D-mannoheptulose and perseitol in ‘Hass’ avocado: Metabolism in seed and mesocarp tissue

• Published in: South African journal of botany 2012-03, Vol.79, p.159-165
• Main Authors: Tesfay, S.Z., Bertling, I., Bower, J.P.
• Format: Article
• Language: English
• Subjects: avocados; bark; cotyledons; D-mannoheptulose; energy; etiolation; fructose-bisphosphate aldolase; hexoses; mesocarp; metabolism; Persea americana Mill; Perseitol; polyols; seedlings; seeds
• ISSN: 0254-6299; 1727-9321
• DOI: 10.1016/j.sajb.2011.10.006

Mature avocado plants produce a higher amount of the heptose D-mannoheptulose and its polyol form, perseitol, than hexoses. These heptoses have various functions as anti-oxidants and energy sources. Although C7 sugars are abundantly produced in avocado, knowledge of their metabolism and synthesis is limited. Therefore the synthesis of these sugars in seed and mesocarp tissue was investigated, in particular the interconversions of perseitol to D- mannoheptulose and of D-mannoheptulose to perseitol were followed in different tissues. The ability of cotyledons of etiolated seedlings to convert perseitol to D-mannoheptulose was significantly higher than that of the cotyledons of germinating seedlings grown in the light as well as that of dormant seeds. The cotyledons of light‐grown, germinating plants had a greater ability to convert D-mannoheptulose to perseitol than the cotyledons of etiolated plants and the dormant seed. The cotyledons of etiolated plants showed higher activity in converting perseitol to D-mannoheptulose than the cotyledons of light-grown plants; whereas the cotyledons of light-grown plants had a greater ability to convert D-mannoheptulose to perseitol than cotyledons of etiolated seedlings. In mesocarp tissue, a higher percentage of perseitol was converted to D-mannoheptulose than vice versa. Furthermore, aldolases could be identified in mesocarp as well as in dry seed tissue. Therefore, the production of these heptoses and their enzymatic inter-conversion is dependent on the plant's ontogenic stage. This paper describes the following basic findings a) conversion of D-mannoheptulose to perseitol as well as conversion of perseitol to D-mannoheptulose in dry seed, cotyledons and mesocarp tissue b) detection of aldolase enzymes and c) interconversion of perseitol and D-mannoheptulose in bark exudates. ▶ Avocado seed predominantly stores perseitol. ▶ Perseitol can be converted to D-mannoheptulose as an energy- providing compound as well as a transport carbohydrate. ▶ The aldolase may also play a role in the formation of mannoheptulose and perseitol.