Postulated Physiological Roles of the Seven-carbon Sugars, Mannoheptulose, and Perseitol in Avocado

Abstract

<jats:p>Avocado (<jats:italic>Persea americana</jats:italic> Mill.) tissues contain high levels of the seven-carbon (C7) ketosugar mannoheptulose and its polyol form, perseitol. Radiolabeling of intact leaves of `Hass' avocado on `Duke 7' rootstock indicated that both perseitol and mannoheptulose are not only primary products of photosynthetic CO<jats:sub>2</jats:sub> fixation but are also exported in the phloem. In cell-free extracts from mature source leaves, formation of the C7 backbone occurred by condensation of a three-carbon metabolite (dihydroxyacetone-P) with a four-carbon metabolite (erythrose-4-P) to form sedoheptulose-1,7-bis-P, followed by isomerization to a phosphorylated <jats:sc>d</jats:sc>-mannoheptulose derivative. A transketolase reaction was also observed which converted five-carbon metabolites (ribose-5-P and xylulose-5-P) to form the C7 metabolite, sedoheptulose-7-P, but this compound was not metabolized further to mannoheptulose. This suggests that C7 sugars are formed from the Calvin Cycle, not oxidative pentose phosphate pathway, reactions in avocado leaves. In avocado fruit, C7 sugars were present in substantial quantities and the normal ripening processes (fruit softening, ethylene production, and climacteric respiration rise), which occurs several days after the fruit is picked, did not occur until levels of C7 sugars dropped below an apparent threshold concentration of ≈20 mg·g<jats:sup>-1</jats:sup> fresh weight. The effect of picking could be mimicked by girdling the fruit stalks, which resulted in ripening on the tree. Again, ripening followed a decline in C7 sugars to below an apparent threshold level. Taken together, these data indicate that the C7 sugars play important roles in carbon allocation processes in the avocado tree, including a possible novel role as phloem-mobile ripening inhibitors.</jats:p>

Fatty acid and sugar composition of avocado, cv. Hass, in response to treatment with an ethylene scavenger or 1-methylcyclopropene to extend storage life

“…a in 3 days shelf life. This decrease in C7 sugars content during low temperature storage and during ripening of avocado has already been reported by others (Bertling & Bower, 2005;Liu et al, 1999bLiu et al, , 2002Meyer & Terry, 2008) and supports the hypothesis that avocado fruit may have an enzymatic mechanism to metabolize C7 sugars (Liu et al, 1999b). It has long been known that avocado fruit do not ripen on the tree but only after detachment from the tree, possibly due to a ripening inhibitor transported from tree to fruit.…”

“…Sucrose, mannoheptulose and the alcohol perseitol were the main soluble components present in avocado mesocarp tissue (Tables 4-6), as previously reported before (Bertling & Bower, 2005;Cowan, 2004;Liu et al, 1999bLiu et al, , 2002Meyer & Terry, 2008). Fructose and glucose were detected, but were at or below the limit of quantification in almost all samples and thus they were not considered.…”

“…Despite the importance of reserve carbohydrates as an energy source for the respiratory processes during fruit storage and ripening (Kozlowski, 1992), little is known about the nature of carbon substrates which support the respiratory process in avocado fruit. Moreover, the exact function of heptose sugars in avocado fruit is not completely understood and the number of related research papers published in the past decade remain limited (Bertling & Bower, 2005;Cowan, 2004;Liu et al, 1999aLiu et al, , 1999bLiu et al, , 2002Meyer & Terry, 2008). In the present study, relatively low amounts of mannoheptulose were found in late season fruit stored at 5°C (on average 2.92 mg g À1 residue; 1.11 mg g À1 DW) with most values below quantification, regardless of treatments or storage time (Exp.…”

“…Hass (Liu, Robinson, Madore, Witney, & Arpaia, 1999a). Liu, Robinson, Madore, Witney, and Arpaia (1999b) and others (Liu, Sievert, Arpaia, & Madore, 2002) have suggested a possible association between C7 sugar metabolism and fruit ripening, possibly by acting as a factor of ripening inhibition.…”

Intrinsic changes in photosynthetic parameters of carrot leaves under increasing CO₂ concentrations and soil moisture regimes

“…2) show the CO 2 limited and RuBPCO active phase of photosynthesis and are strongly represented by the α values in our model (Sage 1994). The second stage is the capacity of thylakoid reactions to provide ATP and NADPH for regeneration of RuBP determined by the electron transport rate or Calvin cycle capacity or triose phosphate regeneration capacity (Rajasekaran et al 1997, Liu et al 2002. The P Nmax value represents the acclimation phase of the curve in our model.…”