The Conservation Of Two Arbuscular Mycorrhiza Symbiotic Genes In Land Plants And Their Loss/ Pseudolization In Brassicaceae

Most land plants can form a root symbiosis with arbuscular mycorrhizal (AM) fungi from Glomeromycota for efficient absorption of phosphate, nitrate and water in the soil. A series of complex morphological and physiological changes occur during the formation of AM symbiosis. Genes involved in AM symbiosis signal transduction are called SYM genes. POLLUX/DMI1 and CASTOR are two paralogous SYM genes encoding ion channel proteins localized in nuclear envelope, regulating the ion flux. The two genes are essential to the AM symbiosis pathway. Studies on structures and functions of POLLUX/DMI1 and CASTOR in model legume plants have been widely reported, whereas little research has focused on the molecular evolution of the two genes. In this article, we searched genome data of 35 land plants and 5 green algae plants which have been whole genome sequenced and received 37 POLLUX/DMI1 homologs and 33 CASTOR homologs.6 homologs derived from moss and green algae which are neither POLLUX/DMIl nor CASTOR are defined as P/C-Like genes. We revised the annotation of POLLUX/DMI1 and CASTOR through sequence comparison and analysis. Through our result, the two genes diversified after bryophytes, forming two independent clades in land plants. POLLUX/DMI1 homologs are mostly conserved in land plants, while many CASTOR homologs become pseudogenes. In 5 Brassicaceae genomes, CASTOR has been completely lost.Members of the Brassicaceae, including the model plant Arabidopsis thaliana, seem to have removed themselves from the ancestral symbiosis with AM fungi and lost some of the SYM genes. In order to clarify the molecular mechanism of POLLUX/DMIl and CASTOR in Brassicaceae, we obtain 33 POLLUX/DM1 copies from 23 cruciferous species (9 species have multiple copies) through RT-PCR and TA cloning.18 of the 33 sequences are pseudogenes. Further analysis shows 9 types of InDel (Insertion/Deletion) occur in these POLLUX/DMI1 copies,7 of which are found on exon-intron boundaries while 6 types cause appearance of stop codon. The results presented here show that, contributed by alternative splicing, POLLUX/DMI1 of Brassicaceae seem to be in the process of loss/pseudolization. Additionally, we use a new method to stain roots of 8 wild cruciferous species, confirming the lack of AM structure in Brassicaceae.