Regulation of basal resistance by a powdery mildew-induced cysteine-rich receptor-like protein kinase in barley

The receptor-like protein kinases (RLKs) constitute a large and diverse group of proteins controlling numerous plant physiological processes, including development, hormone perception and stress responses. The cysteine-rich RLKs (CRKs) represent a prominent subfamily of transmembrane-anchored RLKs. We have identified a putative barley (Hordeum vulgare) CRK gene family member, designated HvCRK1. The mature putative protein comprises 645 amino acids, and includes a putative receptor domain containing two characteristic 'domain 26 of unknown function' (duf26) domains in the N-terminal region, followed by a rather short 17-amino-acid transmembrane domain, which includes an AAA motif, two features characteristic of endoplasmic reticulum (ER)-targeted proteins and, finally, a characteristic putative protein kinase domain in the C-terminus. The HvCRK1 transcript was isolated from leaves inoculated with the biotrophic fungal pathogen Blumeria graminis f.sp. hordei (Bgh). HvCRK1 transcripts were observed to accumulate transiently following Bgh inoculation of susceptible barley. Transient silencing of HvCRK1 expression in bombarded epidermal cells led to enhanced resistance to Bgh, but did not affect R-gene-mediated resistance. Silencing of HvCRK1 phenocopied the effective penetration resistance found in mlo-resistant barley plants, and the possible link between HvCRK1 and MLO was substantiated by the fact that HvCRK1 induction on Bgh inoculation was dependent on Mlo. Finally, using both experimental and in silico approaches, we demonstrated that HvCRK1 localizes to the ER of barley cells. The negative effect on basal resistance against Bgh and the functional aspects of MLO- and ER-localized HvCRK1 signalling on Bgh inoculation are discussed.

Transcriptional regulation by an NAC (NAM-ATAF1,2-CUC2) transcription factor attenuates ABA signalling for efficient basal defence towards Blumeria graminis f. sp. hordei in Arabidopsis

ATAF1 is a member of a largely uncharacterized plant-specific gene family encoding NAC transcription factors, and is induced in response to various abiotic and biotic stimuli in Arabidopsis thaliana. Previously, we showed that a mutant allele of ATAF1 compromises penetration resistance in Arabidopsis with respect to the non-host biotrophic pathogen Blumeria graminis f. sp. hordei (Bgh). In this study, we have used genome-wide transcript profiling to characterize signalling perturbations in ataf1 plants following Bgh inoculation. Comparative transcriptomic analyses identified an over-representation of abscisic acid (ABA)-responsive genes, including the ABA biosynthesis gene AAO3, which is significantly induced in ataf1 plants compared to wild-type plants following inoculation with Bgh. Additionally, we show that Bgh inoculation results in decreased endogenous ABA levels in an ATAF1-dependent manner, and that the ABA biosynthetic mutant aao3 showed increased penetration resistance to Bgh compared to wild-type plants. Furthermore, we show that ataf1 plants show ABA-hyposensitive phenotypes during seedling development and germination. Our data support a negative correlation between ABA levels and penetration resistance, and identify ATAF1 as a new stimuli-dependent attenuator of ABA signalling for the mediation of efficient penetration resistance in Arabidopsis upon Bgh attack.

Carbon partitioning in leaves and tubers of transgenic potato plants with reduced activity of fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase

The role of fructose-2,6-bisphosphate (Fru-2,6-P(2)) in regulation of carbon metabolism was investigated in transgenic potato plants (Solanum tuberosum L. cv Dianella) transformed with a vector containing a cDNA-sequence encoding fructose-6-phosphate,2-kinase (F6P,2-K, EC 2.7.1.105)/fructose-2,6-bisphosphatase (F26BPase, EC 3.1.3.46) in sense or antisense direction behind a CaMV 35S promoter. The activity of F6P,2-K in leaves was reduced to 5% of wild-type (WT) activity, and the level of Fru-2,6-P(2) was reduced both in leaves (10% of the WT level) and in tubers (40% of the WT level). Analysis of photosynthetic (14)CO(2) metabolism, showed that in plant lines with reduced Fru-2,6-P(2) level the carbon partitioning in the leaves was changed in favour of sucrose biosynthesis, and the soluble sugars-to-starch labelling ratio was doubled. The levels of soluble sugars and hexose phosphates also increased in leaves of the transgenic plants. Most notably, the levels of hexoses were four- to six-fold increased in the transgenic plants. In tubers with reduced levels of Fru-2,6-P(2) only minor effects on carbohydrate levels were observed. Furthermore, carbon assimilation in tuber discs supplied with [U-(14)C]-sucrose showed only a moderate increase in labelling of hexoses and a decreased labelling of starch. Similar results were obtained using [U-(14)C]-glucose. No differences in growth of the transgenic lines and the WT were observed. Our data provide evidences that Fru-2,6-P(2) is an important factor in the regulation of photosynthetic carbon metabolism in potato leaves, whereas the direct influence of Fru-2,6-P(2) on tuber metabolism was limited.

Structure and heterologous expression of a gene encoding fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase from Arabidopsis thaliana

A full-length cDNA clone encoding fructose-6-phosphate, 2-kinase/fructose-2,6-bisphosphatase from Arabidopsis thaliana (AtF2KP) was isolated. The encoded protein is composed of two different regions: (i) a 400 amino acid COOH-terminal region, covering the catalytic region of the protein which is homologous to enzymes from other eukaryotes. This region is highly conserved among plant species (88% identity to spinach F2KP). (ii) A 345 amino acid plant-specific NH(2)-terminal region, with 59% identity to spinach F2KP, which is composed of homologous motifs and intermittent variable sequences. Western blots show that F2KP from several plant species migrates in sodium dodecyl sulphate-polyacrylamide gel electrophoresis as a similar sized (93 kDa) protein. AtF2KP was expressed in Escherichia coli as a full length and a truncated (without the NH(2)-terminal region) fusion protein. Both forms had kinase as well as phosphatase activity, but presence of the NH(2)-terminal region influenced the ratio between the two activities. It is suggested that the NH(2)-terminal region represents a regulatory region, which defines specific properties of the plant enzymes. A genomic clone for the corresponding gene, AtF2KP, was isolated. The clone (9519 bp) included 23 exons, 22 introns and the promoter sequence. Southern blot analysis showed only one copy of the gene in the A. thaliana genome.