Abstract
Santalum album (sandalwood) is one of the economically important plant species in the Santalaceae for its production of highly valued perfume oils. Sandalwood is also a hemiparasitic tree that obtains some of its water and simple nutrients by tapping into other plants through haustoria which are highly specialized organs in parasitic angiosperms. However, an understanding of the molecular mechanisms involved in haustorium development is limited. In this study, RNA sequencing (RNA-seq) analyses were performed to identify changes in gene expression and metabolic pathways associated with the development of the S. album haustorium. A total of 56,011 non-redundant contigs with a mean contig size of 618 bp were obtained by de novo assembly of the transcriptome of haustoria and non-haustorial seedling roots. A substantial number of the identified differentially expressed genes were involved in cell wall metabolism and protein metabolism, as well as mitochondrial electron transport functions. Phytohormone-mediated regulation might play an important role during haustorial development. Especially, auxin signaling is likely to be essential for haustorial initiation, and genes related to cytokinin and gibberellin biosynthesis and metabolism are involved in haustorial development. Our results suggest that genes encoding nodulin-like proteins may be important for haustorial morphogenesis in S. album. The obtained sequence data will become a rich resource for future research in this interesting species. This information improves our understanding of haustorium development in root hemiparasitic species and will allow further exploration of the detailed molecular mechanisms underlying plant parasitism.
Highlights
The Santalaceae is a widely distributed family of flowering plants and its members, like other species of the Santalales, are hemiparasitic plants which can produce photosynthetic products for themselves, but need to obtain water and solutes from the host
A transcriptomic analysis of haustorium development in S. album was performed using RNA sequencing (RNA-seq) technology to understand metabolic pathways and transcriptional regulatory networks during haustorial development aided by cytomorphological features
A total of 56,011 contigs were generated from a combination of haustoria and seedling root libraries, and nearly half of them could be annotated against the NCBI NR database
Summary
The Santalaceae is a widely distributed family of flowering plants and its members, like other species of the Santalales, are hemiparasitic plants which can produce photosynthetic products for themselves, but need to obtain water and solutes from the host. The sandalwood tree, is one of the economically important members in this family. At least 300 species, including S. album itself, can act as hosts of sandalwood tree, supplying water and nutrients through a unique organ termed the haustorium, especially during early phases of development (Fineran, 1963; Nagaveni and Vijayalakshmi, 2003). Parasitic plants depend on host root-derived chemical signals to induce seed germination and the formation of haustoria. Most parasitic species will develop a functional haustorium depending on a second chemical signal derived from the host exudates, such as 2,6-dimethoxy-p-benzoquinone (DMBQ), phenolic acids, and flavonoids which have been identified as haustoria-inducing factors (HIFs) (Cook et al, 1966; Albrecht et al, 1999; Yoneyama et al, 2008). Little is known about the molecular mechanisms involved in haustorial development in S. album
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