Internal morphological changes of floret primordia in wheat (Triticum aestivum L.) in response to changes to photoperiod: When and why the floret die?

MIRALLES, D.J. GARCÍA, G.A, GONZALEZ, F.G. MALDONADO SA, VAZQUEZ M.D.

Saint Petersburgh

Congreso; 8 International wheat conference; 2010

The number of fertile florets at anthesis has been identified as one of the most important trait associated with the number of grains in wheat. The total number of floret primordia initiated within each spikelet in the spike exceeds the number of floret primordia that reach the stage of fertile floret. as 40-80% of the initiated florets  die previous to anthesis.  Floret mortality is the main bottleneck to obtain a high number of grains per spike as the grain setting (i.e. the step from fertile florets to the beginning of grain growth) is normally high in wheat (i.e. more than 90%). The competition for assimilates during the period in which stem and spike are growing together determine assimilates allocated in the spikes at anthesis, which would determine the fate of the floret primordia previously initiated. Nevertheless,  the timing and the fine tuning reasons associated with floret primordia mortality are still unknown. Recently exploring the gene expression associated with floral primordia mortality when photoperiod is altered during stem elongation, it was concluded that autophagy determined the cell death of the florets that did not survive to anthesis. Nevertheles, in that study the anatomical signs of cell death were observed in a two extreme stages of floral development (i.e. viable and aborted primordia). In the present study we aimed to understand  the progress of changes in tissue/cell morphology associated with floret primordia mortality in different floret positions within a spikelet  from the primordia initiation to the fertile floret stage. The experiment was carried out during two consecutive years (2006 and 2007) at the University of Buenos Aires, Dpt. Plant Production, Argentina. The photoperiod response cultivar Mercia, was vernalized in growth chamber during 45 days (5 ºC and 8 hs photoperiod) previous to the transplant into the field . When plants began  stem elongation (BSE), photoperiod treatments were applied until anthesis (An). Treatments consisted on: natural photoperiod (NP), extended (NP+3) and shortening photoperiod (NP-3) during 3 hs over the natural photoperiod,. Extended photoperiods were applied by a mixture of incandescent and fluorescent lamps of extremely low intensity and shortening of photoperiod was applied by automatic tunnel that covered the plot during 1.5 h during both twilights. Radiation intercepted by the crop and canopy temperature was not significantly affected by the treatments. From BSE to An, three times a week,  spikes from the main stems were dissected to determine the floret stages (scale Waddington) in all primordia present in  the central spikelets. Flowers were fixed in 0.5% v/v glutaraldehyde and 2.5% v/v paraformaldehyde in 0.1 M phosphate buffer, pH 7.2, at 4_C for 3 h and prepared to be observed under electron microscopy Zeiss EM 109 turbo transmission electron microscope.  As it was expected, the extended photoperiod reduced the length of the BSE-An phase and enhanced floret development while short photoperiod induced to a slower floret development. When plants were exposed to extended photoperiod the ovary and anthers of distal primordia stopped their development earlier than the basal floret and also earlier than the same floret position in the NP and/or NP-3 treatments. The microscopy observations of the tissues,  of  those distal floret primordia that stopped development (and finally could not complete their development) showed a lack of disorganization at cellular level, without signs of macroautophagy or necrosis. The typical signs of autohagy as condensation of chromatin, disappearance of the nucleolus, formation of numerous vacuoles that converge to form a large vacuole, and the presence of autophagosomes were not observed. Thus the development of florets stopped prior to any observation of autophfagy suggesting that this process represents the final step of floret death. It remains unclear the molecular mechanisms that causes floret mortality when photoperiod is altered during stem elongation phase.