Small RNAs are abundant in plant reproductive tissues, especially 24-nucleotide (nt) small interfering RNAs (siRNAs). Most 24-nt siRNAs are dependent on RNA Pol IV and RNA-DEPENDENT RNA POLYMERASE 2 (RDR2) and establish DNA methylation at thousands of genomic loci in a process called RNA-directed DNA methylation (RdDM). In Brassica rapa, RdDM is required in the maternal sporophyte for successful seed development. Here, we demonstrate that a small number of siRNA loci account for over 90% of siRNA expression during B. rapa seed development. These loci exhibit unique characteristics with regard to their copy number and association with genomic features, but they resemble canonical 24-nt siRNA loci in their dependence on RNA Pol IV/RDR2 and role in RdDM. These loci are expressed in ovules before fertilization and in the seed coat, embryo, and endosperm following fertilization. We observed a similar pattern of 24-nt siRNA expression in diverse angiosperms despite rapid sequence evolution at siren loci. In the endosperm, siren siRNAs show a marked maternal bias, and siren expression in maternal sporophytic tissues is required for siren siRNA accumulation. Together, these results demonstrate that seed development occurs under the influence of abundant maternal siRNAs that might be transported to, and function in, filial tissues.

Terpenes are economically and ecologically important phytochemicals. Their synthesis is controlled by the terpene synthase (TPS) gene family, which is highly diversified throughout the plant kingdom. The plant family Myrtaceae are characterised by especially high terpene concentrations, and considerable variation in terpene profiles. Many Myrtaceae are grown commercially for terpene products including the eucalypts Corymbia and Eucalyptus. Eucalyptus grandis has the largest TPS gene family of plants currently sequenced, which is largely conserved in the closely related E. globulus. However, the TPS gene family has been well studied only in these two eucalypt species. The recent assembly of two Corymbia citriodora subsp. variegata genomes presents an opportunity to examine the conservation of this important gene family across more divergent eucalypt lineages. Manual annotation of the TPS gene family in C. citriodora subsp. variegata revealed a similar overall number, and relative subfamily representation, to that previously reported in E. grandis and E. globulus. Many of the TPS genes were in physical clusters that varied considerably between Eucalyptus and Corymbia, with several instances of translocation, expansion/contraction and loss. Notably, there was greater conservation in the subfamilies involved in primary metabolism than those involved in secondary metabolism, likely reflecting different selective constraints. The variation in cluster size within subfamilies and the broad conservation between the eucalypts in the face of this variation are discussed, highlighting the potential contribution of selection, concerted evolution and stochastic processes. These findings provide the foundation to better understand terpene evolution within the ecologically and economically important Myrtaceae.

Terpenes are economically and ecologically important phytochemicals. Their synthesis is controlled by the terpene synthase (TPS) gene family, which is highly diversified throughout the plant kingdom. The plant family Myrtaceae are characterised by especially high terpene concentrations, and considerable variation in terpene profiles. Many Myrtaceae are grown commercially for terpene products including the eucalypts Corymbia and Eucalyptus. Eucalyptus grandis has the largest TPS gene family of plants currently sequenced, which is largely conserved in the closely related E. globulus. However, the TPS gene family has been well studied only in these two eucalypt species. The recent assembly of two Corymbia citriodora subsp. variegata genomes presents an opportunity to examine the conservation of this important gene family across more divergent eucalypt lineages. Manual annotation of the TPS gene family in C. citriodora subsp. variegata revealed a similar overall number, and relative subfamily representation, to that previously reported in E. grandis and E. globulus. Many of the TPS genes were in physical clusters that varied considerably between Eucalyptus and Corymbia, with several instances of translocation, expansion/contraction and loss. Notably, there was greater conservation in the subfamilies involved in primary metabolism than those involved in secondary metabolism, likely reflecting different selective constraints. The variation in cluster size within subfamilies and the broad conservation between the eucalypts in the face of this variation are discussed, highlighting the potential contribution of selection, concerted evolution and stochastic processes. These findings provide the foundation to better understand terpene evolution within the ecologically and economically important Myrtaceae.

Corymbia citriodora is a member of the predominantly Southern Hemisphere Myrtaceae family, which includes the eucalypts (Eucalyptus, Corymbia and Angophora; ~800 species). Corymbia is grown for timber, pulp and paper, and essential oils in Australia, South Africa, Asia, and Brazil, maintaining a high-growth rate under marginal conditions due to drought, poor-quality soil, and biotic stresses. To dissect the genetic basis of these desirable traits, we sequenced and assembled the 408 Mb genome of Corymbia citriodora, anchored into eleven chromosomes. Comparative analysis with Eucalyptus grandis reveals high synteny, although the two diverged approximately 60 million years ago and have different genome sizes (408 vs 641 Mb), with few large intra-chromosomal rearrangements. C. citriodora shares an ancient whole-genome duplication event with E. grandis but has undergone tandem gene family expansions related to terpene biosynthesis, innate pathogen resistance, and leaf wax formation, enabling their successful adaptation to biotic/abiotic stresses and arid conditions of the Australian continent.