Strong extracellular nuclease activity displayed by barley (Hordeum vulgare L) uninucleate microspores

Electroporation is becoming an increasingly important technique for plant transformation. Nevertheless, no positive results were achieved in barley when uninucleate microspores were used as target cells. Since it was previously demonstrated that electric shocks create pores in the microspore cell wall; experiments were designed to verify the presence of nucleases in the electroporation mix. Aliquots of all the solutions used for microspore extraction, purification and transformation were collected and analysed using supercoiled pBI 221 as a substrate; a nuclease activity was detected in all samples. Though microspore rinsing removed most nucleolytic activity in the supernatants, DNA preservation in the electroporation buffer was difficult to achieve, because microspores appeared capable of synthesising and releasing endonucleases at any time. Microspore chilling at 0 degrees C was fairly effective in reducing nuclease secretion in the mix, whereas 1% PEG or 10 mM EDTA maintained most of the DNA in a supercoiled or circular relaxed form. EDTA effects were counterbalanced by Mg2+, but not Ca2+ or Zn2+, and enhanced by Mn2+. Barley microspore nucleases actively degraded different DNAs as well as TMV RNA, and apparently had a molecular weight above 30 kDa. Nuclease inactivation with EDTA did not alter microspore viability and allowed a transient expression of the uidA gene in electroporated barley microspores.