Understanding the molecular mechanism of postharvest seed coat darkening in pinto beans

Nishat Shayala Islam, The University of Western Ontario

Abstract

Pinto bean (Phaseolus vulgaris) is one of the leading market classes of dry beans that is affected by postharvest seed coat darkening. Bean producers and vendors encounter significant crop value loss due to the decreased consumer preference for the darker beans. Darker beans also display poor canning quality and longer cooking times. Special storage conditions can slow down bean darkening to some extent, however, maintaining such condition is expensive. The available slow darkening (SD) pinto cultivars can overcome the darkening issues of existing regular darkening (RD) cultivars, however, the SD pintos show poor agronomic performances in different environmental conditions. These sought to identify the gene/s responsible for slow darkening in pinto beans to develop improved varieties.

As part of my initial research objective, a basic-Helix-Loop-Helix (bHLH) transcription factor encoding the P (Pigment) gene was identified in pinto beans. P protein in a regulator of proanthocyanidins (PAs) biosynthesis. Using genetic complementation, transcript abundance, metabolite analysis, and inheritance study, I found a novel allele of P, P^sd, whose partial loss-of-function slows down seed coat darkening by reducing the PA levels. Compared to P, P^sd contains an additional glutamate residue and an arginine to histidine substitution and produces less PAs by downregulating the expression of DIHYDROFLAVONOL 4-REDUCTASE and ANTHOCYANIDIN REDUCTASE. Then, I identified that P interacts with PvMYB3A, PvMYB10B and PvWD9 proteins to form a protein complex. Additionally, the interaction study also highlighted the necessity of other plant component(s) in complex formation and its activity. Overexpression of P and PvMYB3A accumulated an elevated level of PAs in pinto bean hairy roots, that constitutively express PvWD9. Finally, a genome-wide analysis for MATE genes and their detailed characterization in common bean identified PvMATE8 as a transporter of PA monomers. PvMATE8 is able to transport epicatechin-3´-O-glucosides, not epicatechin aglycons from cytosol to the vacuole and thus ensuring substrate flow to maintain a feedback regulation for upstream epicatechin biosynthesis. Knowing the mechanism and committed steps of the PA pathway will not only help tackle postharvest darkening issues but also will make it easy to incorporate other beneficial traits in SD beans.