Abstract
BackgroundAdventitious rooting is an organogenic process by which roots are induced from differentiated cells other than those specified to develop roots. In forest tree species, age and maturation are barriers to adventitious root formation by stem cuttings. The mechanisms behind the respecification of fully differentiated progenitor cells, which underlies adventitious root formation, are unknown.ResultsHere, the GRAS gene family in pine is characterized and the expression of a subset of these genes during adventitious rooting is reported. Comparative analyses of protein structures showed that pine GRAS members are conserved compared with their relatives in angiosperms. Relatively high GRAS mRNA levels were measured in non-differentiated proliferating embryogenic cultures and during embryo development. The mRNA levels of putative GRAS family transcription factors, including Pinus radiata’s SCARECROW (SCR), PrSCR, and SCARECROW-LIKE (SCL) 6, PrSCL6, were significantly reduced or non-existent in adult tissues that no longer had the capacity to form adventitious roots, but were maintained or induced after the reprogramming of adult cells in rooting-competent tissues. A subset of genes, SHORT-ROOT (PrSHR), PrSCL1, PrSCL2, PrSCL10 and PrSCL12, was also expressed in an auxin-, age- or developmental-dependent manner during adventitious root formation.ConclusionsThe GRAS family of pine has been characterized by analyzing protein structures, phylogenetic relationships, conserved motifs and gene expression patterns. Individual genes within each group have acquired different and specialized functions, some of which could be related to the competence and reprogramming of adult cells to form adventitious roots.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0354-8) contains supplementary material, which is available to authorized users.
Highlights
Adventitious rooting is an organogenic process by which roots are induced from differentiated cells other than those specified to develop roots
Adventitious root formation is an organogenic process induced in stem cuttings, or in intact plants, by which roots are induced from differentiated cells other than those specified to develop roots
Hypocotyl or epicotyl cambial cells from 90-day-old seedlings respond to the presence of exogenous auxin by dividing, but the re-orientation of the divisional planes needed for the direct organization of a root meristem does not occur or occurs infrequently
Summary
Adventitious rooting is an organogenic process by which roots are induced from differentiated cells other than those specified to develop roots. Adventitious root formation is an organogenic process induced in stem cuttings, or in intact plants, by which roots are induced from differentiated cells other than those specified to develop roots. Cells competent to form adventitious roots are confined to the cambial region, which is mostly located centrifugal to the resin canal at the xylem poles of the hypocotyl from 21-day-old seedlings. These cells exhibit rapid division and the re-orientation of divisional planes to directly organize a root meristem in response to exogenous auxin, without becoming a developmentally non-identified callus cell. The mechanisms that enable a somatic differentiated cell to become a pluripotent or totipotent cell, which can develop a root, shoot, or embryo, or repair damaged tissues, are unknown
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