Abstract
The ETYHLENE RESPONSE FACTOR/APETALA2 (ERF/AP2) transcription factors have been shown to control a wide range of developmental and environmental responses in plants. These include hormonal responses to ethylene and Abscisic Acid (ABA) as well as to cold and drought. In Actinidia chinensis (kiwifruit), ripening is unusual: although it is sometimes classed as a climacteric fruit (ethylene-associated ripening), much of fruit ripening occurs independently from autocatalytic ethylene production. Initiation of ripening appears to be strongly developmentally controlled and modulated by low temperature. In this study, fruit treated with different temperatures showed an increase in soluble sugar accumulation, and a corresponding increase in ß-AMYLASE (BAM) genes (predominantly BAM3.2 and BAM9) with lower temperatures. To investigate the potential role of the AP2/ERF gene family in the control of fruit ripening in kiwifruit this family was investigated further. Using the new genome annotation and further genome sequence analysis we identified 226 ERF-like genes, 10 AP2L/RAV-like genes and 32 AP2-like genes. An RNA-seq screen from kiwifruit of different maturities, and following treatment with ethylene and temperatures between 0 and 16°C, revealed 4%, 26% and 18% of the ERF-like genes were upregulated by maturation, ethylene and cold temperatures, respectively. Focusing on the C-REPEAT/DRE BINDING FACTOR (CBF) cold master regulators, nine potential genes were identified based on sequence similarity. Five of these CBF-like genes were found in a copy number variant (CNV) cluster of six genes on chromosome 14. Expression analysis showed that two homeologous genes (ERF41 and ERF180) increased in abundance with cold and ethylene, while the cluster of CNV CBF-like genes had lost the ability to respond to cold and increased only with ethylene, suggesting an evolutionary progression of function of these genes.
Highlights
Kiwifruit berry ripening is unusual; it is sometimes classified as a climacteric fruit, much of the fruit ripening occurs independently from autocatalytic ethylene production [1]
Acc03601.1 is likely to have been given two models because of sequence differences in the two species from which they were initially cloned: AdERF11 was identified from an EST from A. chinensis var. deliciosa ‘Hayward’ and ERF108 was from the A. chinensis var. chinensis ‘Hongyang’ gene model
While cold storage is very successful for fruit storage, it is becoming clear that the cold as a treatment in itself can have a profound influence on kiwifruit ripening beyond slowing metabolism [3, 4]
Summary
Kiwifruit berry ripening is unusual; it is sometimes classified as a climacteric fruit (ethylene associated ripening), much of the fruit ripening occurs independently from autocatalytic ethylene production [1]. As the fruit develops on the vine it progresses from maturation into the first phase of ripening (Phase 1) independently of autocatalytic ethylene and involves a progression of starch breakdown, colour change, and loss of firmness [1]. Phase 1 ripening may be influenced by both fruit development and the environment. After the fruit reaches physiological maturity, if the fruit is exposed to ethylene, Phase 1 ripening is accelerated [2, 7] and, depending on the stage of development and the amount of ethylene, the fruit can be pushed into Phase 2 ripening
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