Changes in anatomical structure and cell wall metabolism of 'Hom Thong' Banana peel following hot water treatment
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Fruit softening is an important change during the ripening process that affects fruit quality and postharvest shelf life. Cell wall degrading enzymes are considered to be the main factor responsible for cell wall disassembly and texture changes in fruit. To elucidate the effect of hot water treatment (HWT, 50 oC, 10 min) on the softening of ‘Hom Thong’ banana (Musa acuminata, AAA group), firmness, cell wall composition, the activities of cell wall degrading enzymes, and the expression of related genes were investigated in the peel. The decrease in firmness was delayed in hot water treated fruit as compare to control fruit during storage at 25 oC. Application of hot water could delay the increase of water-soluble pectin correlated to fruit firmness during storage. A higher level of HCl-soluble pectin was detected in treated fruits, indicating the reduction of cell wall polymer solubilization. Loss of hemicellulose was also slowed down in treated fruits. HWT did not affect the amount of EDTA-soluble pectin and cellulose. For enzyme activity and gene expression, treatment of intact fruit with hot water reduced activities of pectate lyase (PL) and β-galactosidase (β-Gal) in the peel, and also decreased the accumulation of their mRNAs (MaPL1 and MaGAL, respectively). Polygalacturonase (PG) enzyme activity was inhibited by heat treatment, and the increase in MaPG1 expression was delayed. HWT had little effect on pectin methylesterase (PME) or β-1,3 glucanase (Glu). After treatment, reduced PME activity was measured, but activity returned to levels similar to that of control fruits. PME activity varied during storage and did not correlate well with MaPME1 expression. There was no significant difference in Glu enzyme activity or MaGLU expression between treated and control fruit. Further investigation in cell wall structural changes following HWT through microscopic observation showed that structural alterations in peel of hot water treated banana fruits being slower as compared to untreated fruits. The reduction of peel thickness and number of cell layer occurred during storage was retarded by exposure to hot water. The formation of air space in ground tissue of hot water treated fruit was slower than that of control fruits. These results suggest that postharvest HWT was effective in preventing cell wall degradation of ‘Hom Thong’ banana fruit and reduced activities and gene expression of some enzymes associated with cell wall modification leading to maintainence of firmness in ‘Hom Thong’ banana.
果实软化是果实成熟过程中的重要生理变化，可显著影响果实品质与采后货架期。细胞壁降解酶被认为是介导果实细胞壁解离与质地改变的核心因素。为阐明热水处理（HWT, 50℃, 10 min）对红通香蕉（Hom Thong, Musa acuminata，AAA群）软化的调控效应，本研究对果皮硬度、细胞壁组分、细胞壁降解酶活性以及相关基因的表达水平进行了系统检测。 在25℃贮藏条件下，热水处理组果实的硬度下降速率较对照组显著延缓。热水处理可延缓贮藏期间与果实硬度密切相关的水溶性果胶（water-soluble pectin）含量上升。处理组果实中盐酸可溶性果胶（HCl-soluble pectin）水平显著高于对照组，表明细胞壁聚合物的溶解过程受到有效阻滞。半纤维素的降解进程同样在处理组中得到明显延缓。热水处理对乙二胺四乙酸可溶性果胶（EDTA-soluble pectin）与纤维素的含量无显著影响。 在酶活性与基因表达层面，完整果实经热水处理后，果皮内果胶酸裂解酶（PL, pectate lyase）与β-半乳糖苷酶（β-Gal, β-galactosidase）的活性显著降低，其对应编码基因MaPL1与MaGAL的mRNA积累量也同步下调。多聚半乳糖醛酸酶（PG, polygalacturonase）的活性受到热处理抑制，MaPG1基因的表达上调过程也被延缓。热水处理对果胶甲酯酶（PME, pectin methylesterase）与β-1,3-葡聚糖酶（Glu, β-1,3 glucanase）的影响相对有限：处理初期虽检测到PME活性下降，但贮藏后期该酶活性恢复至与对照组相近的水平；贮藏期间PME活性存在明显波动，且与MaPME1的表达量相关性较弱。处理组与对照组果实的Glu酶活性以及MaGLU的表达量均无显著差异。 通过显微观测对热水处理后的果皮细胞壁结构变化进行进一步研究后发现，热水处理组红通香蕉果皮的结构重塑速率较未处理组更为缓慢。贮藏期间出现的果皮厚度降低与细胞层数减少的现象，因热水处理而得到显著延缓。热水处理组果实基本组织中气腔的形成速率也明显慢于对照组。 上述结果表明，采后热水处理可有效延缓红通香蕉果实的细胞壁降解，下调部分参与细胞壁修饰的酶的活性与基因表达水平，从而维持红通香蕉的果实硬度。
