The Rubisco Chaperone BSD2 May Regulate Chloroplast Coverage in Maize Bundle Sheath Cells

Area of Science:

• Plant Biology
• Molecular Biology
• Photosynthesis Research

Background:

• Bundle Sheath Defective2 (BSD2) is essential for Rubisco biogenesis and bundle sheath (BS) cell differentiation in maize (Zea mays).
• Despite similar BSD2 levels in mesophyll (M) and BS chloroplasts, Rubisco accumulates only in BS chloroplasts, suggesting potential additional roles for BSD2 in cell development.

Purpose of the Study:

• To investigate the role of BSD2 in M and BS cells by creating transgenic maize lines with altered BSD2 expression.
• To determine if BSD2 has functions beyond Rubisco accumulation, particularly in cell development and chloroplast regulation.

Main Methods:

• Generation of transgenic maize lines overexpressing or underexpressing BSD2 in M and BS cells using cell type-specific promoters (RBCS, PEPC) or the ubiquitin promoter.
• Complementation analysis of the bsd2 mutation with transgenic lines.
• Assessment of Rubisco accumulation, chloroplast coverage, and photosynthetic rates (gas exchange).

Main Results:

• All transgenes complemented the bsd2 mutation, unexpectedly indicating BSD2 is not vital in M cells and BS BSD2 is in excess for Rubisco accumulation.
• Altered BSD2 levels significantly impacted chloroplast coverage in BS cells, with underexpression decreasing it by 30-50% and overexpression increasing it by 25%.
• Reduced chloroplast coverage in underexpressing lines correlated with decreased photosynthetic rates, suggesting a role for BSD2 in chloroplast size regulation and CO2 diffusion.

Conclusions:

• BSD2's primary role is in Rubisco biogenesis within BS cells; it has minimal essential function in M cells.
• BSD2 plays an ancillary role in BS cells, influencing chloroplast size and coverage, which affects photosynthetic efficiency.
• Photosynthetic limitations in altered BSD2 lines may stem from factors beyond chloroplast volume, such as Rubisco biogenesis or turnover rates.