Altered patterns of senescence and ripening in gf, a stay-green mutant of tomato (Lycopersicon esculentum Mill.)

The gf tomato mutant, which retains chlorophyll during ripening, has been found to be affected in leaf senescence. The leaves of the gf mutant show an absolute stay-green phenotype. As leaf senescence and fruit ripening proceed, there is a marked difference in chlorophyll content between wild-type and gf. In both attached and detached leaf studies, or after treatment with ethylene, the leaves withered and abscised in gf with only slight loss of chlorophyll and carotenoids. Total protein content declined and free amino acids increased during leaf senescence in wild-type and gf, but Western analysis showed that LHCII polypeptides were retained at higher levels in gf. Expression of senescence-related mRNAs increased normally in gf whereas those for cab, rbcS and rbcL declined in both mutant and wild-type. The mutant possesses enzyme activity for chlorophyllase, the formation of phaeophorbide a by the action of Mg-dechelatase and the oxygenolytic opening of the porphyrin macrocycle. Analysis of chlorophyll breakdown products in fruit indicated that gf, like other stay-green mutants, accumulates chlorophyllides a and b, but phaeophorbide a does not accumulate in vivo. This may indicate that, in the mutant, in vivo the action of phaeophorbide a-oxygenase is somehow prevented, either by altered accessibility or transport of components required for thylakoid disassembly or the absence of another facto

The nature of floral signals in Arabidopsis. I. Photosynthesis and a far-red photoresponse independently regulate flowering by increasing expression of FLOWERING LOCUS T (FT)

Arabidopsis flowers in long day (LD) in response to signals transported from the photoinduced leaf to the shoot apex. These LD signals may include protein of the gene FLOWERING LOCUS T (FT) while in short day (SD) with its slower flowering, signalling may involve sucrose and gibberellin. Here, it is shown that after 5 weeks growth in SD, a single LD up-regulated leaf blade expression of FT and CONSTANS (CO) within 4–8 h, and flowers were visible within 2–3 weeks. Plants kept in SDs were still vegetative 7 weeks later. This LD response was blocked in ft-1 and a co mutant. Exposure to different LD light intensities and spectral qualities showed that two LD photoresponses are important for up-regulation of FT and for flowering. Phytochrome is effective at a low intensity from far-red (FR)-rich incandescent lamps. Independently, photosynthesis is active in an LD at a high intensity from red (R)-rich fluorescent lamps. The photosynthetic role of a single high light LD is demonstrated here by the blocking of the flowering and FT increase on removal of atmospheric CO2 or by decreasing the LD light intensity by 10-fold. These conditions also reduced leaf blade sucrose content and photosynthetic gene expression. An SD light integral matching that in a single LD was not effective for flowering, although there was reasonable FT-independent flowering after 12 SD at high light. While a single photosynthetic LD strongly amplified FT expression, the ability to respond to the LD required an additional but unidentified photoresponse. The implications of these findings for studies with mutants and for flowering in natural conditions are discussed