Accumulation of a 22‐kDa protein and its mRNA in the leaves of <i>Raphanus sativus</i> in response to salt stress or water deficit

Abstract

The response of 10‐day‐old seedlings of Raphanus sativus L. cv. Fakir to salt stress (100 m M to 200 m M NaCl) was investigated. Three weeks after initiation of salt treatment, the fresh weight of the shoots of salt‐treated plants was half that of untreated plants. The salt stress resulted in the accumulation of Na + , preferably in the old leaves. The K + level was reduced by as much as 50% in the old leaves of NaCl‐treated plants, whereas this reduction was only 20–25% in the young leaves. Free proline accumulated in all aerial organs, and the highest levels were found in the young leaves. Patterns of total proteins extracted from the leaves of control or salt‐treated plants were compared. The most obvious change concerned a 22‐kDa, pl 7.5 polypeptide, which accumulated after exposure of the plants to NaCl. The appearance of this polypeptide was also mediated by a rapid drought stress, and sequencing indicated that it is related to the Künitz protease inhibitor family. A cDNA clone corresponding to the radish 22‐kDa polypeptide was obtained and sequenced. Northern blot analysis showed that salt stress induces a large accumulation of this mRNA in the leaves of radish.