Flowering in Tobacco Needs Gibberellins but is not Promoted by the Levels of Active GA1 and GA4 in the Apical Shoot

Abstract

Flowering of Nicotiana tabacum cv Xhanti depends on gibberellins because gibberellin-deficient plants, due to overexpression of a gibberellin 2-oxidase gene (35S:NoGA2ox3) or to treatment with the gibberellin biosynthesis inhibitor paclobutrazol, flowered later than wild type. These plants also showed inhibition of the expression of molecular markers related to floral transition (NtMADS-4 and NtMADS-11). To investigate further the role of gibberellin in flowering, we quantified its content in tobacco plants during development. We found a progressive reduction in the levels of GA1 and GA4 in the apical shoot during vegetative growth, reaching very low levels at floral transition and beyond. This excludes these two gibberellins as flowering-promoting factors in the apex. The evolution of active gibberellin content in apical shoots agrees with the expression patterns of gibberellin metabolism genes: two encoding gibberellin 20-oxidases (NtGA20ox1 = Ntc12, NtGA20ox2 = Ntc16), one encoding a gibberellin 3-oxidase (NtGA3ox1 = Nty) and one encoding a gibberellin 2-oxidase (NtGA2ox1), suggesting that active gibberellins are locally synthesized. In young apical leaves, GA1 and GA4 content and the expression of gibberellin metabolism genes were rather constant. Our results support that floral transition in tobacco, in contrast to that in Arabidopsis, is not regulated by the levels of GA1 and GA4 in apical shoots, although reaching a threshold in gibberellin levels may be necessary to allow meristem competence for flowering.

Efecto de la sobreexpresión y del silenciamiento de genes del metabolismo de giberelinas sobre el desarrollo de tabaco

“…This led to a flowering delay of hybrid 3ox/20ox-OE plants compared with parental plants. This observation is consistent with a previous report where an excess of active gibberellin in tobacco apical shoots was claimed to have an inhibitory effect on floral transition (Gallego-Giraldo et al 2007). …”

“…PCRs were performed in an ABI PRISM Õ 7000 Sequence detection System using SYBR Green to monitor double-stranded DNA synthesis (Applied Biosystems). PCR conditions and data analysis were performed as described in Gallego-Giraldo et al (2007). The analyses were carried out using two biological replicates.…”

“…Differential and tissuespecific regulation of gibberellin gene family members is frequently found, presumably to maintain appropriate levels of gibberellins in different organs during development (Mitchum et al 2006). In tobacco, two GA20ox, two GA3ox and three GA2ox genes are known to be differentially and tissue specifically expressed (Itoh et al 1999, Sakamoto et al 2001. Gallego-Giraldo et al 2007).…”

A Redox-Mediated Modulation of Stem Bolting in TransgenicNicotiana sylvestrisDifferentially Expressing the External Mitochondrial NADPH Dehydrogenase

“…5). On searching of databases, we found two floral induction-associated MADS box genes in N. sylvestris (Jang et al, , 2002Gallego-Giraldo et al, 2007). The NsMADS2 transcript was 3-fold higher in the overexpressor lines (S2 and S6) than in the suppressor line (S8), with the wild type being intermediate (Fig.…”

“…NsMADS1 and NsMADS2 belong to the SQUAMOSA subfamily of floral meristem and organ identity proteins, including Arabidopsis APETALA1 (AP1; Becker and Theissen, 2003;Smykal et al, 2007). NsMADS1 is an ortholog to the floral transition marker NtMADS11 (Jang et al, 2002;Gallego-Giraldo et al, 2007). NsMADS2 is a transcriptional activator that upon overexpression in N. tabacum causes early flowering, and the ortholog NtMADS5/NAP1-2 is tran- Figure 7.…”