RNA-seq insights into primary root apical meristem exhaustion and determinate root growth in Pachycereus pringlei (Cactaceae)

Many Cactaceae species exhibit determinate growth of the primary root as a consequence of root apical meristem (RAM) exhaustion. The genetic regulation of this root growth pattern is unknown. 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 was characterized and used to evaluate differential transcript expression, identify RT-qPCR reference genes, and infer a transcriptional regulatory network. We generated a robust and comprehensive transcriptome of the primary root apex of P. pringlei and identified putative orthologs of Arabidopsis regulators of RAM maintenance, as well as putative lineage-specific transcripts. Furthermore, the transcriptome contained putative orthologs of most proteins involved in housekeeping processes, hormone signaling, and metabolic pathways. Specific transcriptional programs operate in the root apex at specific developmental time points. The transcriptional state of the P. pringlei root apex as the RAM becomes exhausted is comparable to that of cells from the meristematic, elongation, and differentiation zones of Arabidopsis roots. We suggest that the genetic program underlying the drought stress response is induced during development of the Cactaceae root, and that lineage-specific transcripts could contribute to root apical meristem exhaustion in Cactaceae. 1 mm of the apex of the Pachycereus pringlei primary root was collected. 350, 450, and 600 root tips were pooled per samples (developmental stage) at the initial, intermediate, or terminal growth stage, respectively. Two biological replicates were included per developmental stage. Total RNA was extracted and mRNA-seq was performed on the Illumina HiSeq Platform at BGI-Tech, Hong Kong.