Modification of gibberellin signalling (metabolism & signal transduction) in sugar beet: analysis of potential targets for crop improvement

Abstract

Sugar beet, Beta vulgaris spp. vulgaris is a biennial long day plant with an obligate requirement for vernalization (prolonged exposure to low temperature). As a spring crop in temperate European climates, it is vulnerable to vernalization-induced premature bolting and flowering, resulting in reduced crop yield and quality. Gibberellins (GAs) play important roles in key physiological processes including stem elongation (bolting) and flowering and are, therefore, potential targets for controlling reproductive growth in sugar beet. We show that the BvGA20ox gene, which encodes an enzyme necessary for GA biosynthesis, was transcriptionally activated in apices of sugar beet plants after vernalization and that GA metabolism can be manipulated to delay bolting in vernalized plants. We demonstrate that down-regulation of GA responses by transformation with the Arabidopsis thaliana gai gene (which represses GA signalling), under its own promoter (pgai::gai) or deactivation of GA by over-expression of the Phaseolus coccineus (bean) GA2ox1 gene, which inactivates GA, increased the required post vernalization thermal time (an accurate and stable measure of physiological time), to bolt by approximately 300 degrees Cd. This resulted in agronomically significant bolting time delays of approximately 2 weeks and 3 weeks in the pgai::gai and 35S::PcGA2ox1 plants, respectively. Our data represent the first transgenic sugar beet model to (1) show that GA signalling can be used to improve crops by manipulation of the transition to reproductive growth; and (2) provide evidence that GA is required for seed set in sugar beet.

A new RNASeq-based reference transcriptome for sugar beet and its application in transcriptome-scale analysis of vernalization and gibberellin responses

“…Further, our previous studies (unpublished) have indicated that vernalization-dependent GA-induced developmental processes leading to reproductive growth appear to be localized to the apical shoot meristematic tissues. The role of GA in floral regulatory networks is well established [16] and has been demonstrated for bolting and flowering in sugar beet [8,17]. To perform transcriptome-scale analysis of associated changes in gene expression, we harvested between 30-50 plants per treatment and micro-dissected apical tissues (Figure 1B), under the stereo microscope.…”

Bolting and flowering control in sugar beet: relationships and effects of gibberellin, the bolting gene B and vernalization

“…Therefore, we cannot conclude with certainty whether the B allele has a direct or an indirect role in the temporal control of the floral phase transitions during bolting in sugar beet. In inductive LDs neither the B allele nor GA affected the rate of flower development when measured as the time taken from floral bud formation to flower opening, and the final bolt height was not affected by GA. We know, however, from previous studies that although GA is not limiting for these processes, it has important roles in both (Mutasa-Göttgens et al , 2009). Here, we also detected gene dose effects from the dominant B allele, resulting in delayed bolting in the heterozygous annual types.…”

“…What is not clear is the role of GA or its interactions with the B allele. GA promotes cell elongation as required for bolting and is not normally considered to limit the ability of plants to bolt since, in LDs, the bolting frequency among plants with reduced levels of bioactive GA is not significantly affected (Mutasa-Göttgens et al , 2009). Based on apex height to root ratio values, our results demonstrate that, under inductive LDs, GA has no significant major effects on the induction of bolting ( P > 0.19), which is instead influenced by the B allele ( P < 0.05), without significant interactive effects between the two ( P > 0.22).…”

“…In sugar beet, GAs are important for reproductive growth (Radley, 1975; Debenham, 1999; Sorce et al , 2002). Endogenous levels of GA are not limiting for bolting (Mutasa-Göttgens et al , 2009) and are also in the related B. vulgaris spp. maritima sufficient for vegetative bolt induction (Van Dijk, 2009).…”

Alternative splicing of the maize Ac transposase transcript in transgenic sugar beet (Beta vulgaris L.)

“…Currently, breeding programs aim to reduce impurities such as potassium salts, amino acids, and betain that negatively affect sugar extraction (Bosemark 1993). Recent biotechnological improvements using transformation technologies for sugar beet include salt tolerance and the delay of bolting time (Liu et al 2008; Mutasa-Gottgens et al 2009). Furthermore, tissue specific and storage induced genes and their promoters were identified (Kloos et al 2002; Oltmanns et al 2006; Rotthues et al 2008; Stahl et al 2004).…”