Localized gene expression changes during adventitious root formation in black walnut (Juglans nigra L.)

Area of Science:

• Plant Biology
• Molecular Biology
• Forestry & Horticulture

Background:

• Adventitious root formation is vital for clonal propagation of superior genotypes in forestry and horticulture.
• Black walnut (Juglans nigra L.) exhibits recalcitrance to adventitious rooting, hindering the propagation of elite trees.
• Molecular mechanisms regulating adventitious root development are not well understood, particularly in recalcitrant species.

Purpose of the Study:

• To elucidate transcriptional changes during adventitious root development in black walnut softwood cuttings.
• To identify key genes involved in root formation and their spatial expression patterns.
• To differentiate gene expression between root-forming and non-forming tissues.

Main Methods:

• Quantitative real-time polymerase chain reaction (qRT-PCR) was used to measure the expression of nine rooting-related genes.
• Laser capture microdissection (LCM) was optimized to isolate RNA from specific cell types (cortical, phloem fiber, phloem parenchyma).
• Gene expression analysis was performed on juvenile and mature black walnut cuttings throughout adventitious root development following auxin treatment.

Main Results:

• mRNA abundance of all nine tested rooting-related genes was altered by auxin treatment in both juvenile and mature cuttings.
• SCARECROW LIKE-1 (SCL) showed a significant increase (24- and 23-fold) in juvenile, rooting-competent cells at days 16 and 18.
• AUXIN RESPONSE FACTOR (ARF)6, ARF8, and SHORTROOT expression increased in rooting-competent tissues, with highest transcript abundance localized to root progenitor cells.

Conclusions:

• Specific genes, including SCL, ARF6, ARF8, and SHORTROOT, play critical roles in adventitious root development in black walnut.
• Rooting-competent cells exhibit distinct gene expression patterns compared to non-competent cells, with localized transcript abundance.
• This study provides high-resolution, site-specific insights into the molecular regulation of adventitious rooting in a commercially important woody species.