Fruit Bearing Suppresses Citrus FLOWERING LOCUS T Expression in Vegetative Shoots of Satsuma Mandarin (Citrus unshiu Marc.)

Abstract

Citrus trees alternate between rich and poor crops and are known to be alternate-bearing crops. Alternate bearing results from suppression of flowering due to bearing of fruits. To understand the molecular mechanism how fruit bearing affects flowering, we investigated the relationship between fruits and a flowering-related gene, citrus FLOWERING LOCUS T (CiFT). On trees with different amounts of fruits, the fruit weight/leaf area ratio at harvest was negatively and highly correlated with CiFT expression in the vegetative shoots during fall and winter, which is the period of floral induction. In addition, CiFT expression levels during fall and winter were positively and highly correlated with the flower number the following spring. These results indicate that fruit growth suppresses CiFT expression and decreases the flower number the next spring. In another experiment conducted to determine the effect of the period of fruit bearing on CiFT expression, trees having 3 primary scaffold branches were analyzed. From each branch in 1 tree, all the flowers or fruits were harvested at different times. In November, CiFT was expressed at different levels in each branch, with a tendency to be low in the stem of vegetative shoots from branches that bore fruits for longer periods. This result indicates that a long fruit-bearing period suppresses CiFT expression in vegetative shoots. CiFT expression was detected at much higher levels in fruit-bearing shoots than in vegetative shoots in September. In January, the high levels of CiFT expression in bearing shoots decreased to levels lower than those found in vegetative shoots. Thus, in fruit-bearing shoots, the CiFT expression of an unknown relationship to floral induction is observed during the period before floral induction and the seasonal change of CiFT expression in fruit-bearing shoots is different from that in vegetative shoots.

The Effect of Fruit Bearing on Low-molecular-weight Metabolites in Stems of Satsuma Mandarin (<i>Citrus unshiu</i> Marc.)

“…In our experiments, the accumulation of CiFT mRNA was suppressed in fruiting trees and closely related to the fruit load and number of flower buds the following spring (Table 1); these results corroborated previously reported data (Nishikawa et al, 2012). The suppression of CiFT expression by fruit bearing indicates that CiFT expression is correlated to alternate bearing.…”

“…It has been indicated that citrus FLOWERING LOCUS T (CiFT) plays an important role in the flowering of citrus and that fruit bearing suppresses floral induction via the suppression of CiFT expression (Nishikawa, 2013;Nishikawa et al, 2007Nishikawa et al, , 2012. In recent years, transcriptomic and proteomic analyses have been conducted regarding alternate bearing in citrus (Muñoz-Fambuena et al, 2013a, b;Shalom et al, 2012Shalom et al, , 2014.…”

Regulation of Floral Induction in Citrus

“…Fruit removal also promotes floral induction (GarciaLuis et al, 1986;Iwasaki, 1959;Nishikawa et al, 2012;Okuda et al, 1996). In satsuma mandarin, the presence of fruit on the tree results in a gradual inhibition of floral induction.…”

“…In satsuma mandarin, the presence of fruit on the tree results in a gradual inhibition of floral induction. Early thinning of fruit increases flower number in the following spring (Garcia-Luis et al, 1986;Iwasaki, 1959;Okuda et al, 1996) and the number of fruit on the tree is negatively correlated with flower number in the following year (Nishikawa et al, 2012).…”

“…Recently, changes in gene expression in response to fruit bearing have been reported in 'Murcott' (C. reticulata Blanco) (Shalom et al, 2012) -Fambuena et al, 2011), and satsuma mandarin (Nishikawa et al, 2012). In these genotypes, the levels of CiFT expression seem to be high in light-fruiting "off" trees and low in heavy-fruiting "on" trees during the floral inductive period.…”