those challenges. In this review, we discuss recent research pertaining to the diseases Fusarium basal rot and Iris yellow spot, the insect pest onion thrips, onion pungency, and dormancy. Recent research for screening onion bulbs for Fusarium basal rot resistance has resulted in improved screening techniques and germplasm exhibiting less disease when inoculated with the disease-causing pathogen. Improved screening methods have resulted in germplasm exhibiting fewer and less severe Iris yellow spot symptoms when onion thrips and conducive environmental conditions are present. Onion germplasm with less and differing compositions of epicuticular wax on their leaves have shown a nonpreference for thrips feeding and have the potential for developing thrips tolerant cultivars. Conventional breeding efforts and genetic manipulation of the genes producing alliinase and lachrymatory factor synthase has resulted in low pungency, tearless onions. In long-day onions, an annual generation time has been achieved by artificially breaking bulb dormancy early while ensuring proper vernalization has been completed. Genetic improvement of these and many other onion traits will continue and result in better production in the future.

Structure of Allium lachrymatory factor synthase elucidates catalysis on sulfenic acid substrate

Natural lachrymatory effects are invoked by small volatile S-oxide compounds. They are produced through alkene sulfenic acids by the action of lachrymatory factor synthase (LFS). Here we present the crystal structures of onion LFS (AcLFS) revealed in solute-free and two solute-stabilized forms. Each structure adopts a single seven-stranded helix-grip fold possessing an internal pocket. Mutagenesis analysis localized the active site to a layer near the bottom of the pocket, which is adjacent to the deduced key residues Arg71, Glu88, and Tyr114. Solute molecules visible on the active site have suggested that AcLFS accepts various small alcohol compounds as well as its natural substrate, and they inhibit this substrate according to their chemistry. Structural homologs have been found in the SRPBCC superfamily, and comparison of the active sites has demonstrated that the electrostatic potential unique to AcLFS could work in capturing the substrate in its specific state. Finally, we propose a rational catalytic mechanism based on intramolecular proton shuttling in which the microenvironment of AcLFS can bypass the canonical [1,4]-sigmatropic rearrangement principle established by microwave studies. Beyond revealing how AcLFS generates the lachrymatory compound, this study provides insights into the molecular machinery dealing with highly labile organosulfur species.Significance statementCrushing of onion liberates a volatile compound, syn-propanethial S-oxide (PTSO), which causes lachrymatory effect on humans. We present the crystal structures of onion LFS (AcLFS), the enzyme responsible for natural production of PTSO. AcLFS features a barrel-like fold, and mutagenic and inhibitory analyses revealed that the key residues are present in the central pocket, harboring highly concentrated aromatic residues plus a dyad motif. The architecture of AcLFS is widespread among proteins with various biological functions, such as abscisic acid receptors and polyketide cyclases, and comparisons with these homologs indicate that unique steric and electronic properties maintain the pocket as a reaction compartment. We propose the molecular mechanism behind PTSO generation and shed light on biological decomposition of short-lived sulfur species.