Abstract
To identify specific genes determining the initiation and formation of adventitious roots (AR), a microarray-based transcriptome analysis in the stem base of the cuttings of Petunia hybrida (line W115) was conducted. A microarray carrying 24,816 unique, non-redundant annotated sequences was hybridized to probes derived from different stages of AR formation. After exclusion of wound-responsive and root-regulated genes, 1,354 of them were identified which were significantly and specifically induced during various phases of AR formation. Based on a recent physiological model distinguishing three metabolic phases in AR formation, the present paper focuses on the response of genes related to particular metabolic pathways. Key genes involved in primary carbohydrate metabolism such as those mediating apoplastic sucrose unloading were induced at the early sink establishment phase of AR formation. Transcriptome changes also pointed to a possible role of trehalose metabolism and SnRK1 (sucrose non-fermenting 1- related protein kinase) in sugar sensing during this early step of AR formation. Symplastic sucrose unloading and nucleotide biosynthesis were the major processes induced during the later recovery and maintenance phases. Moreover, transcripts involved in peroxisomal beta-oxidation were up-regulated during different phases of AR formation. In addition to metabolic pathways, the analysis revealed the activation of cell division at the two later phases and in particular the induction of G1-specific genes in the maintenance phase. Furthermore, results point towards a specific demand for certain mineral nutrients starting in the recovery phase.
Highlights
Adventitious root formation (AR formation) is the key developmental process for asexual propagation of most ornamental plants
The largest categories were amino acid metabolism (19%), energy metabolism (17%), carbohydrate metabolism (16%) and biosynthesis of secondary metabolites (12%)
Based on the presented data and focusing on changes in transcript abundances of genes involved in primary metabolism, membrane transporters, cell division or signalling during various phases of adventitious roots (AR) formation following working model has been set up (Fig. 6)
Summary
Adventitious root formation (AR formation) is the key developmental process for asexual propagation of most ornamental plants. Adventitious roots arise from tissues other than the root pericycle [1] and can either be formed naturally on intact plants in dependence on the developmental program and environmental stimuli or develop in response to injury for example at the wounding site of cuttings [2]. Cambium or adjacent vascular tissues are involved in AR formation in cuttings. Biochemical and especially histochemical analyses revealed that AR formation should be considered as a complex multi-step process which is affected by endogenous factors, including phytohormones with a central role of auxin [5], and environmental factors, such as wounding or light [6]. The influence of carbohydrates and hormonal crosstalk [7], [8], of nitrogen supply or free amino acids [9], [10], of general mineral nutrition [11] or of antioxidative enzymes [12] on root formation has been investigated in many plants
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