Abstract:
Magnoliids are the third-largest group of angiosperms and occupy a critical position in
angiosperm evolution. In the past years, due to the lack of sequenced genomes, the
disease resistance gene (R gene) profile of magnoliids remains poorly understood. By
the genome-wide identification of 1,832 NLR genes from seven magnoliid genomes, we
built a framework for the evolution of magnoliid R genes. TNL genes were completely
absent from five magnoliids, presumably due to immune pathway deficiencies. A total of
74 ancestral R genes (70 CNLs, 3 TNLs, and 1 RNL) were recovered in a common
ancestor of magnoliids, from which all current NLR gene repertoires were derived.
Tandem duplication served as the major drive for NLR genes expansion in seven
magnoliid genomes, as most surveyed angiosperms. Due to recent rapid expansions,
most magnoliids exhibited “a first expansion followed by a slight contraction and
a further stronger expansion” evolutionary pattern, while both Litsea cubeba and
Persea americana showed a two-times-repeated pattern of “expansion followed by
contraction.” The transcriptome analysis of seven different tissues of Saururus chinensis
revealed a low expression of most NLR genes, with some R genes displaying a relatively
higher expression in roots and fruits. Overall, our study sheds light on the evolution
of NLR genes in magnoliids, compensates for insufficiency in major angiosperm
lineages, and provides an important reference for a better understanding of angiosperm
NLR genes.
