Fig. 8

Hypothetical model of seed coat permeability in dormant (PR9920–171) and non-dormant (TARS-HT1) beans. Higher expression of pectin acetylesterase-8 in physically dormant seeds causes higher rate of acetyl group removal from the pectin polymers. This facilitates replacement of calcium ions and crosslinking of pectin polymers that convert pectin to an insoluble and strong form. Strong adhesion of pectin polymers resists separation of palisade cells against imposed tension of lens structure. However, in non-dormant seeds (TARS-HT1), a 5-bp insertion causes loss-of-function mutation in pectin acetylesterase-8-2. This loss-of-function result in remaining of acetyl groups that cause a steric hindrance with calcium ions. Lower calcium-mediated crosslinking among pectin polymers in this condition resulted in a looser cell wall that is more prone to the micro-fissures. Tension from swelling parts of lens causes the palisade cell in the lens groove to separate from each other and develop micro-cracks in loose non-dormant seed coat. a Schematic representation of pectin fibers within cell wall. Black solid lines represent pectin fibers. Green transparent circle represent acetyl groups and red circles represents calcium ions that facilitates crosslinking of pectin polymers. b Schematic representation of the lens on the seed coat. Black arrows indicate the possible tension imposed by the swelling structure of the lens on the lens groove cells. c Palisade cells located within the lens groove. Black lines indicate the possible tension imposed by the swelling structure of the lens on these cells that cause separation of cells in non-dormant seed coat. d The palisade cells separation in non-dormant seeds results in the appearance of micro-cracks within the lens groove