A Temperature-Sensitive Chlorophyll b-Deficient Mutant of Sweetclover (Melilotus alba)

Area of Science:

• Plant Biology
• Molecular Genetics
• Biochemistry

Background:

• The ch4 mutant of Melilotus alba exhibits partial chlorophyll deficiency and an altered chlorophyll a/b ratio.
• Previous studies confirmed these traits at permissive temperatures (≤23°C).

Purpose of the Study:

• To investigate the effect of nonpermissive temperatures (≥26°C) on the ch4 sweetclover mutant.
• To determine the specificity of the mutation's impact on chlorophyll content and related photosynthetic components.

Main Methods:

• Growth of ch4 mutant and normal sweetclover at permissive and nonpermissive temperatures.
• Measurement of chlorophyll content and chlorophyll a/b ratios.
• Assay of Ribulose 1,5-bisphosphate carboxylase activity.
• High-Performance Liquid Chromatography (HPLC) analysis of carotenoid content.
• Investigation of thylakoid protein kinase activity.

Main Results:

• At nonpermissive temperatures (26°C), ch4 mutants developed small, yellow leaves with drastically reduced chlorophyll content (1/20th of normal).
• Newly developed leaves at nonpermissive temperatures showed the yellow phenotype, while previously grown leaves did not recover chlorophyll.
• Ribulose 1,5-bisphosphate carboxylase activity was halved in mutants at nonpermissive temperatures, suggesting a specific effect on chlorophyll synthesis rather than general protein synthesis.
• Carotenoid content was less affected than chlorophyll, and thylakoid protein kinase activity remained unaltered.

Conclusions:

• The ch4 mutation confers temperature-sensitive chlorophyll deficiency in sweetclover, primarily affecting chlorophyll accumulation.
• The mutation's impact appears specific to chlorophyll biosynthesis or stability, not general protein synthesis or carotenoid levels.
• Photoperiod may influence the temperature-dependent phenotypic expression of the ch4 mutation.