Investigating the role of polygalacturonase in cotyledonal cracking of green beans (Phaseolus vulgaris L.)

Abstract

Transverse cotyledonal cracking (TVC) was characterised at physiological, biochemical and molecular levels in three common bean (Phaseolus vulgaris L.) cultivars: Imbali, Tongaat and Tokai. The incidence of TVC was determined visually on ten-day old seedlings and was expressed as the number of cracks per plant. The effect of Ca++ on TVC incidence was examined by enhancing the calcium content of seed cotyledons using calcium salts in seed priming and coating. EDAX was used to quantify the cotyledonal calcium content. Activities of the pectinolytic enzymes polygalacturonase (PG) and pectin methylesterase (PME) were assayed at dry seed, VC, R4 and R6 phenophases of the bean plant, and zymogram electrophoresis was used to identify the two enzymes. The results showed that cultivars Imbali, Tongaat, and Tokai were susceptible resistant and intermediate, respectively, with respect to TVC incidence. Calcium uptake was higher in Tongaat cultivar (P= 0.05) compared with Imbali and Tokai, irrespective of calcium salt enhancement. Vigour was significantly reduced (P< 0.01) by both coating and priming. TVC significantly lowered yield (P= 0.05) in the susceptible Imbali cultivar. Zymogram analysis identified lytic bands at ~45kDa (PG gel) and ~30kDa (PME gel). The activity profile of PME was similar for all cultivars. However, PG activity of susceptible Imbali was high in dry seeds and at the R4 and R6 stages of plant development, whereas the more resistant cultivars displayed high activity at the VC stage only. It is concluded that high PG activity at R4 stage is a more reliable determinant of green bean propensity to cotyledonal cracking.