Identification of a homolog of Arabidopsis DSP4 (SEX4) in chestnut: its induction and accumulation in stem amyloplasts during winter or in response to the cold_

Oligosaccharide synthesis is an important cryoprotection strategy used by woody plants during winter dormancy. At the onset of autumn, starch stored in the stem and buds is broken down in response to the shorter days and lower temperatures resulting in the buildup of oligosaccharides. Given that the enzyme DSP4 is necessary for diurnal starch degradation in Arabidopsis leaves, this study was designed to address the role of DSP4 in this seasonal process in Castanea sativa Mill. The expression pattern of the CsDSP4 gene in cells of the chestnut stem was found to parallel starch catabolism. In this organ, DSP4 protein levels started to rise at the start of autumn and elevated levels persisted until the onset of spring. In addition, exposure of chestnut plantlets to 4 °C induced the expression of the CsDSP4 gene. In dormant trees or cold-stressed plantlets, the CsDSP4 protein was immunolocalized both in the amyloplast stroma and nucleus of stem cells, whereas in the conditions of vegetative growth, immunofluorescence was only detected in the nucleus. The studies indicate a potential role for DSP4 in starch degradation and cold acclimation following low temperature exposure during activity–dormancy transition.

Allocation of 14C assimilated in late spring to tissue and biochemical stem components of cork oak (Quercus suber L.) over the seasons

Carbon distribution in the stem of 2-year-old cork oak plants was studied by 14CO2 pulse labeling in late spring in order to trace the allocation of photoassimilates to tissue and biochemical stem components of cork oak. The fate of 14C photoassimilated carbon was followed during two periods: the first 72 h (short-term study) and the first 52 weeks (long-term study) after the 14CO2 photosynthetic assimilation. The results showed that 14C allocation to stem tissues was dependent on the time passed since photoassimilation and on the season of the year. In the first 3 h all 14C was found in the polar extractives. After 3 h, it started to be allocated to other stem fractions. In 1 day, 14C was allocated mostly to vascular cambium and, to a lesser extent, to primary phloem; no presence of 14C was recorded for the periderm. However, translocation of 14C to phellem was observed from 1 week after 14CO2 pulse labeling. The phellogen was not completely active in its entire circumference at labeling, unlike the vascular cambium; this was the tissue that accumulated most photoassimilated 14C at the earliest sampling. The fraction of leaf-assimilated 14C that was used by the stem peaked at 57% 1 week after 14CO2 plant exposure. The time lag between C photoassimilation and suberin accumulation was ∼8 h, but the most active period for suberin accumulation was between 3 and 7 days. Suberin, which represented only 1.77% of the stem weight, acted as a highly effective sink for the carbon photoassimilated in late spring since suberin specific radioactivity was much higher than for any other stem component as early as only 1 week after 14C plant labeling. This trend was maintained throughout the whole experiment. The examination of microautoradiographs taken over 1 year provided a new method for quantifying xylem growth. Using this approach it was found that there was more secondary xylem growth in late spring than in other times of the year, because the calculated average cell division time was much shorter.