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Stably Transformed Lotus japonicus Plants Overexpressing Phytoglobin LjGlb1-1 Show Decreased Nitric Oxide Levels in Roots and Nodules as Well as Delayed Nodule Senescence

Fukudome, Mitsutaka, Watanabe, Eri, Osuki, Ken-Ichi, Imaizumi, Ryujiro, Aoki, Toshio, Becana, Manuel, Uchiumi, Toshiki
Plant & cell physiology 2019 v.60 no.4 pp. 816-825
1-aminocyclopropane-1-carboxylic acid, Lotus corniculatus var. japonicus, Mesorhizobium loti, abscisic acid, carboxylic acids, enzyme activity, ethylene, gene overexpression, genes, legumes, nitric oxide, nitrogenase, root nodules, signal transduction, symbiosis
The class 1 phytoglobin, LjGlb1-1, is expressed in various tissues of the model legume Lotus japonicus, where it may play multiple functions by interacting with nitric oxide (NO). One of such functions is the onset of a proper symbiosis with Mesorhizobium loti resulting in the formation of actively N₂-fixing nodules. Stable overexpression lines (Ox1 and Ox2) of LjGlb1-1 were generated and phenotyped. Both Ox lines showed reduced NO levels in roots and enhanced nitrogenase activity in mature and senescent nodules relative to the wild-type (WT). Physiological and cytological observations indicated that overexpression of LjGlb1-1 delayed nodule senescence. The application to WT nodules of the NO donor S-nitroso-N-acetyl-dl-penicillamine (SNAP) or the phytohormones abscisic acid (ABA) and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) repressed nitrogenase activity, induced the expression of three senescence-associated genes and caused cytological changes evidencing nodule senescence. These effects were almost completely reverted by the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. Our results reveal that overexpression of LjGlb1-1 improves the activity of mature nodules and delays nodule senescence in the L.japonicus–M.loti symbiosis. These beneficial effects are probably mediated by the participation of LjGlb1-1 in controlling the concentration of NO that may be produced downstream in the phytohormone signaling pathway in nodules.