Calcium involved in the poly(γ-glutamic acid)-mediated promotion of Chinese cabbage nitrogen metabolism

Plant growth can reportedly be promoted by poly(γ-glutamic acid) (γ-PGA). However, the underlying mechanism is unknown. To reveal the mechanism of γ-PGA, we designed an experiment that investigated the effect of γ-PGA on the nitrogen metabolism of Chinese cabbage hydroponic cultured at different calcium (Ca) levels and varied exogenous Ca²⁺ inhibitors. The results showed that nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase, and glutamate dehydrogenase activities in leaves and roots were obviously enhanced by γ-PGA at the normal Ca²⁺ level (4.0mM). Meanwhile, γ-PGA increased the content of total nitrogen, soluble protein, and soluble amino acids in leaves. However, the promotional effect of γ-PGA on fresh weight weakened when Ca²⁺ was inadequate. Moreover, γ-PGA not only induced the influx of extracellular Ca²⁺ and Ca²⁺ in organelles into cytoplasm, but also increased the Ca²⁺-ATPase level to modify Ca²⁺ homeostasis in plant cells. In addition, exogenous Ca²⁺ inhibitors significantly suppressed the γ-PGA-mediated promotion of cytoplasmic free Ca²⁺ level, calmodulin (CaM) content, GS and glutamate dehydrogenase activities. In summary, γ-PGA accelerated the nitrogen metabolism of plants through the Ca²⁺/CaM signaling pathway, thereby improving the growth of the plant.