Abstract
The Calcium-Dependent Protein Kinase (CDPK)-Related Kinase family (CRKs) consists of eight members in Arabidopsis. Recently, AtCRK5 was shown to play a direct role in the regulation of root gravitropic response involving polar auxin transport (PAT). However, limited information is available about the function of the other AtCRK genes. Here, we report a comparative analysis of the Arabidopsis CRK genes, including transcription regulation, intracellular localization, and biological function. AtCRK transcripts were detectable in all organs tested and a considerable variation in transcript levels was detected among them. Most AtCRK proteins localized at the plasma membrane as revealed by microscopic analysis of 35S::cCRK-GFP (Green Fluorescence Protein) expressing plants or protoplasts. Interestingly, 35S::cCRK1-GFP and 35S::cCRK7-GFP had a dual localization pattern which was associated with plasma membrane and endomembrane structures, as well. Analysis of T-DNA insertion mutants revealed that AtCRK genes are important for root growth and control of gravitropic responses in roots and hypocotyls. While Atcrk mutants were indistinguishable from wild type plants in short days, Atcrk1-1 mutant had serious growth defects under continuous illumination. Semi-dwarf phenotype of Atcrk1-1 was accompanied with chlorophyll depletion, disturbed photosynthesis, accumulation of singlet oxygen, and enhanced cell death in photosynthetic tissues. AtCRK1 is therefore important to maintain cellular homeostasis during continuous illumination.
Highlights
Changes in environmental conditions represent a continuous stimulation for plants, which need adequate responses in metabolic adjustment, growth, and development
The Calcium-Dependent Protein Kinase (CDPK) kinase family have been identified throughout the plant kingdom and represents a large calcium-sensing subfamily of serine/threonine protein kinases that have been identified only in the plant kingdom but not found in animals [1,5,7]
The CDPK-Related Kinase Calmodulin-Dependent Protein Kinase-Related Kinases (CRKs) family is closely related to CDPKs but—in contrast to CDPKs—only limited amount of information is available on their biological functions [2,4,13,15,23,49]
Summary
Changes in environmental conditions represent a continuous stimulation for plants, which need adequate responses in metabolic adjustment, growth, and development. CDPKs and the Ca2+/Calmodulin-Dependent Protein Kinase-Related Kinases (CRKs) are Ser/Thr protein kinases, which have regulatory functions in diverse processes like plant growth and development, abiotic and biotic stress responses [1,2,3,4,5,13] as well as in phytohormone regulation [6]. The CDPKs have a variable N-terminal, a catalytic, an autoinhibitory domain, and a calmodulin-like (CaM) domain at their C-terminus The latter domain (EF-hand) of CDPKs is able to bind calcium mediating environmental effects to downstream signals [2,3,4,5]. To reveal information on their biological function, we carried out a systematic characterization of other AtCRK members We determined their intracellular localization through microscopic study of 35S::cCRK-eGFP gene fusions and cell fractionation. The length of illumination had an undetectable or only slight influence on the transcript levels of the AtCRK genes tested
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