Two papers in this issue discuss flowering - one addresses its timing, and the other the specification of flower organ identity. On p. 3213, Hoecker and colleagues have investigated how the SPA protein family helps plants to adjust their development to the environment. The researchers have previously shown that SPA proteins are required for skotomorphogenesis (the growth that occurs when seedlings are kept in the dark); now they investigate the role of SPA proteins in photoperiodic flowering (the timing of flowering in response to day length). They show that all four SPA proteins interact with CONSTANS (CO),which is essential for the early flowering that occurs in response to long days. CO transcription is regulated by the circadian clock and its protein is stabilised by light, which together allow CO protein to accumulate only when days are long. The presence of high levels of CO protein in spatriple mutants lead the authors to speculate that SPA proteins control the stability of CO in response to light.
The mechanisms by which floral organs develop are largely well characterised - A, B, C and E class genes combine to direct the development of the four floral whorls into stamens, carpals, petals and sepals - but some crucial aspects remain unclear. On p. 3159, Liu and colleagues describe a new model for the regulation of the class C gene AGAMOUS(AG), which specifies stamen and carpel development in the inner two whorls. The researchers previously identified two transcriptional co-repressors called SEUSS (SEU) and LEUNIG (LUG) that prevent ectopic expression of AG. The authors now show that these co-repressors interact with the MADs box proteins SEPALLATA3 (SEP3) and APETALA1 (AP1), converting them from activators into repressors of AG expression. So although previous models hold that AG is activated in all four whorls and repressed in the outer two, these researchers suggest that SEU and LUG repress AG in all four whorls, but that this is then antagonised by AG activation in the inner whorls.