Submitted to: Genome
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/15/2006
Publication Date: 8/15/2006
Citation: Chung, S., Gordon, V., Staub, J.E. 2006. Sequencing of cucumber (Cucumis sativus l.) chloroplast genomes identifies putative candidate genes for chilling tolerance. Genome. 50:215-225. Interpretive Summary: Cold temperatures during seed germination and/or plant growth can severely reduce the yield potential of crop species. Chilling temperature stress (above 32o F but lower than 50o F) is a major abiotic stress in cucumber during vegetative and reproductive growth. Cells are affected in different ways when challenged with chilling temperatures. In cucumber, it is known that a specific part of the cell, the chloroplast (the organelle that is the site of photosynthesis), is compromised under chilling temperatures resulting in stunting and/or death of the plant. The genetic control (the hereditary factors that govern the degree of response) of chilling injury in cucumber is centered in the chloroplast. Biotechnology (tools that allow for the dissection of genes and genetic components) can be used to determine the genetic basis for chilling response (how genes which are hereditary units which are in linear arrays on chromosomes). Therefore, biotechnical tools were used to identify possible genes responsible for chilling injury in cucumber. Data indicate that there are three genes that have specific cellular functions and could be involved with chilling response. These results are the first step in the identification of the gene(s) for chilling injury in cucumber. It is now possible for plant geneticists to focus in the DNA (material that compose genes) structure of these three candidate genes to determine which genes control chilling in cucumber. This will increase our understanding of the genetics of chilling injury such that plant breeders can more efficiently develop chilling tolerant cucumber cultivars. Chilling tolerant cucumbers increase the likelihood of growth under cool Spring temperatures that will allow U.S. growers to reduce early yield losses and thus become more effective in producing a crop to meet early market demands. The ability to more consistently meet such market targets will allow the U.S. grower to become more globally competitive.
Technical Abstract: Chilling injury in cucumber (Cucumis sativus L.) is conditioned by maternal factors and the sequencing of its chloroplast (cp) genome could lead to the identification of economically important candidate genes. Complete sequencing of cucumber cpDNA was facilitated by the development of 414 consensus chloroplast sequencing primers (CCSPs) from conserved cpDNA sequences of Arabidopsis (Arabidopsis thaliana L.), spinach (Spinacia oleracea L.), and tobacco (Nicotiana tabacum L.) cpDNAs using degenerative primer technologies. Genomic sequence analysis led to the construction of 301 CCSPs and 72 cucumber chloroplast specific sequencing primers (CSSPs) that were used for the complete sequencing of cpDNA of line Gy14 (155,525 bp) and ‘Chipper’ (155,524 bp) cucumber which are respectively susceptible and tolerant to chilling injury (40C for 5.5 hr) in the first leaf-stage. Comparative cpDNA sequence analyses revealed that one sequence span (located between genes trnK and rps16) and two nucleotides (located in genes atpB and ycf1) differed between chilling susceptible and tolerant lines. These sequence differences correspond to previously reported, maternally inherited differences in chilling response between reciprocal F1 progeny derived from these lines. Sequence differences at these three cpDNA sites were also detected in a genetically diverse array of cucumber germplasm with differing chilling responses. These and previously reported results suggest that one or several of these sequences could be responsible for the observed response to chilling injury in cucumber. The comprehensive sequencing of cpDNA of cucumber produced CCSP and CSSP have immediate application in dicotyledonous species for cpDNA sequencing and investigation of evolutionary relationships.