Molecular genetic analysis of cold-regulated gene
transcription.
Viswanathan C, Zhu JK.
Department of Plant Sciences, University of Arizona, Tucson, AZ 85721, USA.
Chilling and freezing temperatures adversely affect the productivity and quality
of crops. Hence improving the cold hardiness of crop plants is an important goal
in agriculture, which demands a clear understanding of cold stress signal
perception and transduction. Pharmacological and biochemical evidence shows that
membrane rigidification followed by cytoskeleton rearrangement, Ca(2+) influx
and Ca(2+) -dependent phosphorylation are involved in cold stress signal
transduction. Cold-responsive genes are regulated through
C-repeat/dehydration-responsive elements (CRT/DRE) and abscisic acid
(ABA)-responsive element cis elements by transacting factors C-repeat binding
factors/dehydration-responsive element binding proteins (CBFs/DREBs) and basic
leucine zippers (bZIPs) (SGBF1), respectively. We have carried out a forward
genetic analysis using chemically mutagenized Arabidopsis plants expressing
cold-responsive RD29A promoter-driven luciferase to dissect cold signal
transduction. We have isolated the fiery1 (fry1) mutant and cloned the FRY1
gene, which encodes an inositol polyphosphate 1-phosphatase. The fry1 plants
showed enhanced induction of stress genes in response to cold, ABA, salt and
dehydration due to higher accumulation of the second messenger, inositol
(1,4,5)- triphosphate (IP(3)). Thus our study provides genetic evidence
suggesting that cold signal is transduced through changes in IP(3) levels. We
have also identified the hos1 mutation, which showed super induction of
cold-responsive genes and their transcriptional activators. Molecular cloning
and characterization revealed that HOS1 encodes a ring finger protein, which has
been implicated as an E3 ubiquitin conjugating enzyme. HOS1 is present in the
cytoplasm at normal growth temperatures but accumulates in the nucleus upon cold
stress. HOS1 appears to regulate temperature sensing by the cell as
cold-responsive gene expression occurs in the hos1 mutant at relatively warm
temperatures. Thus HOS1 is a negative regulator, which may be functionally
linked to cellular thermosensors to modulate cold-responsive gene transcription.