25. In mature Arabidopsis embryo, root apical meristem consists of cells derived from
    (1) embryo and apical suspensor cells
    (2) embryo only
    (3) suspensor only.
    (4) hypophysis only.

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Introduction

The root apical meristem (RAM) is a crucial stem cell niche responsible for root growth and development in plants. In Arabidopsis thaliana, embryogenesis establishes the RAM early on through specialized cell divisions. Among key players is the hypophysis, a unique embryonic cell critical in constructing the future root tip and quiescent center.

Hypophysis: Source of Root Apical Meristem Cells

• The hypophysis is the uppermost cell of the suspensor, positioned adjacent to the embryonic proembryo during early development.

• It undergoes an asymmetric division generating the quiescent center cells and root cap initials, forming the core of the RAM.

• The root cap and quiescent center derived from the hypophysis maintain stem cell fate and organize root tissue patterning in the mature root meristem.

• The remainder of the RAM progenitor cells arise from the embryo itself, but the hypophysis uniquely contributes the organizing cells critical for root formation.

Contrasting Other Origins

• The suspensor as a whole (Option 1) contributes only the hypophysis to RAM, not the entire structure.

• The embryo alone (Option 2) contributes other tissues, but not the organizer cells of the RAM.

• The hypophysis (Option 4) uniquely forms the RAM’s quiescent center and lower tier meristem cells, crucial for root growth and regeneration.

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Summary Table: Origin of Root Apical Meristem in Arabidopsis Embryo

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Conclusion

The root apical meristem cells in mature Arabidopsis embryo are primarily derived from the hypophysis, the specialized cell in the suspensor that organizes stem cell fate essential for root formation.