Flowering of the Grass Lolium perenne. Effects of Vernalization and Long Days on Gibberellin Biosynthesis and Signaling

Abstract

Almost 50 years ago, it was shown that gibberellin (GA) applications caused flowering in species normally responding to cold (vernalization) and long day (LD). The implication that GAs are involved with vernalization and LD responses is examined here with the grass Lolium perenne. This species has an obligatory requirement for exposure to both vernalization and LD for its flowering (inflorescence initiation). Specific effects of vernalization or LD on GA synthesis, content, and action have been documented using four treatment pairs: nonvernalized or vernalized plants exposed to short days (SDs) or LDs. Irrespective of vernalization status, exposure to two LDs increased expression of L. perenne GA 20-oxidase-1 (LpGA20ox1), a critical GA biosynthetic gene, with endogenous GAs increasing by up to 5-fold in leaf and shoot. In parallel, LD led to degradation of a DELLA protein, SLENDER (within 48 h of LD or within 2 h of GA application). There was no effect on GA catabolism or abscisic acid content. Loss of SLENDER, which is a repressor of GA signaling, confirms the physiological relevance of increased GA content in LD. For flowering, applied GA replaced the need for LD but not that for vernalization. Thus, GAs may be an LD, leaf-sourced hormonal signal for flowering of L. perenne. By contrast, vernalization had little impact on GA or SLENDER levels or on SLENDER degradation following GA application. Thus, although vernalization and GA are both required for flowering of L. perenne, GA signaling is independent of vernalization that apparently impacts on unrelated processes.

Regulation of Flowering in the Long-Day Grass Lolium temulentum by Gibberellins and the FLOWERING LOCUS T Gene

“…Growth-active GAs, such as GA 1 and GA 4 , accumulate in the leaf of L. temulentum and Lolium perenne after exposure to 2 LD (Gocal et al, 1999;MacMillan et al, 2005). Here, particularly because we sampled frequently and overnight, we detected increases in leaf GA 1 during the single LD (Fig.…”

“…The first biochemical and gene expression differences are also evident at the shoot apex at this time (CT 24 and later; Evans and Rijven, 1967;Gocal et al, 1999). Thus, GA 5 could well be one transported florigen of L. temulentum, a conclusion that also applies to L. perenne, which requires both vernalization and LD exposure and where leaf GA content is considerably elevated after exposure to two LDs (MacMillan et al, 2005).…”

“…There are many similar reports of FR-rich LD increasing 20-oxidase expression in leaves or petioles of spinach (Lee and Zeevaart, 2002), Arabidopsis (Wu et al, 1996;Xu et al, 1997;Hisamatsu et al, 2005), potato (Solanum tuberosum; Carrera et al, 1999), and L. perenne (MacMillan et al, 2005). Furthermore, in landmark studies, not just gene expression, but 20-oxidase activity itself, increased in spinach petioles exposed to LD (Gilmour et al, 1986).…”

“…The functional characterization of expressed protein of LpGA20ox1 was documented by MacMillan et al (2005); it converts GA 53 to GA 20 via GA 44 and GA 19 (MacMillan et al, 2005). The protein of Lp2ox was capable of 2-oxidation of GA 20 as a substrate when transiently expressed in a reticulocyte assay as described by Spielmeyer et al (2004).…”

The plant stress hormone ethylene controls floral transition via DELLA-dependent regulation of floral meristem-identity genes

“…Our studies identify the ''GA pathway'' of floral control (1, 2) as a major regulator of flowering in response to environmental signals (see also ref. 48).…”