Abstract
Plants are unique in their ability to continuously produce new meristems and organ primordia. In Arabidopsis, the transcription factor LEAFY (LFY) functions as a master regulator of a gene network that is important for floral meristem and organ specification. UNUSUAL FLORAL ORGANS (UFO) is a co-activator of LEAFY and is required for proper activation of APETALA3 in the floral meristem during the specification of stamens and petals. The ufo mutants display defects in other parts of the flower and the inflorescence, suggestive of additional roles. Here we show that the normal determinacy of the developing Arabidopsis leaves is affected by the expression of a gain-of-function UFO fusion protein with the VP16 transcriptional activator domain. In these lines, the rosette and cauline leaf primordia exhibit reiterated serration, and upon flowering produce ectopic meristems that develop into flowers, bract leaves and inflorescences. These striking phenotypes reveal that developing leaves maintain the competency to initiate flower and inflorescence programs. Furthermore, the gain-of-function phenotypes are dependent on LFY and the SEPALLATA (SEP) MADS-box transcription factors, indicative of their functional interactions with UFO. The findings of this study also suggest that UFO promotes the establishment of the lateral meristems and primordia in the peripheral zone of the apical and floral meristems by enhancing the activity of LFY. These novel phenotypes along with the mutant phenotypes of UFO orthologs in other plant species suggest a broader function for UFO in plants.
Highlights
The continuous production of new meristems is a characteristic feature in plants and accounts for their distinctive indeterminate growth
UNUSUAL FLORAL ORGANS (UFO) is a transcriptional co-activator To assess the functions of UFO, Arabidopsis plants were transformed with the UFO gene under the control of the cauliflower mosaic virus 35S (CaMV 35S) promoter. 22% of the transgenic plants produced serrate leaves and flowers with reduced sepals and abnormalities in the development of the gynoecium valves and the style
Flowers of the ask1-1 mutant showed fewer petals compared to wild type and were often replaced with petal-stamen chimeras, which resemble the flowers of an intermediate ufo-1 mutant implying a genetic interaction between these two genes (Figure 1C, D) [12,26]
Summary
The continuous production of new meristems is a characteristic feature in plants and accounts for their distinctive indeterminate growth. The apical and axillary meristems are usually indeterminate [2], whereas the leaf primordia are most often determinate [3]. During the course of its life cycle, the plant produces multiple meristems and primordia and these will acquire different identities to give rise to different organs. The identities are determined by the combinatorial expression and functions of specific meristem and organ identity genes controlled spatially and temporally by preprogrammed genetic networks [4]. LFY is a plant specific transcription factor and activates several key floral organ identity genes including the ABC class MADS-box genes [5]. The lfy mutant is impaired in the floral fate specification of the meristems produced by the inflorescence meristem, and as a consequence, new meristems default towards a co-inflorescence fate, resulting in a leafy appearance [6,7]
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