Abstract
The dwarf or compact plant architecture is an important trait in plant breeding. A number of genes controlling plant height have been cloned and functionally characterized which often involve in biosynthesis or signaling of plant hormones such as brassinosteroids (BRs). No genes for plant height or vine length have been cloned in cucurbit crops (family Cucurbitaceae). From an EMS-induced mutagenesis population, we identified a super compact (SCP) mutant C257 which was extremely dwarf due to practically no internode elongation. Under dark growing condition, C257 did not undergo skotomorphogenesis and its mutant phenotype could be rescued with exogenous application of brassinolide (BL), suggesting SCP might be a BR-deficient mutant. Segregation analysis revealed a single recessive gene scp-1 that was responsible for the SCP mutation. Map-based cloning combined with a modified MutMap identified CsCYP85A1, a member of the plant cytochrome P450 monooxygenase gene family, as the most possible candidate gene for scp-1, which encodes a BR-C6-oxidase in the BR biosynthesis pathway. We show that a SNP within the second exon of scp-1 candidate gene caused the SCP phenotype. Three copies of the CsCYP85A gene are present in the cucumber genome, but only the scp-1/CsCYP85A1 gene seemed active. The expression of CsCYP85A1 was higher in flowers than in the leaves and stem; its expression in the wild type (WT) was feedback regulated by BL application. Its expression was reduced in C257 as compared with the WT. This was the first report of map-based cloning of a plant height gene in cucurbit crops. The research highlighted the combined use of linkage mapping, an improved MutMap method and allelic diversity analysis in natural populations in quick cloning of simply inherited genes in cucumber. The roles of CsCYP85A1 in regulation of internode elongation in cucumber was discussed.
Highlights
Dwarf plant mutations have played important roles in elucidating the regulatory molecular mechanisms of plant growth and development (Tanabe et al, 2005)
We show that scp-1 encodes a cucumber cytochrome P450 gene CsCYP85A1 that played an important role in the BR biosynthesis pathway
We examined the homologous sequences of cucumber SCP1/CsCYP85A1 protein in other plants species including Arabidopsis thaliana, tomato, rice, maize, and soybean, and found this protein sequence was highly conserved with amino acid identity varying from 61 to 80% among those compared (Supplementary Table S4)
Summary
Dwarf plant mutations have played important roles in elucidating the regulatory molecular mechanisms of plant growth and development (Tanabe et al, 2005). Genes underlying dwarf mutations in a number of plant species have been cloned and functionally characterized, which in many cases are associated with biosynthesis and response pathways of plant hormones that regulate cell elongation and division (Clouse and Sasse, 1998; Richards et al, 2001; Sakamoto et al, 2004; Yamaguchi, 2008; Akter et al, 2015; Tamiru et al, 2015). A remarkable example is the green revolution genes (sd-1 in rice, and Rht-B1b and Rht-D1b in wheat for semi-dwarf varieties) that are involved in GA biosynthesis and signaling pathways (Peng et al, 1999; Spielmeyer et al, 2002; Hedden, 2003; Zeng et al, 2011). From the plant breeding perspective, the loss-of-function mutations of these genes are not useful due to the extreme dwarf phenotypes; instead, mutants with moderate changes in BR levels by manipulating the expression of the BR biosynthesis or signaling genes may be more meaningful for crop improvement (Choe et al, 2001; Liu et al, 2007; Divi and Krishna, 2009)
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