AUTHOR=Mitalo Oscar W. , Asiche William O. , Kang Seung W. , Ezura Hiroshi , Akagi Takashi , Kubo Yasutaka , Ushijima Koichiro 

TITLE=Examining the Role of Low Temperature in Satsuma Mandarin Fruit Peel Degreening via Comparative Physiological and Transcriptomic Analysis

JOURNAL=Frontiers in Plant Science

VOLUME=Volume 13 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.918226

DOI=10.3389/fpls.2022.918226

ISSN=1664-462X

ABSTRACT=Peel degreening is the most conspicuous aspect of fruit ripening in many citrus fruit because of its importance for marketability. In this study, peel degreening in response to propylene (an ethylene analog), and at varied storage temperatures was characterized in satsuma mandarin (Citrus unshiu Marc.) fruit. Propylene treatment triggered rapid peel degreening (within 4–6 d), indicated by an increase in citrus color index (CCI) and chlorophyll loss. Peel degreening was also observed in fruit at 10 and 15ºC after 28–42 d, with gradual CCI increase and chlorophyll reduction. However, fruit at 5, 20 and 25ºC remained green and no substantial changes in peel CCI and chlorophyll content were recorded during the 42-d storage duration. The transcriptomes of peels of fruit treated with propylene for 4 d and those stored at varied temperatures for 28 d were then analyzed by RNA-Seq. We identified three categories of differentially expressed genes which were regulated by: (i) propylene (and by analogy, ethylene) alone, (ii) low temperature (5, 10 or 15ºC vs 25ºC) alone, and (iii) either propylene or low temperature. Genes encoding proteins associated with chlorophyll degradation (such as CuSGR1, CuNOL, CuACD2, CuCAB2 and CuLHCB2), and a transcription factor (CuERF114) were differentially expressed by propylene or low temperature. To further examine temperature-induced pathways, we also monitored gene expression during on-tree fruit maturation vs postharvest. The onset of on-tree peel degreening coincided with autumnal drops in field temperatures, and it was accompanied by differential expression of low temperature-regulated genes. Contrarily, genes that were exclusively regulated by propylene (such as CuCOPT1 and CuPOX-A2) displayed insignificant expression changes during on-tree peel degreening. These findings indicate that low temperature could be involved in the fruit ripening-related peel degreening independently of ethylene.