by Timo Hytönen
Department of Agricultural Sciences, University of Helsinki, Helsinki, Finland
Strawberry is one of the most economically-important berry crops in the world. It is a rosette plant that reproduces both generatively and vegetatively through stolons called runners. There is a strong trade-off between flowering and runnering, but runners are important because fruit production is based on clonally propagated plants. In a diploid woodland strawberry, two classical mutants affecting flowering and runnering are known. Recessive mutations in Seasonal flowering locus (SFL) and Runnering locus (R) cause perpetual flowering and runnerless phenotypes, respectively (Figure 1; Brown and Wareing 1965). SFL encodes a major floral repressor, the woodland strawberry homolog of TERMINAL FLOWER1 (Koskela et al. 2012; Iwata et al. 2012), which mediates photoperiodic and temperature signals to control seasonal flowering (Rantanen et al. 2015). The molecular nature of R, however, has remained elusive.
Figure 1. Classical mutations in woodland strawberry. Recessive mutations in SFL and R genes cause perpetual flowering and inability to produce runners, respectively (left), whereas the plant with dominant alleles (right) is seasonal flowering and produces runners.
Guttridge and Thompson (1964) showed that exogenous gibberellin (GA) treatment induces runner formation and suppresses flowering in a runnerless perpetual flowering mutant of woodland strawberry, indicating that GA may play a role in the trade-off between flowering and runnering. Now, over 50 years later, two studies provided molecular evidence for the role of GA in the control of axillary bud differentiation to runners or branch crowns. Tenreira et al. (2017) identified a gene encoding a GA biosynthetic enzyme, GA20-oxidase, as a plausible candidate for R (see also commentary by Lockhart 2017), and Caruana et al. (2017) reported a single functional DELLA protein that suppresses runner formation in woodland strawberry.
Tenreira et al. (2017) identified FvGA20ox4 as a candidate gene for R by genetic mapping and whole-genome sequencing of a pooled mutant sample. They found a 9-bp deletion in the second exon of the gene and showed by enzyme assays that only non-mutated FvGA20ox4 was able to convert GA12 to GA20 which is the precursor of active GA1. Furthermore, in situ hybridization experiments showed FvGA20ox4 expression in axillary meristems. Together with previous growth regulator experiments (e.g. Guttridge and Thompson 1964; Hytönen et al. 2009), these new data provide strong evidence for FvGA20ox4 being the R gene. However, the role of four other GA20-oxidase encoding genes in axillary bud differentiation remains unresolved. Mutant complementation or targeted mutagenesis of FvGA20ox4 is still required to obtain a final proof.
In another recent study, Caruana et al. (2017) performed a mutagenesis screen in a runnerless woodland strawberry. They found a mutant that continuously produced runners and, using a mapping-by-sequencing strategy, they identified a gene encoding a DELLA growth repressor FvRGA1, as the prime candidate. Next, they generated an inducible dominant negative version of the corresponding DELLA protein and showed that it was able to suppress the formation and elongation of runners in woodland strawberry indicating that a single DELLA protein controls axillary bud fate.
Does GA regulate the trade-off between flowering and runnering in strawberry then? The studies discussed here provide solid evidence for a role of the GA pathway in the control of axillary bud fate, and based on the presented evidence the following working model can be proposed: FvGA20ox4 likely encodes a rate-limiting enzyme of the GA biosynthetic pathway in the axillary bud. In the presence of an active GA20ox enzyme, GA20 is produced and further converted to GA1 by GA3-oxidases; GA1 then causes the degradation of FvRGA1 leading to runner growth, whereas the reduction of GA1 level leads to the accumulation of this DELLA protein and the differentiation of axillary buds to branch crowns. GA also indirectly affects flowering by controlling the number of shoots capable of producing an inflorescence (Tenreira et al. 2017; Caruana et al. 2017), but additional signals are required for floral induction in apical meristems of the crowns.
Brown T, Wareing PF. 1965. The genetical control of the everbearing habit and three other characters in varieties of Fragaria vesca. Euphytica 14: 97-112. https://doi.org/10.1007/BF00032819
Caruana JC, Sittmann JW, Wang W, Liu Z. 2017. Suppressor of Runnerless encodes a DELLA protein that controls runner formation for asexual reproduction in strawberry. Molecular Plant http://dx.doi.org/10.1016/j.molp.2017.11.001
Guttridge CG, Thompson PA. 1964. The effect of gibberellins on growth and flowering of Fragaria and Duchesnea. Journal of Experimental Botany 15: 631–646. https://doi.org/10.1093/jxb/15.3.631.
Hytönen T, Elomaa P, Moritz T, Junttila O. 2009. Gibberellin mediates daylength controlled differentiation of vegetative meristems in strawberry (Fragaria x ananassa Duch.). BMC Plant Biology 9:18. doi: 10.1186/1471-2229-9-18
Iwata H, Gaston A, Remay A, Thouroude T, Jeauffre J, Kawamura K, Oyant LHS, Araki T, Denoyes B, Foucher F. 2012. The TFL1 homologue KSN is a regulator of continuous flowering in rose and strawberry. Plant Journal 69: 116–125. http://doi.org/10.1111/j.1365-313X.2011.04776.x
Koskela E, Mouhu K, Albani MC, Kurokura T, Rantanen M, Sargent D, Battey NH, Coupland G, Elomaa P, Hytönen T. 2012. Mutation in TERMINAL FLOWER1 reverses the photoperiodic requirement for flowering in the wild strawberry, Fragaria vesca. Plant Physiology 159: 1043-1054. http://doi.org/10.1111/tpj.12809
Lockhart J. 2017. Flowering versus runnering: uncovering the protein behind a trait that matters in strawberry. Plant Cell 29: 2080-2081. https://doi.org/10.1105/tpc.17.00709
Rantanen M, Kurokura T, Jiang P, Mouhu K, Hytönen T. 2015. Strawberry homolog of TERMINAL FLOWER1 integrates photoperiod and temperature signals to inhibit flowering. Plant Journal 82: 163-173. http://doi.org/10.1111/tpj.12809
Tenreira T, Lange MJP, Lange T, Bres C, Labadie M, Monfort A, Hernould M, Rothan C, Denoyes B. 2017. A specific gibberellin 20-oxidase dictates the flowering-runnering decision in diploid strawberry. Plant Cell 29: 2168-2182. https://doi.org/10.1105/tpc.16.00949