Abstract
Many Cactaceae species exhibit determinate growth of the primary root as a consequence of root apical meristem (RAM) exhaustion. The genetic regulation of this growth pattern is unknown. Here, we de novo assembled and annotated the root apex transcriptome of the Pachycereus pringlei primary root at three developmental stages, with active or exhausted RAM. The assembled transcriptome is robust and comprehensive, and was used to infer a transcriptional regulatory network of the primary root apex. Putative orthologues of Arabidopsis regulators of RAM maintenance, as well as putative lineage-specific transcripts were identified. The transcriptome revealed putative orthologues of most proteins involved in housekeeping processes, hormone signalling, and metabolic pathways. Our results suggest that specific transcriptional programs operate in the root apex at specific developmental time points. Moreover, the transcriptional state of the P. pringlei root apex as the RAM becomes exhausted is comparable to the transcriptional state of cells from the meristematic, elongation, and differentiation zones of Arabidopsis roots along the root axis. We suggest that the transcriptional program underlying the drought stress response is induced during Cactaceae root development, and that lineage-specific transcripts could contribute to RAM exhaustion in Cactaceae.
Highlights
Plant growth and organogenesis are postembryonic processes sustained by the presence and activity of meristems, which act as reservoirs of pluripotent cells
These data, together with the incidence of root apical meristem (RAM) exhaustion in lateral roots and the determinate growth of roots regenerated from calli[12], indicate that RAM exhaustion in Cactaceae species is a genetically regulated developmental program, rather than a response to environmental conditions
Our knowledge of the genetic regulation of RAM establishment and maintenance is mainly derived from studies of Arabidopsis thaliana (Arabidopsis) mutants with short root phenotypes
Summary
Contig features Average length (nt) Max. length (nt) Min. length (nt) 400–2,000 nt 2,000–4,000 nt >4,000 nt. The primary root of the plt[1] mutant of Arabidopsis exhibits subtle alterations of the cell division pattern in the QC and root cap cells, while plt[1] plt[2] double mutants develop a much shorter primary root that stops growing soon after germination due to RAM exhaustion, exhibiting determinate growth[15]. In addition to their well-characterized roles in the radial organization of the root, the GRAS transcription cofactors SCARECROW (SCR) and SHORT-ROOT (SHR) are involved in RAM maintenance. The P. pringlei root apex transcriptome was used to assess differential gene expression and to infer a transcriptional regulatory network, with the aim of expanding our knowledge of RAM exhaustion in Cactaceae species and, enhancing our understanding of RAM maintenance in angiosperms
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