Transcriptomics Reveals the ERF2-bHLH2-CML5 Module Responses to H₂S and ROS in Postharvest Calcium Deficiency Apples

Area of Science:

• Plant Physiology
• Biochemistry
• Molecular Biology

Background:

• Calcium deficiency accelerates postharvest fruit quality deterioration, but the underlying mechanisms remain unclear.
• Bitter pit development in apples is a significant postharvest quality issue linked to calcium deficiency.

Purpose of the Study:

• To elucidate the molecular mechanisms behind calcium deficiency-induced bitter pit in postharvest apples.
• To investigate the physiological and molecular changes associated with calcium deficiency in apple peels.

Main Methods:

• Physiological analyses of apple peels (ROS, MDA, phenol, flavonoid, H2S, anthocyanin, protein content, enzyme activities).
• Principal Component Analysis (PCA) to identify key factors differentiating calcium-deficient and sufficient peels.
• Transcriptome analysis, RT-qPCR, transient expression assays, and dual-luciferase reporter system to study gene expression and interactions.

Main Results:

• Calcium-deficient apple peels exhibited bitter pit, increased ROS and MDA, higher PPO activity, and reduced H2S production, anthocyanin, and protein content.
• PCA identified calcium content, ROS, and H2S as critical factors. Differentially expressed genes included calmodulin-like proteins (CMLs) and transcription factors (AP2/ERFs, bHLHs).
• CML5 expression positively correlated with ERF2/17, bHLH2, and H2S-related genes, with CML5, ERF2, and bHLH2 demonstrating transcriptional co-activation.

Conclusions:

• Calcium deficiency triggers bitter pit in apples via complex physiological and molecular pathways involving ROS and H2S.
• The interaction between Ca2+ and endogenous H2S, mediated by CML5, ERF2, and bHLH2, plays a crucial role in postharvest apple quality decline.