Genotypic differences in pod wall and seed growth relate to invertase activities and assimilate transport pathways in asparagus bean

Background and Aims

Coordination of sugar transport and metabolism between developing seeds and their enclosing fruit tissues is little understood. In this study the physiological mechanism is examined using two genotypes of asparagus bean (Vigna unguiculata ssp. sesquipedialis) differing in pod wall and seed growth rates. Pod growth dominates over seed growth in genotype ‘Zhijiang 121’ but not in ‘Zhijiang 282’ in which a ‘bulging pod’ phenotype is apparent from 8 d post-anthesis (dpa) onward.

Methods

Seed and pod wall growth rates and degree of pod-bulging were measured in the two genotypes together with assays of activities of sucrose-degrading enzymes and sugar content in pod wall and seed and evaluation of cellular pathways of phloem unloading in seed coat using a symplasmic fluorescent dye, 5(6)-carboxyfluorescein (CF).

Key Results

Activities of cell wall, cytoplasmic and vacuolar invertases (CWIN, CIN and VIN) were significantly smaller in pod walls of ‘282’ than in ‘121’ at 10 dpa onwards. Low INV activities were associated with weak pod wall growth of ‘282’. In seed coats, CF was confined within the vasculature in ‘282’ but moved beyond the vasculature in ‘121’, indicating apoplasmic and symplasmic phloem unloading, respectively. Higher CWIN activity in ‘282’ seed coats at 6–8 dpa correlated with high hexose concentration in embryos and enhanced early seed growth. However, CWIN activity in ‘282’ decreased significantly compared with ‘121’ from 10 dpa onwards, coinciding with earlier commencement of nuclei endoreduplication in their embryos.

Conclusions

The study shows genotypic differences between ‘bulging pod’ and ‘non-bulging’ phenotypes of asparagus bean in sucrose metabolism in relation to the pathway of phloem unloading in developing seed coats, and to pod and seed growth. Low INV activity in pod wall corresponds to its shortened and weak growth period; by contrast, the apoplasmic path in the seed coat is associated with high CWIN activity and strong early seed growth.