Abstract
Adventitious root formation at the base of plant cuttings is an innate de novo organogenesis process that allows massive vegetative propagation of many economically and ecologically important species. The early molecular events following shoot excision are not well understood. Using whole-genome microarrays, we detected significant transcriptome remodeling during 48 h following shoot removal in Populus tremula x Populus alba softwood cuttings in the absence of exogenous auxin, with 27% and 36% of the gene models showing differential abundance between 0 and 6 h and between 6 and 24 h, respectively. During these two time intervals, gene networks involved in protein turnover, protein phosphorylation, molecular transport, and translation were among the most significantly regulated. Transgenic lines expressing a constitutively active form of the Populus type-B cytokinin response regulator PtRR13 (DeltaDDKPtRR13) have a delayed rooting phenotype and cause misregulation of CONTINUOUS VASCULAR RING1, a negative regulator of vascularization; PLEIOTROPIC DRUG RESISTANCE TRANSPORTER9, an auxin efflux transporter; and two APETALA2/ETHYLENE RESPONSE FACTOR genes with sequence similarity to TINY. Inappropriate cytokinin action via DeltaDDKPtRR13 expression appeared to disrupt adventitious root development 24 h after shoot excision, when root founder cells are hypothesized to be sensitive to the negative effects of cytokinin. Our results are consistent with PtRR13 acting downstream of cytokinin to repress adventitious root formation in intact plants, and that reduced cytokinin signaling after shoot excision enables coordinated expression of ethylene, auxin, and vascularization pathways leading to adventitious root development.
Highlights
Adventitious root formation at the base of plant cuttings is an innate de novo organogenesis process that allows massive vegetative propagation of many economically and ecologically important species
Adventitious root formation can be seen as a threestage process: (1) activation, where the cells originating the root primordia become competent to respond to the rhizogenic action of auxin; (2) induction, comprising the determination of root primordia and initial cell divisions; and (3) outgrowth, where root primordia elongate and vascular connections are established to preexisting vasculature within the stem
Because early physiological and biochemical evidence indicates that there are significant changes in endogenous hormone pools, including ethylene, auxin, and cytokinin, during the first 48 h after excision (Maldiney et al, 1986; Selby et al, 1992; Hausman, 1993; De Klerk et al, 1997), we predicted that changes in gene expression during this time frame would provide information about the early and poorly defined molecular networks involved in adventitious root formation
Summary
Adventitious root formation at the base of plant cuttings is an innate de novo organogenesis process that allows massive vegetative propagation of many economically and ecologically important species. Using whole-genome microarrays, we detected significant transcriptome remodeling during 48 h following shoot removal in Populus tremula 3 Populus alba softwood cuttings in the absence of exogenous auxin, with 27% and 36% of the gene models showing differential abundance between 0 and 6 h and between 6 and 24 h, respectively. During these two time intervals, gene networks involved in protein turnover, protein phosphorylation, molecular transport, and translation were among the most significantly regulated. We previously identified 22 genes in Populus that exhibit the typical features of plant RRs (Ramirez-Carvajal et al, 2008)
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