High Ribonuclease Activity in the Testa of Common Bean Seedlings during Germination: Implication and Characterization of the Ribonuclease T2 PvRNS3

T2 ribonucleases are endoribonucleases that are found in every organism and that carry out important biological functions. In plants, T2 ribonucleases are organized into multi-gene families, and each member is thought to have a specific function. In this study, the ribonuclease activity has been analyzed in common bean seedlings during germination and it was found that the activity was very high in the testa during this process. This high activity correlated with a high level of expression of the S-like ribonuclease T2 PvRNS3. The protein encoded by this gene was overexpressed in Escherichia coli and characterized. The purified protein showed ribonuclease activity with RNA and not with DNA, confirming that PvRNS3 encodes a ribonuclease. PvRNS3 is an acidic ribonuclease with remarkable heat stability, of which activity is inhibited by Cu and Zn, as well as by ditiotreitol (DTT). PvRNS3 expression was also selectively induced in some stress situations, such as salt stress in radicles and wounded leaves. The high level of expression in the testa and high ribonuclease activity suggest an additional role for the testa in common bean germination, apart from being a protective barrier for embryos in seeds. The putative role of this ribonuclease in the extracellular space after seed hydration and release to the surrounding space to improve seedling fitness is discussed.