Phloem transport of sulfur in Ricinus

Area of Science:

• Plant Physiology
• Biochemistry
• Plant Molecular Biology

Background:

• Understanding sulfur transport in plants is crucial for nutrient allocation and metabolic processes.
• Phloem transport is the primary pathway for distributing nutrients throughout the plant.
• The specific forms of sulfur transported via phloem in Ricinus communis require detailed investigation.

Purpose of the Study:

• To investigate the forms of sulfur compounds exported from mature leaves of Ricinus communis via phloem transport.
• To quantify the proportion of labeled sulfur transported as inorganic sulfate versus organic sulfur compounds.
• To identify the major organic sulfur compounds involved in phloem translocation.

Main Methods:

• Mature Ricinus communis leaves were supplied with radio-labeled sulfate ((35)SO4(2-)).
• Phloem exudates were collected and analyzed for the distribution of (35)S.
• The chemical forms of transported sulfur, including inorganic sulfate and organic compounds, were identified and quantified.

Main Results:

• Only a small fraction (1-2%) of absorbed radiosulfur was exported to the stem within 2-3 hours.
• Approximately 12% of the recovered (35)S was in a reduced form.
• The bulk of sulfur was transported as inorganic sulfate, with 20-40% in organic sulfur compounds, primarily glutathione, followed by methionine and cysteine.

Conclusions:

• Ricinus communis phloem primarily transports sulfur as inorganic sulfate.
• Organic sulfur compounds, notably glutathione, methionine, and cysteine, also contribute to phloem sulfur transport.
• Reduced forms of glutathione and cysteine are predominantly found in sieve tubes, indicating their role in sulfur distribution.