Iron homeostasis regulates maturation of tomato (climacteric) and capsicum (non-climacteric) fruits

Abstract

Endogenous iron quickly interchanges its oxidation states (Fe III and Fe II), which modulate production of reactive oxygen species (ROS) and nitric oxide (NO), and also serve as co-factor for various enzymes. A sharp increase in Fe(III) content in mature fruits of tomato (climacteric) is a unique observation from the present work. Iron homeostasis in non-climacteric capsicum fruits, however, accompanies a balancing of Fe(III) and Fe(II) levels during turning and ripe stages. Heme oxygenase-1 (HO-1), which mobilizes toxic heme and produces Fe(II), apparently regulates iron homeostasis in tomato. Nearly static HO-1 activity in capsicum indicates that HO-1 probably does not have a major role in iron homeostasis in non-climacteric fruit maturation. Endogenous NO content is much higher during tomato fruit maturation than in capsicum. ROS production is highest in green (unripe) tomato and capsicum fruits and it sharply declines with fruit maturation. Tomato fruits exhibit very high β-cyanoalanine synthase (β-CAS) activity in turning stage and mature fruits, highlighting its crucial role in maintaining a static level of cyanide (CN) generated as a byproduct of ethylene biosynthesis during fruit maturation. In contrast, β-CAS activity is very low in maturing capsicum fruits. Thus, a coordination between iron signal and, NO and ROS accumulation in climacteric and non-climacteric fruits is apparently perceived by hormonal (ethylene) signals getting altered differentially during fruit ripening.