L-Ascorbic acid is accumulated in source leaf phloem and transported to sink tissues in plants

Area of Science:

• Plant Physiology
• Biochemistry
• Molecular Biology

Background:

• L-Ascorbic acid (AsA), a vital antioxidant, plays a critical role in plant growth and development.
• Understanding the transport mechanisms of AsA is essential for optimizing plant health and productivity.

Purpose of the Study:

• To investigate the phloem loading and translocation of L-Ascorbic acid (AsA) in plants.
• To determine the role of AsA transport in supplying sink tissues.
• To explore the regulation of AsA biosynthesis and its impact on sink demand.

Main Methods:

• Application of L-Ascorbic acid (L-{[14C]}AsA) to source leaves of intact plants.
• Analysis of AsA distribution using autoradiography and High-Performance Liquid Chromatography (HPLC).
• Direct detection of AsA in sieve tube sap.
• Feeding experiments with L-galactono-1,4-lactone (GAL-L), the AsA precursor, to assess its impact on AsA levels.

Main Results:

• L-Ascorbic acid (AsA) is actively loaded into the phloem of source leaves and transported to sink organs like roots, shoots, and flowers.
• Direct detection of AsA in aphid stylet sap confirmed its presence in sieve tube sap.
• Supplementation with L-galactono-1,4-lactone (GAL-L) significantly increased AsA levels in both treated leaves and untreated sink tissues.
• Source leaves exhibit higher AsA biosynthetic capacity but lower turnover rates compared to sink tissues.

Conclusions:

• Phloem translocation of AsA is a key mechanism for supplying non-photosynthetic sink tissues with this essential compound.
• AsA biosynthesis in source leaves appears limited by substrate availability rather than enzymatic capacity.
• Source production and phloem transport of AsA can influence sink development, highlighting AsA's role in cell division and growth.