Quantitative proteomics reveals dual effects of calcium on radicle protrusion in soybean

To reveal calcium-mediated germination in soybean, a gel-free/label-free proteomics was performed in radicle of seed imbibed with CaCl₂. Morphological analysis presented promoting and suppressing performance of seed growth under 5 and 50 mM CaCl₂, respectively. A total of 106 and 581 proteins were identified in response to 5 and 50 mM CaCl₂, respectively. Among 33 proteins, which were simultaneously affected by 5 and 50 mM CaCl₂ imbibition, proteins related to protein metabolism, cell, development, and stress showed reversed abundance in response to CaCl₂ on dose-dependent manner. Notably, protein abundance of late embryogenesis abundant (LEA) 4-5, LEA4, and dehydrin decreased and increased by 5 and 50 mM CaCl₂, respectively, consistent with the transcript level. Moreover, inhibited biosynthesis of gibberellic acid repressed growth of 5 mM CaCl₂-imbibed soybean, while inhibition of abscisic acid biosynthesis released the suppressing effects of 50 mM CaCl₂. Taken together, these results suggest that decreased or increased protein abundance of LEA4-5, LEA4, and dehydrin might determine promoting or suppressing effects of low or high level of calcium on soybean through enhancing seed sensitivity to gibberellic acid or abscisic acid during radicle protrusion. SIGNIFICANCE: Calcium serves as a versatile signal in plant growth; however, calcium-mediated germination on dose-dependent manner remains elusive. In this study, dual effects of calcium on radicle protrusion in soybean were investigated using proteomic approach. Radicle growth of germinating seed was improved by 5 mM CaCl₂; however, it was retarded by 50 mM CaCl₂. Late embryogenesis abundant (LEA) 4-5, LEA4, and dehydrin displayed converse profiles in response to low and high concentrations of CaCl₂ at both protein abundance and gene expression level. Inhibited biosynthesis of gibberellic acid (GA) significantly impeded radicle protrusion in presence of low concentration of CaCl₂, while inhibiting of abscisic acid (ABA) biosynthesis released suppression induced by high concentration of CaCl₂. These findings suggest that LEA proteins are associated with calcium-mediated radicle protrusion on dose-dependent manner, and seed sensitivity to GA and ABA might determine promoting and suppressing effects of calcium on radicle protrusion in soybean.