Differential homogalacturonan metabolic mechanisms in cellular regions of fruits during postharvest ripening and senescence

INTRODUCTION: Postharvest fruit textural changes, driven by cell wall remodeling, primarily manifest as softening or lignifying. Methylesterification of homogalacturonan (HG) pectin directly impacts cell wall biomechanics, yet its in-situ spatiotemporal role in shaping fruit textural characteristics during ripening and senescence remains poorly understood. OBJECTIVES: This study aimed to trace HG remodeling in different fruits during ripening and senescence, and elucidate how spatial and temporal modifications of HG shape postharvest texture traits, bridging the gap between molecular-level modifications and agronomically relevant fruit texture. METHODS: Two peach varieties (melting/non-melting) and two loquat varieties (lignification-sensitive/insensitive) were selected to compare their physiological and biochemical changes related to texture and pectin metabolism during postharvest storage at 20 °C. Immunohistochemical imaging using antibodies JIM7, LM20, JIM5, LM19, and 2F4 was employed to specifically identify HG with varying methylesterification and HG dimer in the distinct cellular regions of peach and loquat fruits. RESULTS: Melting peaches lost 91.80% firmness over 7 days, while non-melting retained 65.39% firmness over 18 days. In contrast, lignification-sensitive loquat rose 64.19% firmness over 18 days, whereas lignification-insensitive loquat declined slightly until day 12 before recovering. Pectin methylesterase-mediated demethylesterification in exocarp and parenchyma cells of fruits was suggested to exhibit distinct putative patterns, consistent with random action in peaches and processive (blockwise) action in loquats. Moreover, the softening rate of peaches correlated with the difference between the production and further degradation of low-methylesterified HG, whereas loquat firmness was associated with both HG dimer formation and lignin deposition. Notably, vascular bundles in all fruits were inferred to exhibit a putative blockwise mode of demethylesterification, distinct from other cellular regions. CONCLUSION: These findings reveal cell‑level HG metabolic mechanisms driving texture divergence in peach and loquat. The putative mode‑specific HG dynamics identified provide a targeted framework for developing strategies to improve shelf-life management in these two fruits.