Search for Candidate Genes Causing the Excessive Ca Accumulation in Roots of Tipburn-Damaged Lisianthus (Eustoma grandiflorum) Cultivars

Abstract

Occurrence of tipburn is a severe problem in the production of lisianthus cultivars. Previous studies have shown excessive Ca accumulation in the roots of tipburn-damaged cultivars, where the distribution of Ca to the tips of the top leaves is inhibited. However, few studies have investigated the association between Ca accumulation and gene expression in horticultural crops. To provide a list of candidate target genes that might be causing the excessive Ca accumulation in roots, we focused Ca2+ transporter and pectin methylesterase (PME) genes and RNA-seq of upper leaves and roots in tipburn-occurrence cultivar (“Voyage peach”: VP) and non-occurrence cultivar (“Umi honoka”: UH) was conducted. In both the upper leaves and roots of VP, genes encoding the glutamate receptors (GLRs), cation/Ca2+ exchangers 4 (CCX4), Na+/Ca2+ exchanger-like protein (NCL), and PMEs were upregulated, and a gene encoding the cyclic nucleotide-gated ion channel 9 (CNGC9) was downregulated. In contrast, genes encoding the vacuolar cation/proton exchanger 5 (CAX5), calcium-transporting ATPase 1 and 12 (ACA1 and ACA12) showed differential expression in each organ. Among them, only CAX5 was upregulated and ACA12 was downregulated in the roots of VP. Based on these results, we suggested that CAX5 and ACA12 are the candidate genes causing the excessive Ca accumulation in the roots of tipburn-occurrence lisianthus cultivars. Future studies should investigate the temporal changes in gene expression using quantitative PCR and conduct functional analysis of candidate genes in tipburn-damaged lisianthus cultivars.