Breakdown of Chlorophyll in Higher Plants—Phyllobilins as Abundant, Yet Hardly Visible Signs of Ripening, Senescence, and Cell Death

Abstract

Fall colors have always been fascinating and are still a remarkably puzzling phenomenon associated with the breakdown of chlorophyll (Chl) in leaves. As discovered in recent years, nongreen bilin‐type Chl catabolites are generated, which are known as the phyllobilins. Collaborative chemical‐biological efforts have led to the elucidation of the key Chl‐breakdown processes in senescent leaves and in ripening fruit. Colorless and largely photoinactive phyllobilins are rapidly produced from Chl, apparently primarily as part of a detoxification program. However, fluorescent Chl catabolites accumulate in some senescent leaves and in peels of ripe bananas and induce a striking blue glow. The structural features, chemical properties, and abundance of the phyllobilins in the biosphere suggest biological roles, which still remain to be elucidated.

A Role for TIC55 as a Hydroxylase of Phyllobilins, the Products of Chlorophyll Breakdown during Plant Senescence

“…Most of the phyllobilins that have been identified to date as degradation products of chlorophyll in higher plants carry a hydroxyl group at the C3 2 position (Christ and Hörtensteiner, 2014;Kräutler, 2014Kräutler, , 2016Scherl et al, 2016). Furthermore, hydroxylated phyllobilins have been found in each species analyzed so far ( Figure 8A) and in many cases have been shown to represent the major phyllobilin fraction (Oberhuber et al, 2003;Christ et al, 2013).…”

“…This implies that phyllobilins could possibly play a role beyond being merely degradation products. In line with this, phyllobilins have been shown to be efficient antioxidants and may act as light filters or optical brighteners (Kräutler, 2016). To what extent such physicochemical properties may serve for a biological role and to what extent phyllobilin modification is important in this respect remain unclear so far.…”

“…fluorescent chlorophyll catabolites) as the result of nonenzymatic isomerization inside the acidic vacuolar sap (Oberhuber et al, 2003;Christ et al, 2013). Since the first identification of an NCC in senescent leaves of barley (Hordeum vulgare) in 1991 (Kräutler et al, 1991), phyllobilins were found in senescent leaves from more than 20 different angiosperm species (Kräutler, 2016) but are also formed during fruit ripening Moser et al, 2009). Although the principal linear tetrapyrrole backbone of all phyllobilins is identical, they exhibit rather great structural diversity among the investigated species.…”

Human Colors—The Rainbow Garden of Pathology: What Gives Normal and Pathologic Tissues Their Color?

“…Once a leaf or a fruit ripens or dies, chlorophyll is degraded, and yellow, orange, and/or red carotenes become apparent. 4 This is why leaves change color during fall, and why a banana (Musa acuminata) turns yellow when it's ripe. Xanthines (Greek xanthos ¼ yellow) are yellow pigments (zeaxanthin, lutein, canthaxanthin) that give color to several organisms (Figure 1, A).…”

In Search of Bioactivity – Phyllobilins, an Unexplored Class of Abundant Heterocyclic Plant Metabolites from Breakdown of Chlorophyll

“…Chl breakdown has been studied most extensively in senescent leaves, in which colorless PBs accumulate as seemingly ‘useless’ products of a tightly controlled mere Chl detoxification process . Structure elucidation of PBs has prompted a change of paradigm eventually: due to the bilin‐type build‐up of their core structures and their specific further decoration with (glycosidic) functional groups, PBs are considered, nowadays, to represent complex heterocyclic natural products with structures of significant pharmaceutical interest, and with a vast potential for physiological activity in plants …”