Soybean NADP-Malic Enzyme Functions in Malate and Citrate Metabolism and Contributes to Their Efflux under Al Stress

Abstract

Malate accumulation has been suggested to balance Al-induced citrate synthesis and efflux in soybean roots. To test this hypothesis, characteristics of Al-induced accumulation and efflux of citrate and malate were compared between two soybean genotypes combining a functional analysis of GmME1 putatively encode a cytosolic NADP-malic enzyme. Similar amounts of citrate were released, and root elongation was equally inhibited before 8 h of Al treatment of Jiyu 70 and Jiyu 62 cultivars. Jiyu 70 began to secrete more citrate and exhibited higher Al resistance than did Jiyu 62 at 12 h. A sustained increase in internal malate and citrate concentrations was observed in Jiyu 70 at 24 h of Al treatment. However, Jiyu 62 decreased its malate concentration at 12 h and its citrate concentration at 24 h of Al treatment. GmME1 localized to the cytoplast and clustered closely with cytosolic malic enzymes AtME2 and SgME1 and was constitutively expressed in the roots. Al treatment induced higher NADP-malic enzyme activities and GmME1 expression levels in Jiyu 70 than in Jiyu 62 within 24 h. Compared with wild-type hairy roots, over-expressing GmME1 in hairy roots (GmME1-OE) produced higher expression levels of GmME1 but did not change the expression patterns of either of the putative citrate transporter genes GmAACT1 and GmFRDL or the malate transporter gene GmALMT1, with or without Al treatment. GmME1-OE showed a higher internal concentration and external efflux of both citrate and malate at 4 h of Al stress. Lighter hematoxylin staining and lower Al contents in root apices of GmME1-OE hairy roots indicated greater Al resistance. Comprehensive experimental results suggest that sustaining Al-induced citrate efflux depends on the malate pool in soybean root apices. GmME1 encodes a cytosolic malic enzyme that contributes to increased internal malate and citrate concentrations and their external efflux to confer higher Al resistance.

Metabolic and physiological adjustments of maize leaves in response to aluminum stress

“…In fact, mitochondrial malate and citrate concentrations have been demonstrated to improve Al tolerance in tobacco (Nicotiana tabacum), papaya (Carica papaya) and soybean (Glycine max) (De la Fuente et al 1997;Zhou et al 2018). Thus, the canalization of the metabolic flux to improve only malate and citrate concentrations suggests their central role in the enhancement of plant Al tolerance (Nunes-Nesi et al 2014).…”

Roles of malic enzymes in plant development and stress responses

“…GmME 1 encodes a cytoplasmic ME that helps to increase the concentrations and efflux of malate and citric acid in the body, thereby improving aluminum resistance in plants and helping them to adapt better to the conditions in which they live. 41 Moreover, NADP-ME is also an important player in plantbased defense in Arabidopsis. After pathogen-associated molecular patterns (PAMPs) treatment and pathogen infection, total NADP-ME activity and NADP-ME2 transcript levels are enhanced, and NADP-ME2 loss-of-function mutants (nadp-me2) shows enhanced susceptibility.…”

Heterologous Expression of a Glycine soja C2H2 Zinc Finger Gene Improves Aluminum Tolerance in Arabidopsis

“…In recent years, there have been more and more reports on the use of soybean hairy root transformation to verify the function of the soybean Al resistance gene. For example, the dyed GmME1-OX hairy root tip was shallow, and the content of Al was low, indicating strong Al resistance [49]. After Al treatment, the hairy roots of over-expression were found to have the shallow color compared to the control, with the deep color observed of RNAi roots compared to the control, which showed that the Al ion content of GsGIS3-OX hairy root was lower than control ( Figure 6).…”