Leaf seasonal accumulation of a 47-kDa dehydrin and changes in its cryoprotective activity in Nothofagus dombeyi (Mirb.) Blume

Resumo

Nothofagus dombeyi (Mirb.) Blume is an evergreen tree, pioneer in habitats subject to unfavorable environmental conditions
as nighttime freezing temperatures and low water content. It inhabits zones characterized by cold Winter with freezing
temperature and Spring frosts reaching to -5ºC. Furthermore, it has the capacity to maintain a functional photosynthetic
apparatus at low temperatures. We postulated that N. dombeyi is capable to accumulate dehydrins in leaves in response to
cold. Seasonal variation in dehydrins, soluble total proteins, and cryoprotective activity in vitro of protein extracts of N.
dombeyi leaves were analyzed during the years 2004 and 2005. A dehydrin-like protein of 47-kDa was immunologically
identified; this protein was present only in Winter and Spring. The highest accumulation of this protein and the maximal
in vitro cryoprotective activity was found during Spring. The leaf total protein extract of this season preserved the 100%
of lactate dehydrogenase (LDH) enzyme activity after 5 freeze/thaw cycles in liquid nitrogen adding 10 ?g/ml of total
proteins extract. The soluble protein concentration increased from 1.8 mg/g in Summer to 4.8 mg/g in Fall on base of
fresh weight, and declined through Winter and Spring. The 47-kDa dehydrin may be related to cold hardiness of leaf
tissue in overwintering N. dombeyi. We conclude that cell dehydration caused by freezing is the main factor involved in
the triggering of 47-kDa dehydrin accumulation in leaves of N. dombeyi. The accumulation of reactive oxygen species
(ROS) by oxidative stress product of high irradiance and frosts during Spring, and their role in the triggering of the 47-kDa
dehydrin accumulation in leaves of N. dombeyi is discussed.