Expression patterns of several floral genes during flower initiation in the apical buds of apple (Malus × domestica Borkh.) revealed by in situ hybridization

Abstract

The apple (Malus × domestica Borkh.) is one of the commercially important fruit crops in the worldwide. The apple has a relatively long juvenile period (up to 4 years) and a long reproductive period between the flower initiation and the mature fruit (14-16 months), which prevent the fruit breeding. Therefore, the understanding of the flowering system is important to improve breeding efficiency in the apple. In this study, to examine the temporal and spatial expression patterns of the floral genes, MdTFL1, MdAP1 (MdMASD5), AFL2, and MdFT, we conducted in situ hybridization analysis in the apple shoot apex. In vegetative phase, MdTFL1 was expressed on the rib meristem zone. When vegetative meristem began converting into inflorescence meristem, the expression level of MdTFL1 was drastically decreased. At the early stage of inflorescence meristem, the expression levels of AFL2, MdFT, and MdAP1 were up-regulated in the leaf primordia and the upper region of cell layers on the shoot apex. In late stage, the expression levels of AFL2 and MdAP1 were up-regulated in the young floral primordia. At a more advanced stage, high expression of MdAP1 was observed in the inflorescence primordium through the inner layer of sepal primordia and the outer layer of receptacle primordia and floral axis. Our results suggest that AFL2, MdFT, and MdAP1 affect to convert from the vegetative meristem into the inflorescence meristem after the decline of MdTFL1 expression. After that, AFL2 and MdAP1 promote the formation of the floral primordia and floral organs.

Overexpression of Arabidopsis FT gene in apple leads to perpetual flowering

“…It was unclear if the considerably increased expression of AFL2 was affected by AtFT directly or indirectly. AFL2 was expressed in the floral organs of apple, so excess AFL2 might affect the floral organ number and shape (Mimida et al 2011;Wada et al 2002). However, the AFL2-overexpressing transgenic apple trees grew for almost 10 years without developing flowers (data not shown).…”

“…Thus the apple LEAFY ortholog did not induce flowering. In situ hybridization experiments showed that AFL2 was expressed in the shoot apex, floral primordia and floral organs (Mimida et al 2011), and therefore apple LEAFY orthologs are hypothesized to play a role in floral organ development.…”

Identification and expression analysis of APETALA1 homologues in poplar

“…In apple (Malus 9 domestica), the AP1 homologue, MdAP1, is expressed in leaf primordia, the upper cell layers of the shoot apex, inflorescence primordia, both young floral primordia and the inner cell layer of sepal primordia. It is also expressed in the outer cell layer of receptacle primordia, the floral axis, and in developing floral organ primordia (Kotoda et al 2000;Mimida et al 2011). In the present study, the expression patterns of PtAP1-1 and PtAP1-2 in P. tomentosa were similar in that they were highly expressed during the reproductive phase and low transcript levels were observed during the vegetative phase (Fig.…”

Expression Pattern of Several Flowering-related Genes during Flower Bud Formation in Rhododendron × pulchrum ‘Oomurasaki’

“…In apple (Malus domestica), Mimida et al (2011) examined the expression pattern of flowering-related genes during the transition from the vegetative to reproductive phases in apical buds using in situ hybridization. When the committed vegetative meristem is converted into the early inflorescence meristem, a marked decrease in MdTFL1 expression is observed in the rib meristem zone.…”

“…In addition, expression analysis by reverse-transcription-quantitative PCR (RT-qPCR) was performed to understand better the expression pattern of RpFT, RpSOC1, RpLFY, RpTFL1, RpCEN, and RpAP3 during floral bud formation. Although the expression of FT in leaves is essential for flowering in model plants, we analyzed the expression of these flowering-related genes in shoot tips, which comprised the shoot apical parts including the SAM and stem, because Kotoda et al (2010) and Mimida et al (2011) reported the importance of FT-like gene expression in apical buds of apple. A further objective was to determine the putative period of floral bud formation in R. × pulchrum 'Oomurasaki'.…”