Pectin dynamics of alfalfa pollen tube elongation in response to low boron based on proteomics

Boron is an essential micronutrient for plant reproductive growth, and its key function in plants is mediated through cross-linking with the cell wall pectic polysaccharide rhamnogalacturonan II. As an important forage crop, alfalfa suffers from inhibited pollen tube elongation under low boron conditions, leading to floret abortion and reduced seed set. However, the molecular mechanism by which low boron regulates pectin to affect alfalfa pollen-tube elongation remains unclear.

Under low boron condition, methylesterified pectin shifted from an apical to subapical and shank localization, while de-esterified pectin became predominantly concentrated at the pollen tube apex, indicating a boron-dependent reorganization of pectin distribution. Proteomic analysis of pollen tubes grown under varying boron concentrations revealed significant enrichment of pectin methylesterase activity, polygalacturonase activity, and carbohydrate metabolic processes. Among 56 shared differentially expressed proteins, two pectin methylesterase inhibitors - MsPMEI14 and MsPMEI15 - were identified as candidates potentially associated with this response. Chromosomal localization, phylogenetic, and cis-element analyses suggested that both genes were evolutionarily conserved. Subcellular localization analysis demonstrated that both MsPMEI14 and MsPMEI15 localized to the endoplasmic reticulum and acidic vesicular compartments, indicating that both proteins are secreted to the cell wall via the conventional secretory pathway (CPS), where they regulate pectin methylesterification to support pollen tube elongation.

These findings suggest that low boron-induced inhibition of pollen tube elongation is associated with altered spatial distribution of methylesterified and de-esterified pectins, potentially mediated through the regulatory activity of MsPMEI14 and MsPMEI15 on pectin methylesterase activity. Together, this study provides new insights into how pectin dynamics respond to low boron conditions to regulate pollen tube elongation, and highlights MsPMEI14 and MsPMEI15 as promising candidates for future functional characterization aimed at elucidating the molecular basis of low boron-induced pollen tube growth inhibition.