The Cytokinin Type-B Response Regulator PtRR13 Is a Negative Regulator of Adventitious Root Development inPopulus       

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 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. Transgenic lines expressing a constitutively active form of the Populus type-B cytokinin response regulator PtRR13 (DDDKPtRR13) have a delayed rooting phenotype and cause misregulation of CONTINUOUS VASCULAR RING1, a negative regulator of vascularization; PLEIOTROPIC DRUG RESISTANCE TRANS-PORTER9, an auxin efflux transporter; and two APETALA2/ETHYLENE RESPONSE FACTOR genes with sequence similarity to TINY. Inappropriate cytokinin action via DDDKPtRR13 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.

The Physiology of Adventitious Roots

“…(De Klerk and Hanecakova, 2008;Kilkenny et al, 2012), but at low auxin levels, ethylene promoted adventitious rooting in mung bean (De Klerk and Hanecakova, 2008). Ethylene is known to interact with both auxin (R uzicka et al, 2007;Lewis et al, 2011) and cytokinin (Bollmark and Eliasson, 1990;Ramírez-Carvajal et al, 2009), but the precise nature of this interaction in cutting propagation requires further study.…”

Auxin Control in the Formation of Adventitious Roots

“…Transgenic lines expressing an active form of the Populus type-B cytokinin response regulator PtRR13 (ΔDDKPtRR13) have a delayed rooting phenotype and cause misregulation of CONTINUOUS VASCULAR RING1, a negative regulator of vascularization. Inappropriate cytokinin action via ΔDDKPtRR13 expression appeared to disrupt adventitious root develop-ment 24 h after shoot excision, when root founder cells are hypothesized to be sensitive to the negative effects of cytokinin (Ramirez-Carvajal et al, 2009). …”

The AINTEGUMENTA LIKE1 Homeotic Transcription Factor PtAIL1 Controls the Formation of Adventitious Root Primordia in Poplar

“…Our knowledge of adventitious rooting in trees is limited. It has been shown that the expression of SCARECROWlike genes is induced during rooting (Sánchez et al, 2007), whereas the expression of a cytokinin type B response regulator (PtRR13) is negatively modulated during the early steps of adventitious root formation in poplar (Populus trichocarpa) stem cuttings (Ramírez-Carvajal et al, 2009). Moreover, the PtRR13-dependent pathway interfered with auxin transport and inhibited the transcription of two APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) transcription factor genes.…”