Abstract
BackgroundSome plastid-derived metabolites can control nuclear gene expression, chloroplast biogenesis, and chlorophyll biosynthesis. For example, norflurazon (NFZ) induced inhibition of carotenoid biosynthesis in leaves elicits a protoporphyrin IX (Mg-ProtoIX) retrograde signal that controls chlorophyll biosynthesis and chloroplast development. Carotenoid cleavage products, known as apocarotenoids, also regulate plastid development. The key steps in carotenoid biosynthesis or catabolism that can regulate chlorophyll biosynthesis in leaf tissues remain unclear. Here, we established a foliar pigment-based bioassay using Arabidopsis rosette leaves to investigate plastid signalling processes in young expanding leaves comprising rapidly dividing and expanding cells containing active chloroplast biogenesis.ResultsWe demonstrate that environmental treatments (extended darkness and cold exposure) as well as chemical (norflurazon; NFZ) inhibition of carotenoid biosynthesis, reduce chlorophyll levels in young, but not older leaves of Arabidopsis. Mutants with disrupted xanthophyll accumulation, apocarotenoid phytohormone biosynthesis (abscisic acid and strigolactone), or enzymatic carotenoid cleavage, did not alter chlorophyll levels in young or old leaves. However, perturbations in acyclic cis-carotene biosynthesis revealed that disruption of CAROTENOID ISOMERASE (CRTISO), but not ZETA-CAROTENE ISOMERASE (Z-ISO) activity, reduced chlorophyll levels in young leaves of Arabidopsis plants. NFZ-induced inhibition of PHYTOENE DESATURASE (PDS) activity caused higher phytoene accumulation in younger crtiso leaves compared to WT indicating a continued substrate supply from the methylerythritol 4-phosphate (MEP) pathway.ConclusionThe Arabidopsis foliar pigment-based bioassay can be used to differentiate signalling events elicited by environmental change, chemical treatment, and/or genetic perturbation, and determine how they control chloroplast biogenesis and chlorophyll biosynthesis. Genetic perturbations that impaired xanthophyll biosynthesis and/or carotenoid catabolism did not affect chlorophyll biosynthesis. The lack of CAROTENOID ISOMERISATION reduced chlorophyll accumulation, but not phytoene biosynthesis in young leaves of Arabidopsis plants growing under a long photoperiod. Findings generated using the newly customised foliar pigment-based bioassay implicate that carotenoid isomerase activity and NFZ-induced inhibition of PDS activity elicit different signalling pathways to control chlorophyll homeostasis in young leaves of Arabidopsis.
Highlights
IntroductionThe level of photosynthetic pigments (chlorophylls and carotenoids) in leaves is tightly coordinated with chloroplast development and can change during development or in response to environmental stress
The level of photosynthetic pigments in leaves is tightly coordinated with chloroplast development and can change during development or in response to environmental stress
Carotenoid isomerase activity regulates chlorophyll levels in young leaves We investigated if the major rate-limiting step in carotenoid biosynthesis enabled by PHYTOENE SYNTHASE (PSY) could regulate pigment levels in leaves
Summary
The level of photosynthetic pigments (chlorophylls and carotenoids) in leaves is tightly coordinated with chloroplast development and can change during development or in response to environmental stress. Younger Arabidopsis leaves, in comparison to older leaves, harbour actively dividing and expanding cells, leading to a net increase in total cell number and chloroplast capacity per area to store pigments for photosynthesis [21, 30]. This observation correlates with higher rates of photosynthesis in recently emerged leaves compared to mature leaves of Arabidopsis [65]. Developing chloroplasts within the actively dividing cells in younger leaves can alter their biogenesis in response to environmental change, while older leaves that contain fully expanded cells with mature chloroplasts are turned over slowly [21]. We established a foliar pigment-based bioassay using Arabidopsis rosette leaves to investigate plastid signalling processes in young expanding leaves comprising rapidly dividing and expanding cells containing active chloroplast biogenesis
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