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LETTER TO THE EDITOR

Response to Lifschitz Letter

Laboratory of Plant Physiology, Department of Life
Sciences, University of Liège, 4000 Liège, Belgium

Professor Lifschitz's Letter to the Editor is constructed on his assertion that, in our recent article (Thouet et al., 2008), we "restrict the function of SP, well as all TERMINAL FLOWER1 (TFL1) orthologs, to nongrowing axillary meristems," which is not the case. We want to clarify this main point and the answers to other concerns will emerge.

The SELF-PRUNING (SP) gene is known to control sympodial growth in tomato (Solanum lycopersicum), since mutants show successive sympodial units of reduced size and, eventually, determinate growth. Sympodial growth starts when the shoot apical meristem (SAM) initiates the first inflorescence; the uppermost axillary meristem (the sympodial) then takes over. We therefore analyzed the expression pattern of SP around the time of floral transition of the SAM and performed in situ hybridizations in shoot apices, which comprise the SAM, leaf primordia, and their axillary meristems. Since the SP gene is orthologous to TFL1, we were surprised to observe that SP was not expressed in the vegetative or inflorescence SAM, but was restricted to axillary meristems, including the sympodial. Transcript level was high until these meristems started growing.

In any scientific report, the experimental approach defines the context of the discussion and the limits of its conclusions. We analyzed shoot apices, which, obviously, cannot exclude the possibility that SP might be expressed in organs we did not look at, for example, expanded leaves, stems, or roots. We are also aware that other techniques, such as reverse transcription-PCR, could give different results, as reported by Carmel-Goren et al. (2003) or Quinet et al. (2006). In Quinet et al. (2006), the aim was not to perform expression analyses but to select sp mutants. The importance of methodological aspects means that the most relevant comparison of our results was with other shoot apices in situ, namely, the pictures previously published by Professor Lifschitz's lab (Pnueli et al., 1998). Our observations were different and opened a new perspective that we wanted to share with the scientific community.

As stated in the instructions to authors, Scientific Correspondence is "a short contribution providing scientists with a forum to discuss new scientific ideas based on an analysis of the existing literature or a few experiments." This type of communication seemed to match our aims perfectly but imposed strong space limitations. Therefore, when analyzing the existing literature concerning other TFL1 homologs, our purpose was not to make an exhaustive list of tissues or organs in which they are expressed in different species—Professor Lifschitz gives an overview of how well documented and complex this is—but rather to see whether they were detected in axillary meristems. The answer is in our sentence "...the expression of these genes in axillary meristems seems to be the rule in all plants examined so far..." and we list examples thereof. However, in no case do we imply that the expression domain of TFL1 homologs is restricted to axillary meristems "only" or question the major function of TFL1 in inflorescence architecture of many species. Instead, we suggest additional roles in axillary meristems.

Professor Lifschitz titled his letter "Multiple regulatory roles for SELF-PRUNING in the shoot system of tomato"; our results make a further contribution to these roles. Because we observed that SP expression—within the shoot apex—decreased in all axillary meristems after they started to grow, we made the hypothesis that a regulatory role might be linked to shoot branching. It remains to be determined what makes the differences in fate between the sympodial meristem—which, when activated, will produce a short segment of three leaves then an inflorescence—and other axillary meristems that will form lateral branches. Precedence of some buds over others is important in shoot branching and exciting progress has been made recently in understanding the genetic and hormonal control of this process (Ongaro and Leyser, 2008). It was therefore premature to elaborate further before we performed more experiments.

Because sympodial growth and flowering are linked, we fully agree that a broader discussion of SP in the context of florigen is of great interest. Here too, important discoveries have been reported these last few years, and Professor Lifschitz gave compelling evidence that SINGLE FLOWER TRUSS (SFT), the tomato ortholog of FLOWERING LOCUS T (FT), triggers systemic flowering signals (Lifschitz and Eshed, 2006; Lifschitz et al., 2006). In our original manuscript, we had included a discussion (and all relevant references) about how the SP/SFT balance might regulate the "cycling of flowering in the sympodium." We agreed to remove this discussion based on a referee's comment that our Scientific Correspondence did not actually address the control of floral transition. A fundamental challenge is to know how the SAM, the sympodial meristem, and other axillaries respond differently to SFT-derived flowering signals and to correlate this with the expression pattern of SP in these meristems. Most significantly, Conti and Bradley (2007) addressed a similar question in Arabidopsis (Arabidopsis thaliana), in which they observed differential regulation of TFL1 in the SAM versus axillary meristems. We strongly recommend reading their very interesting discussion.

Our Scientific Correspondence has achieved its aim of opening a debate. We hope that good scientific discussion will continue, enriched with new results and perspectives.

	FOOTNOTES

 
www.plantphysiol.org/cgi/doi/10.1104/pp.104.900280

cperilleux{at}ulg.ac.be

	LITERATURE CITED

TOP LITERATURE CITED

 
Carmel-Goren L, Liu YS, Lifschitz E, Zamir D (2003) The SELF PRUNING gene family in tomato. Plant Mol Biol 52: 1215–1222[CrossRef][Web of Science][Medline]

Conti L, Bradley D (2007) TERMINAL FLOWER1 is a mobile signal controlling Arabidopsis architecture. Plant Cell 19: 767–778[Abstract/Free Full Text]

Lifschitz E, Eshed Y (2006) Universal florigenic signals triggered by FT homologues regulate growth and flowering cycles in perennial day-neutral tomato. J Exp Bot 57: 3405–3414[Abstract/Free Full Text]

Lifschitz E, Eviatar T, Rozman A, Shalit A, Goldshmidt A, Amsellem Z, Alvarez JP, Eshed Y (2006) The tomato FT ortholog triggers systemic signals that regulate growth and flowering and substitute for diverse environmental stimuli. Proc Natl Acad Sci USA 103: 6398–6403[Abstract/Free Full Text]

Ongaro V, Leyser O (2008) Hormonal control of shoot branching. J Exp Bot 59: 67–74[Abstract/Free Full Text]

Pnueli L, Carmel-Goren L, Hareven D, Gutfinger T, Alvarez J, Ganal M, Zamir D, Lifschitz E (1998) The SELF-PRUNING gene of tomato regulates vegetative to reproductive switching of sympodial meristems and is the ortholog of CEN and TFL1. Development 125: 1979–1989[Abstract]

Quinet M, Dielen V, Batoko H, Boutry M, Havelange A, Kinet JM (2006) Genetic interactions in the control of flowering time and reproductive structure development in tomato (Solanum lycopersicum L.). New Phytol 170: 701–710[CrossRef][Web of Science][Medline]

Thouet J, Quinet M, Ormenese S, Kinet J-M, Périlleux C (2008) Revisiting the involvement of SELF-PRUNING in the sympodial growth of tomato. Plant Physiol 148: 61–64[Free Full Text]

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