A salvage pathway for phytol metabolism in Arabidopsis

Abstract

Chlorophyll is the most abundant photosynthetic pigment in higher plants. During senescence, chlorophyll is hydrolyzed, resulting in the release of free phytol and chlorophyllide. Although the degradation of chlorophyllide has been studied in depth, the metabolic fate of phytol in plants is less clear. Here, we provide evidence that phytol can be incorporated into chlorophyll, tocopherol, and lipid esters by Arabidopsis seedlings. Phytol is phosphorylated to phytyl-phosphate and phytyl-diphosphate by two successive kinase activities associated with chloroplast envelope membranes of Arabidopsis. Although phytol kinase is CTP-dependent, the second kinase reaction, phytyl-phosphate kinase, shows broader specificity for CTP, GTP, UTP, and ATP. Therefore, in addition to de novo synthesis from geranylgeranyl-diphosphate, phosphorylation of free phytol represents an alternative route for phytyl-diphosphate production as the precursor for chloroplast prenyl lipid synthesis. Lipid esters are produced after feeding phytol to Arabidopsis seedlings, and they also accumulate in large amounts in leaves during senescence. The predominant phytyl ester that accumulates during senescence is hexadecatrienoic acid phytyl ester. Fatty acid phytyl ester synthesis by protein extracts of Arabidopsis is stimulated in the presence of phytol- and acyl-CoA esters. Thus, Arabidopsis contains a distinct enzymatic machinery for redirecting free phytol released from chlorophyll degradation into chloroplast lipid metabolism.