Molecular Research Progress on Xishuangbanna Cucumber (Cucumis sativus L. var. Xishuangbannesis Qi et Yuan): Current Status and Future Prospects

Abstract

The Xishuangbanna (XIS) cucumber is an important botanical variety, accumulating high levels of β-carotene (700 μg/100 g) in the endocarp of mature fruit compared with normal green/white flesh types (25–50 μg/100 g, fresh weight). β-carotene is an essential precursor of provitamin A synthesis required for human health, thus XIS cucumber is an appealing germplasm for vitamin A breeding programs. In this review, we highlighted the molecular research progress of XIS cucumber as well as the bottlenecks undermining its utilization in genetic breeding. The XIS cucumber was first reported in 1983; thereafter, the literature on XIS cucumber was sporadic until 2012 and 2013 following QTL mapping of the Ore gene and subsequent cloning of the CsaBCH1 gene, respectively. Whereas QTL mapping studies underlying its flowering time and fruit quality related traits have been reported, fine mapping of their candidate genes remains unknown. Cucumber fruits are mainly consumed at fresh immature stage; however, XIS cucumber accumulates β-carotene at mature fruit state, thus limiting the utilization of β-carotene derived from it. In our opinion, we believe that the production and commercialization of immature orange-fleshed cucumber would gain wider acceptance among consumers. Additionally, we highlighted a comprehensive breeding strategy, precisely for enhanced β-carotene accumulation based on prior studies of XIS cucumber coupled with those from other crops. In MAS, we proposed schematic molecular backcross breeding strategy using lines possessing both ore and fft1.1 loci. This review, therefore, provides insights of XIS cucumber research and opportunities for further genetic breeding.