Structure and expression of GSL1 and GSL2 genes encoding gibberellin stimulated-like proteins in diploid and highly heterozygous tetraploid potato reveals their highly conserved and essential status.

BACKGROUND: GSL1 and GSL2, Gibberellin Stimulated-Like proteins (also known as Snakin-1 and Snakin-2), are cysteine-rich peptides from potato (Solanum tuberosum L.) with antimicrobial properties. Similar peptides in other species have been implicated in diverse biological processes and are hypothesised to play a role in several aspects of plant development, plant responses to biotic or abiotic stress through their participation in hormone crosstalk, and redox homeostasis. To help resolve the biological roles of GSL1 and GSL2 peptides we have undertaken an in depth analysis of the structure and expression of these genes in potato. RESULTS: We have characterised the full length genes for both GSL1 (chromosome 4) and GSL2 (chromosome 1) from diploid and tetraploid potato using the reference genome sequence of potato, coupled with further next generation sequencing of four highly heterozygous tetraploid cultivars. The frequency of SNPs in GSL1 and GSL2 were very low with only one SNP every 67 and 53 nucleotides in exon regions of GSL1 and GSL2, respectively. Analysis of comprehensive RNA-seq data substantiated the role of specific promoter motifs in transcriptional control of gene expression. Expression analysis based on the frequency of next generation sequence reads established that GSL2 was expressed at a higher level than GSL1 in 30 out of 32 tissue and treatment libraries. Furthermore, both the GSL1 and GSL2 genes exhibited constitutive expression that was not up regulated in response to biotic or abiotic stresses, hormone treatments or wounding. Potato transformation with antisense knock-down expression cassettes failed to recover viable plants. CONCLUSIONS: The potato GSL1 and GSL2 genes are very highly conserved suggesting they contribute to an important biological function. The known antimicrobial activity of the GSL proteins, coupled with the FPKM analysis from RNA-seq data, implies that both genes contribute to the constitutive defence barriers in potatoes. The lethality of antisense knock-down expression of GSL1 and GSL2, coupled with the rare incidence of SNPs in these genes, suggests an essential role for this gene family. These features are consistent with the GSL protein family playing a role in several aspects of plant development in addition to plant defence against biotic stresses.

GSL2 over-expression confers resistance to Pectobacterium atrosepticum in potato.

Over-expression of the potato Gibberellin Stimulated-Like 2 ( GSL2 ) gene in transgenic potato confers resistance to blackleg disease incited by Pectobacterium atrosepticum and confirms a role for GSL2 in plant defence. The Gibberellin Stimulated-Like 2 (GSL2) gene (also known as Snakin 2) encodes a cysteine-rich, low-molecular weight antimicrobial peptide produced in potato plants. This protein is thought to play important roles in the innate defence against invading microbes. Over-expression of the GSL2 gene in potato (cultivar Iwa) was achieved using Agrobacterium-mediated gene transfer of a plant expression vector with the potato GSL2 gene under the regulatory control elements of the potato light-inducible Lhca3 gene. The resulting plants were confirmed as being transgenic by PCR, and subsequently analysed for transcriptional expression of the Lhca3-GSL2-Lhca3 chimeric potato gene. Quantitative RT-PCR analysis demonstrated that the majority of the transgenic potato lines over-expressed the GSL2 gene at the mRNA level. Based on qRT-PCR results and evaluation of phenotypic appearance, eight lines were selected for further characterisation and evaluated in bioassays for resistance to Pectobacterium atrosepticum (formerly Erwinia carotovora subsp. atroseptica), the causal agent of blackleg in potato. Three independent pathogenicity bioassays showed that transgenic lines with significantly increased transcriptional expression of the GSL2 gene exhibit resistance to blackleg disease. This establishes a functional role for GSL2 in plant defence against pathogens in potato.

Towards disease resistance in potatoes using intragenic approaches.

Disease resistance is an important objective of global potato breeding programmes. The use of resistant cultivars is a significant tool for disease management. Recent advances in plant molecular genetics have identified several genes for resistance to potato diseases from within the germplasm pool available to potato breeders. Antimicrobial peptides, such as Snakin-1 (StSN1) and Snakin-2 (StSN2), have been isolated recently from potato tubers. Overexpression of the StSNI and StSN2 genes in potato is known to provide broad spectrum activity against a wide range of bacterial and fungal pathogens. We describe the use of intragenic gene transfer technology towards disease resistance in potatoes. An expression cassette was constructed with the 5' promoter and 3' terminator regions of a potato gene encoding a chlorophyll a/b binding protein (StLhca3). The coding regions of the StSN1 and StSN2 genes of potato were cloned individually between these regulatory regions. The resulting Lhca3-StSNi-Lhca3 and Lhca3-StSN2-Lhco3 chimeric genes were individually cloned into a potato-derived T-DNA-like region for potato transformation. Potato cultivar Iwa was co-cultivated with Agrobocterium harbouring intragenic binary vectors with the StSN1 and StSN2 genes. Regenerated potato plants were screened using PCR to identify lines transformed with the disease resistance genes without the presence of foreign DNA.