A mutation in CsHY2 encoding a phytochromobilin (PFB) synthase leads to a deficiency in phytochrome system and elongated hypocotyl 1(elh1) phenotype in cucumber (Cucumis sativus L.)

Authors: LIANGLIANG, HU - Northwest A&f University; ZHUOSHUAI, JIN - Northwest A&f University; PENG, LIU - Northwest A&f University; JING, SUN - Northwest A&f University; Weng, Yiqun; PENG, CHEN - Tianjin Agricultural University; SHENLI, DU - Tianjin Agricultural University; AIMING, WEL - Tianjin Agricultural University; YUHONG, LI - Tianjin Agricultural University

Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2021
Publication Date: 6/6/2021
Citation: Liangliang, H., Zhuoshuai, J., Peng, L., Jing, S., Weng, Y., Peng, C., Shenli, D., Aiming, W., Yuhong, L. 2021. A mutation in CsHY2 encoding a phytochromobilin (PFB) synthase leads to a deficiency in phytochrome system and elongated hypocotyl 1(elh1) phenotype in cucumber (Cucumis sativus L.). Theoretical and Applied Genetics. 134:2639-2652. https://doi.org/10.1007/s00122-021-03849-4.
DOI: https://doi.org/10.1007/s00122-021-03849-4

Interpretive Summary: Hypocotyl length is a critical determinant in establishing high quality seedlings for successful cucumber production, but knowledge on the molecular regulation of hypocotyl growth in cucumber is very limited. We reported identification and characterization of a long-hypocotyl mutant (elh1). The longer hypocotyl in the mutant was due to long hypocotyl cells than those in normal cucumber line. With the map-based cloning approach, we found that the long hypocotyl in the mutant is caused by a single nucleotide substitution in the CsHY2 gene that encodes a phytochromobilin (PFB) synthase. We expressed this cucumber gene in the Arabidopsis long-hypocotyl mutant, which led to reduced hypocotyl length in the transgenic Arabidopsis plant. The PFB protein was targeted to chloroplasts. The expression levels of CsHY2 and five phytochrome genes were all reduced while several genes promoting cell elongation were increased in the mutant. Examination of the transcriptomes of the mutant and nomral plants revealed differentially expressed genes involved in porphyrin and chlorophyll metabolisms, cell elongation, and plant hormone signal transduction pathways. This study reveals the important role of CsHY2 in regulating hypocotyl length and extends our understanding of the roles of CsHY2 in cucumber.

Technical Abstract: Hypocotyl length is a critical determinant in establishing high quality seedlings for successful cucumber production, but knowledge on the molecular regulation of hypocotyl growth in cucumber is very limited. Here we reported identification and characterization of a cucumber elongated hypocotyl1 (elh1) mutant. We found that the longer hypocotyl in elh1 was due to longitudinal growth of hypocotyl cells. With fine mapping, the elh1 locus was delimited to a 20.9-kb region containing three annotated genes; only one polymorphism was identified in this region between two parental lines, which was a non-synonymous SNP (G28153633A) in the third exon of CsHY2 (CsGy1G030000) that encodes a phytochromobilin (PFB) synthase. Uniqueness of the mutant allele at CsHY2 was verified in 515 cucumber lines. Ectopic expression of CsHY2 in Arabidopsis hy2-1 mutant led to reduced hypocotyl length. The PFB protein was targeted to chloroplasts. The expression levels of CsHY2 and five phytochrome genes, CsPHYA1, CsPHYA2, CsPHYB, CsPHYC and CsPHYE were all significantly down-regulated while several cell elongation related genes were up-regulated in elh1 mutant compared to wild type cucumber, which are correlated with dynamic hypocotyl elongation in the mutant. RNA-seq analysis in the WT and mutant revealed differentially expressed genes involved in porphyrin and chlorophyll metabolisms, cell elongation, and plant hormone signal transduction pathways. This is the first report to characterize and clone the CsHY2 gene in cucumber. This work reveals the important of CsHY2 in regulating hypocotyl length and extends our understanding of the roles of CsHY2 in cucumber.