RNS2: a senescence-associated RNase of Arabidopsis that diverged from the S-RNases before speciation.

Several self-compatible species of higher plants, such as Arabidopsis thaliana, have recently been found to contain S-like RNases. These S-like RNases are homologous to the S-RNases that have been hypothesized to control self-incompatibility in Solanaceous species. However, the relationship of the S-like RNases to the S-RNases is unknown, and their roles in self-compatible plants are not understood. To address these questions, we have investigated the RNS2 gene, which encodes an S-like RNase (RNS2) of Arabidopsis. Amino acid sequence comparisons indicate that RNS2 and other S-like RNases make up a subclass within an RNase superfamily, which is distinct from the subclass formed by the S-RNases. RNS2 is most similar to RNase LE [Jost, W., Bak, H., Glund, K., Terpstra, P., Beintema, J. J. (1991) Eur. J. Biochem. 198, 1-6.], an S-like RNase from Lycopersicon esculentum, a Solanaceous species. The fact that RNase LE is more similar to RNS2 than to the S-RNases from other Solanaceous plants indicates that the S-like RNases diverged from the S-RNases prior to speciation. Like the S-RNase genes, RNS2 is most highly expressed in flowers, but unlike the S-RNase genes, RNS2 is also expressed in roots, stems, and leaves of Arabidopsis. Moreover, the expression of RNS2 is increased in both leaves and petals of Arabidopsis during senescence. Phosphate starvation can also induce the expression of RNS2. On the basis of these observations, we suggest that one role of RNS2 in Arabidopsis may be to remobilize phosphate, particularly when cells senesce or when phosphate becomes limiting.