Genetic engineering of glycinebetaine synthesis in tomato protects seeds, plants, and flowers from chilling damage

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Abstract
Summary: Tomato (Lycopersicon esculentumMill.) plants, which normally do not accumulate glycinebetaine (GB), are susceptible to chilling stress. Exposure to temperatures below 10°C causes various injuries and greatly decreases fruit set in most cultivars. We have transformed tomato (cv. Moneymaker) with a chloroplast‐targetedcodAgene ofArthrobacter globiformis, which encodes choline oxidase to catalyze the conversion of choline to GB. These transgenic plants expresscodA and synthesize choline oxidase, while accumulating GB in their leaves and reproductive organs up to 0.3 and 1.2 μmol g−1fresh weight (FW), respectively. Their chloroplasts contain up to 86% of total leaf GB. Over various developmental phases, from seed germination to fruit production, these GB‐accumulating plants are more tolerant of chilling stress than their wild‐type counterparts. During reproduction, they yield, on average, 10–30% more fruit following chilling stress. Endogenous GB contents as low as 0.1 μmol g−1FW are apparently sufficient to confer high levels of tolerance in tomato plants, as achieved via transformation with thecodAgene. Exogenous application of either GB or H2O2improves both chilling and oxidative tolerance concomitant with enhanced catalase activity. These moderately increased levels of H2O2incodAtransgenic plants, as a byproduct of choline oxidase‐catalyzed GB synthesis, might activate the H2O2‐inducible protective mechanism, resulting in improved chilling and oxidative tolerances in GB‐accumulatingcodAtransgenic plants. Thus, introducing the biosynthetic pathway of GB into tomato through metabolic engineering is an effective strategy for improving chilling tolerance.