Abstract
Abiotic stress stimuli induce the increased synthesis of abscisic acid (ABA), which is generated through the cleavage of xanthophyll precursors. To cope with the increased xanthophyll demand, maize and rice contain a third stress-induced gene copy, coding for phytoene synthase (PSY), which catalyzes the first carotenoid-specific reaction in the pathway. To investigate whether this specific response extends beyond the Poaceae, cassava was analyzed, an important tropical crop known for its drought tolerance. We also found three PSY genes in cassava, one of which (MePSY3) forms a separate branch with the stress-specific Poaceae homologs. However, MePSY3 transcripts were virtually absent in all tissues investigated and did not change upon abiotic stress treatment. In contrast, the two remaining PSY genes contributed differentially to carotenoid biosynthesis in leaves, roots, and flower organs and responded towards drought and salt-stress conditions. Detailed analyses of PSY and 9-cis-epoxycarotenoid cleavage dioxygenase (MeNCED) expression and resulting ABA levels revealed MePSY1 as the main stress-responsive paralog. In the presence of high carotenoid levels in leaves, MePSY1 appeared to support, but not to be rate-limiting for ABA formation; MeNCED represented the main driver. The inverse situation was found in roots where carotenoid levels are low. Moreover, ABA formation and the relative induction kinetics showed discrimination between drought and salt stress. Compared to rice as a drought-intolerant species, the drought response in cassava followed a different kinetic regime. The difference is thought to represent a component contributing to the large differences in the adaptation towards water supply.
Highlights
In plants, carotenoids are involved in a variety of physiologically essential functions
The enzyme phytoene synthase (PSY), which is the subject of the current work, catalyzes the first carotenoidspecific reaction condensing two molecules geranylgeranyl pyrophosphate to form the colorless carotene phytoene, which is converted into the red-colored lycopene through the action of two desaturases and two enzymes involved in isomerisation (Bartley et al 1999; Park et al 2002; Isaacson et al 2002, 2004; Chen et al 2010)
Using the primer set from above, two genomic fragments of different sizes were amplified by PCR, corresponding to two cassava PSY genes, MePSY1 and MePSY2
Summary
Carotenoids are involved in a variety of physiologically essential functions. Carotenoids are integrated in light-harvesting complexes and photosynthetic reaction centers to transfer energy and to mediate photoprotection (Demmig-Adams and Adams 1992, 2002; Niyogi 1999) Another proportion of carotenoids, is being metabolized into at least two classes of phytohormones, abscisic acid (ABA) and the strigolactones both emerging through the action of carotenoid-cleaving dioxygenases (CCDs; Bouvier et al 2005; Auldridge et al 2006). Hydroxylation reactions catalyzed by two P450 and two non-heme iron enzymes (Kim et al 2009) and epoxydation reactions (Niyogi et al 1998; Hieber et al 2000) lead to the formation of lutein as the terminal derivative of the a-carotene derived branch of the pathway, while zeaxanthin, antheraxanthin and violaxanthin are formed within the b-branch. The latter reactions are reversible and represent the xanthophyll cycle which contributes to non-photochemical quenching of chlorophyll fluorescence (Szaboet al. 2005)
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