The homologous HD-Zip I transcription factors HaHB1 and AtHB13 confer cold tolerance via the induction of pathogenesis-related and glucanase proteins.

Plants deal with cold temperatures via different signal transduction pathways. The HD-Zip I homologous transcription factors HaHB1 from sunflower and AtHB13 from Arabidopsis were identified as playing a key role in such cold response. The expression patterns of both genes were analyzed indicating an up-regulation by low temperatures. When these genes were constitutively expressed in Arabidopsis, the transgenic plants showed similar phenotypes including cell membrane stabilization under freezing treatments and cold tolerance. An exploratory transcriptomic analysis of HaHB1 transgenic plants indicated that several transcripts encoding glucanases and chitinases were induced. Moreover, under freezing conditions some proteins accumulated in HaHB1 plants apoplasts and these extracts exerted antifreeze activity in vitro. Three genes encoding two glucanases and a chitinase were overexpressed in Arabidopsis and these plants were able to tolerate freezing temperatures. All the obtained transgenic plants exhibited cell membrane stabilization after a short freezing treatment. Finally, HaHB1 and AtHB13 were used to transiently transform sunflower and soybean leading to the up-regulation of HaHB1/AtHB13-target homologues thus indicating the conservation of cold response pathways. We propose that HaHB1 and AtHB13 are involved in plant cold tolerance via the induction of proteins able to stabilize cell membranes and inhibit ice growth.