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  • br Discussion Light plays a central role

    2022-07-01


    Discussion Light plays a central role in plant growth, development and defence responses. It is required for photosynthesis and other metabolic or transcriptional processes (Kangasjärvi et al., 2012, Zhang and Zhou, 2013, Ballaré, 2014). It is well known that darkness promotes leaf senescence, which is different in the case of detached leaf, in shaded leaves of intact plants or in whole plant exposed to darkness (Weaver and Amasino, 2001, Van Doorn, 2008). Dark-induced senescence results in a significant NIBR189 sale loss, decline in the photosynthetic activity and changes in sugar metabolism, which is regulated by HXKs (Bolouri-Moghaddam et al., 2010). In addition, HXKs play a key role in the control of leaf senescence by inhibiting mitochondrial PT pore (Sarowar et al., 2008, Godbole et al., 2013). However, the expression pattern and activity of HXKs in different leaf positions under dark-induced senescence remain unclear if whole plants are exposed to prolonged darkness. Our results demonstrated that the four identified tomato HXK genes displayed time and leaf age-specific expression under normal photoperiod. The highest expression was found in the mature leaves especially in the case of the mitochondrial SlHXK1 and chloroplastic SlHXK4 transcripts in the late phase of the light period. CREs, which may be responsible for diurnal and circadian rhythm regulation of HXKs, were described not only in tomato but also in maize HXK promoter sequences (Zhang et al., 2014, Poór et al., 2015). The observed expression patterns in the leaves of various ages may determine the activity of HXKs in source and sink organs. Earlier, it was found that dark treatment induced the expression of AtHXK1 in Arabidopsis leaves (Buchanan-Wollaston et al., 2005). Our results demonstrated that the expression of SlHXKs also altered significantly within 6 h after the dark treatment and showed significant changes in a leaf specific manner. The expression of SlHXK1 and SlHXK4 decreased in the mature tomato leaves and was inhibited compared to normal photoperiod in continuous darkness, but transcript abundance of these HXK genes was significantly elevated after 6 and 24 h in the old leaves. In contrast to the 24-h samples, the expression of SlHXK1 and SlHXK4 was suppressed by the 7 day-long dark treatment in the old leaves. However, transcripts of SlHXK3 accumulated significantly in all leaf positions after dark exposure. Different expression patterns in various leaf positions may suggest different hormonal and metabolic control of the expression of HXK genes during the dark-induced senescence process in intact plants. These experiments revealed a time and leaf age-specific expression pattern for HXKs and similar changes were observed in the enzyme activity. Earlier it was found that the HXK activity is regulated by diurnal rhythm, it is slightly increased during the light period and decreased during the night in tobacco leaf (Häusler et al., 2000). Our results not only confirmed this tendency in the case of HXK activity but also demonstrated the differences in the enzyme activities between the leaf maturity stages of intact tomato plant. It was found that the highest HXK activity was detected in the mature leaves and the lowest in the old leaves of tomato under control condition. In mature source leaves, the enzyme activity reached the maximum for the 6th hour of the light period and remained high during the early phase of the dark period. In young, sink leaves the maximum of enzyme activity was found in the middle of the light period suggesting a better carbohydrate supply from source leaves at this period. In contrast, HXK activities were inhibited after a 6-hour-long darkness in all leaf positions. Not only early decline but also long-term decrease was observed in HXK activities in all leaf position after 7 days under continuous dark, but this decline was the smallest in sink leaves.