Background: Iris laevigata Fisch. (Iridaceae) is an important aquatic plant with dark blue flowers. I. laevigata var. alba Ling Wang, L. Su & F. J. Shang is especially rare, it has white perianths with white anthers with a blue tinge. I. laevigata var. alba provides a good reference material for revealing complex and variations in anthocyanin catabolism networks. Modern molecular biology technology will facilitate the study of the mechanisms underlying metabolite variation. However, molecular research on I. laevigata is limited due to the lack of sequence data.
Results: In this study, RNA-Seq was performed on I. laevigata and I. laevigata var. alba at flowering stage. Two libraries were sequenced using Illumina Hiseq 2000 platform. Clean data of each sample reached 7.01 Gb. A total of 64,537 unigenes were obtained after
assembly, including 28,936 unigenes annotated to seven public databases. Then, 143 unigenes were putative homologs to color-related genes, including 1 up-regulated and 12 down-regulated unigenes. Combined with reverse transcript polymerase chain reaction
(RT-PCR), a number of important color-related genes were tested. In perianths, 5 flavonoids (4 anthocyanins and 1 flavone) were detected using HPLC at flowering stage. Then, the putative anthocyanin metabolic process of flowering stage and differential genes related to flower color variation were put forward. A new hypothesis about the absence of phenotypic color of I. laevigata var. alba was proposed. It was suggested that the loss of anthocyanin was due to the interaction of multiple genes. First, upstream metabolic fluxes were redistributed by up-regulated CHI. Second, synergism of down-regulated genes (F3H/DFR/ANS) and competition for substrates between DFR and FLS resulted in relatively low biological flux and multi-shunt of metabolic pathways. As a result, the anthocyanins content in I. laevigata var. alba is very limited, which leads to the variation of flower color phenotype.
Conclusions: This study provides mass sequence data by the deep sequencing of I.laevigata and I. laevigata var. alba for the first time. Combined with the detection and metabolite analysis of flavonoids, it will increase our understanding of the molecular mechanism of the phenotypic variation in I. laevigata var. alba and provide the basis for molecular breeding of unique flower colors.