Cowpea ribonuclease: properties and effect of NaCl-salinity on its activation during seed germination and seedling establishment.

Pitiúba cowpea [Vigna unguiculata (L.) Walp] seeds were germinated in distilled water (control treatment) or in 100 mM NaCl solution (salt treatment), and RNase was purified from different parts of the seedlings. Seedling growth was reduced by the NaCl treatment. RNase activity was low in cotyledons of quiescent seeds, but the enzyme was activated during germination and seedling establishment. Salinity reduced cotyledon RNase activity, and this effect appeared to be due to a delay in its activation. The RNases from roots, stems, and leaves were immunologically identical to that found in cotyledons. Partially purified RNase fractions from the different parts of the seedling showed some activity with DNA as substrate. However, this DNA hydrolyzing activity was much lower than that of RNA hydrolyzing activity. The DNA hydrolyzing activity was strongly inhibited by Cu(2+), Hg(2+), and Zn(2+) ions, stimulated by MgCl(2), and slowly inhibited by EDTA. This activity from the most purified fraction was inhibited by increasing concentrations of RNA in the reaction medium. It is suggested that the major biological role of this cotyledon RNase would be to hydrolyze seed storage RNA during germination and seedling establishment, and it was discussed that it might have a protective role against abiotic stress during later part of seedling establishment.

Alternative oxidase regulation in roots of Vigna unguiculata cultivars differing in drought/salt tolerance.

The alternative oxidase (Aox) was studied at different levels (transcript, protein and capacity) in response to an osmotic shock applied to roots of cowpea (Vigna unguiculata). Two cultivars of V. unguiculata were used, Vita 3 and Vita 5, tolerant and sensitive to drought/saline stress respectively. The seedlings (17-day-old) were grown in hydroponic conditions and submitted to NaCl (100 and 200 mM) or 200.67 g L(-1) PEG 6000 (iso-osmotic condition to 100 mM NaCl). The VuAox1 and VuAox2a mRNA were not detected in either cultivar under all tested conditions while the VuAox2b gene was differently expressed. In the tolerant cultivar (Vita 3), the expression of VuAox2b gene was stimulated by an osmotic stress induced by PEG which was associated with a higher amount and capacity of the Aox protein. In the same cultivar, this gene was under-expressed in salt stress conditions with poor effect on the protein level. In the sensitive cultivar (Vita 5), the transcript level of the VuAox2b was unchanged in response to PEG treatment, even though the protein and the capacity tended to increase. Upon salt stress, the VuAox2b gene was over-expressed. At 100mM NaCl, this VuAox2b gene over-expression led to a higher amount and capacity of Aox. This effect was reduced at 200 mM NaCl. Overall, these results suggest complex mechanisms (transcriptional, translational and post-translational) for Aox regulation in response to osmotic stress.