Differential impact of impaired steryl ester biosynthesis on the metabolome of tomato fruits and seeds

Abstract

Steryl esters (SE) are a storage pool of sterols that accumulates in cytoplasmic lipid droplets and helps to maintaining plasma membrane sterol homeostasis throughout plant growth and development. Ester formation of plant SE is catalyzed by phospholipid:sterol acyltransferase (PSAT) and acyl-CoA:sterol acyltransferase (ASAT), which transfer long-chain fatty acid groups to free sterols from phospholipids and acyl-CoA, respectively. Comparative mass spectrometry-based metabolomic analysis between ripe fruits and seeds of a tomato (<em>Solanum lycopersicum</em> cv Micro-Tom) mutant lacking functional PSAT and ASAT enzymes (<em>slasat1</em>x<em>slpsat1</em>)<em> </em>shows that disruption of SE biosynthesis has a differential impact on the metabolome of these organs, including changes in the relative proportions of free and glycosylated sterols. Significant perturbations were observed in the fruit lipidome in contrast to the mild effect detected in the lipidome of seeds. A contrasting response was also observed in phenylpropanoid metabolism, which is down-regulated in fruits and appears to be stimulated in seeds. Comparison of global metabolic changes using volcano plot analysis suggests that disruption of SE biosynthesis favors a general state of metabolic activation that is more evident in seeds than fruits. Interestingly, there is an induction of autophagy in both tissues, which may contribute along with other metabolic changes to the phenotypes of early seed germination and enhanced fruit resistance to <em>Botrytis cinerea</em> displayed by the <em>slasat1</em>x<em>slpsat1</em> mutant. The results of this study reveal unreported connections between SE metabolism and the metabolic status of plant cells, and lay the basis for further studies aimed at elucidating the mechanisms underlying the observed effects.