Abstract
The secondary growth of a woody stem requires the formation of a vascular cambium at an appropriate position and proper patterning of the vascular tissues derived from the cambium. Class III homeodomain-leucine zipper (HD ZIP) transcription factors have been implicated in polarity determination and patterning in lateral organs and primary vascular tissues and in the initiation and function of shoot apical meristems. We report here the functional characterization of a Populus class III HD ZIP gene, popREVOLUTA (PRE), that demonstrates another role for class III HD ZIPs in regulating the development of cambia and secondary vascular tissues. PRE is orthologous to Arabidopsis (Arabidopsis thaliana) REVOLUTA and is expressed in both the shoot apical meristem and in the cambial zone and secondary vascular tissues. Transgenic Populus expressing a microRNA-resistant form of PRE presents unstable phenotypic abnormalities affecting both primary and secondary growth. Surprisingly, phenotypic changes include abnormal formation of cambia within cortical parenchyma that can produce secondary vascular tissues in reverse polarity. Genes misexpressed in PRE mutants include transcription factors and auxin-related genes previously implicated in class III HD ZIP functions during primary growth. Together, these results suggest that PRE plays a fundamental role in the initiation of the cambium and in regulating the patterning of secondary vascular tissues.
Highlights
The secondary growth of a woody stem requires the formation of a vascular cambium at an appropriate position and proper patterning of the vascular tissues derived from the cambium
The mechanisms leading to the formation of successive cambia in cortical tissues is unknown, as are the mechanisms regulating the patterning of secondary vascular tissues
The functional analyses presented here demonstrate that the Populus class III homeodomain-leucine zipper (HD ZIP), PRE, plays a role in cambium initiation and patterning of secondary vascular tissues
Summary
The secondary growth of a woody stem requires the formation of a vascular cambium at an appropriate position and proper patterning of the vascular tissues derived from the cambium. Genes misexpressed in PRE mutants include transcription factors and auxin-related genes previously implicated in class III HD ZIP functions during primary growth Together, these results suggest that PRE plays a fundamental role in the initiation of the cambium and in regulating the patterning of secondary vascular tissues. Secondary growth in woody plant stems requires several fundamental processes, including the initiation of a vascular cambium and the proper patterning of secondary vascular tissues derived from the cambium. Secondary xylem and phloem are produced in the positions relative to the cambium as primary vascular tissues, maintaining the same polarity relationships. Patterning of Woody Stems successive cambia can initiate additional cambia within cortical parenchyma, which can produce conjunctive tissue as well as secondary xylem and phloem, or even new vascular cambia (Carlquist, 2007). The mechanisms leading to the formation of successive cambia in cortical tissues is unknown, as are the mechanisms regulating the patterning of secondary vascular tissues
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