Molecular and Biochemical Characterization of Postharvest Senescence in Broccoli

Postharvest senescence in broccoli (Brassica oleracea L. var Italica) florets results in phenotypic changes similar to those seen in developmental leaf senescence. To compare these two processes in more detail, we investigated molecular and biochemical changes in broccoli florets stored at two different temperatures after harvest. We found that storage at cooler temperatures delayed the symptoms of senescence at both the biochemical and gene expression levels. Changes in key biochemical components (lipids, protein, and chlorophyll) and in gene expression patterns occurred in the harvested tissue well before any visible signs of senescence were detected. Using previously identified senescence-enhanced genes and also newly isolated, differentially expressed genes, we found that the majority of these showed a similar enhancement of expression in postharvest broccoli as in developmental leaf senescence. At the biochemical level, a rapid loss of membrane fatty acids was detected after harvest, when stored at room temperature. However, there was no corresponding increase in levels of lipid peroxidation products. This, together with an increased expression of protective antioxidant genes, indicated that, in the initial stages of postharvest senescence, an orderly dismantling of the cellular constituents occurs, using the available lipid as an energy source. Postharvest changes in broccoli florets, therefore, show many similarities to the processes of developmental leaf senescence.

西兰花（Brassica oleracea L. var. Italica）花球的采后衰老，会产生与发育型叶片衰老相似的表型变化。为更细致地对比这两个过程，本研究针对采收后分别于两种不同温度下贮藏的西兰花，对其花球的分子与生化变化进行了探究。研究发现，低温贮藏可在生化与基因表达层面延缓衰老症状的出现。在肉眼可见的衰老迹象显现之前，采收组织内的关键生化组分（脂质、蛋白质与叶绿素）以及基因表达模式便已发生改变。借助此前已鉴定的衰老增强表达基因，以及新分离得到的差异表达基因，我们发现其中绝大多数基因在采后西兰花中的表达上调趋势，与发育型叶片衰老中的表现高度一致。在生化层面，室温贮藏条件下，采收后花球的膜脂肪酸会快速流失，但并未伴随脂质过氧化产物水平的相应升高。这一现象，结合保护性抗氧化基因表达量的上调，表明在采后衰老的初始阶段，细胞组分会有序降解，以可利用的脂质作为能量来源。因此，西兰花采后花球的变化过程，与发育型叶片衰老的诸多机制存在高度相似性。