Aconitase activity and expression during the development of lemon fruit

Abstract

Citrus fruits are characterized by the accumulation of high levels of citric acid in the juice sac cells and a decline in acid level toward maturation. It has been suggested that changes in mitochondrial aconitase (EC 4.2.1.3) activity affect fruit acidity. Recently, a cytosolic aconitase (cyt‐Aco) homologous to mammalian iron‐regulated proteins was identified in plants, leading us to re‐evaluate the role of aconitase in acid accumulation. Aconitase activity was studied in 2 contrasting citrus varieties, sweet lime (Citrus limettioides Tan., low acid) and sour lemon (Citrus limon var. Eureka, high acid). Two aconitase isozymes were detected. One declined early in sour lemon fruit development, but was constant throughout sweet lime fruit development. Its reduction in sour lemon was associated with a decrease in aconitase activity in the mitochondrial fraction. Another isozyme was detected in sour lemon toward maturation, and was associated with an increase in aconitase activity in the soluble fraction, suggesting a cytosolic localization. The cyt‐Aco was cloned from lemon juice sac cells, but in contrast to the changes in isozyme activity, its expression was constant during fruit development. We present a model, which suggests that reduction of the mitochondrial aconitase activity plays a role in acid accumulation, while an increase in the cyt‐Aco activity reduces acid level toward fruit maturation.

Reduced Expression of Aconitase Results in an Enhanced Rate of Photosynthesis and Marked Shifts in Carbon Partitioning in Illuminated Leaves of Wild Species Tomato

“…Full-length 2.8-kb clones were obtained from both Lp and Aco-1 accessions of tomato utilizing a PCRbased approach using primers based on a consensus of the publicly available plant aconitase sequences. These cDNAs both encode proteins of molecular masses of 98 kD and exhibit high homology to all sequenced aconitases but particularly to bacterial and mammalian cytoplasmic aconitases (Peyret et al, 1995;Navarre et al, 1999;Sadka et al, 2000;Williams et al, 2002). Using TargetP, an in silico protein targeting prediction tool based on knowledge derived from Swiss-Prot sequence entries (Emanuelsson et al, 2000), we were unable to identify features characteristic of transit peptides in either sequence.…”

Changes in Organic Acid Contents and Related Metabolic Enzyme Activities at Different Stages of Growth of Two Tangerine Cultivars

“…Juice vesicles of tangerine fruit were fractionated into cytosolic or soluble fraction and a crude fraction enriched with mitochondria as described by Sadka et al (2000a) with slight modifications. Citrate synthase activity was determined by using Ellman's reagent [5,5'-dithiobis-(2-nitrobenzoic acid), DTNB].…”

“…High or low citric acid content often reduced the citrus fruit quality, but the moderate concentration of citric acid could make the fruit more palatable (Iglesias et al, 2007). citric acid accumulation and reduction included citrate synthase (EC 2.3.3.1) (Sadka et al, 2001), aconitase (EC 4.2.1.3) (Sadka et al, 2000a) and NADP-IDH (EC 1.1.1.42) (Sadka et al, 2000b).…”

“…Organic acid metabolism has been studied in other fruits, such as sweet lime and sour lemon (Sadka et al, 2000a), pineapple (Saradhuldhat & Paull, 2007), loquat (Chen et al, 2009), melon (Tang et al, 2010) and grape (Sweetman et al, 2009). The data was notably missing any information on the tangerine fruit.…”

“…The data was notably missing any information on the tangerine fruit. Moreover, the regulation and control for citric acid in citrus fruit was not clearly clarified and understood (Sadka et al, 2000a).The objective of this study was to determine the role of citric acid metabolizing enzymes in the control of citric acid accumulation and reductionat different stages of growth of two tangerine cultivars, "Sai Num Phueng" and "See Thong".…”