Reevaluating the involvement of plastidic phosphoglucose isomerase in starch biosynthesis in mesophyll cells

Abstract

It is widely assumed that the whole starch biosynthetic process occurring in leaf mesophyll cells resides exclusively in the chloroplast. According to this view, transitory starch is considered the end-product of a metabolic pathway involving plastidic phosphoglucomutase (pPGM), ADP-glucose pyrophosphorylase (AGP) and starch synthase (SS) that is linked to the Calvin-Benson cycle by means of the plastidic phosphoglucose isomerase (pPGI). In this work we isolated pgi1-3, a mutant totally lacking pPGI activity as a consequence of aberrant splicing of intron 6 of the pPGI encoding gene, PGI1. Starch content in pgi1-3 leaves was ca. 10-15% of that of wild type (WT) leaves, which is similar to that of leaves of pgi1-2, a T-DNA insertion pPGI null mutant. Unexpectedly, microscopy analyses revealed the presence of few starch granules per chloroplast in the mesophyll cells of pgi1-2 and pgi1-3 leaves. Both pgi1-2 and pgi1-3 leaves accumulated WT levels of the starch precursor molecule, ADP-glucose, and displayed reduced SS and high β-amylase activities. Moreover, the two pgi1 mutants displayed a slow growth phenotype and possessed reduced photosynthetic capacities when cultured under continuous light photoperiod. Importantly, pgi1-2 and pgi1-3 leaves accumulated high starch content when plants were cultured in the presence of elevated CO2 concentration. Furthermore, introduction into pgi1-2 and pgi1-3 of a sex1 null mutation impeding β-amylase-mediated starch breakdown reverted the starch-defi cient phenotype of pgi1 mesophyll cells. The overall data (a) show that mesophyll cells of pPGI null mutants accumulate starch, (b) provide strong evidence that the reduced starch content in pgi1 mesophyll cells is largely the consequence of combined factors including reduced photosynthetic capacity and pleiotropic changes in activities of enzymes directly linked to starch metabolism, (c) support the occurrence of important starch biosynthetic pathway(s) alternative to the classic Calvin- Benson cycle-pPGI-pPGM-AGP-SS pathway, and (d) show that pPGI is an important determinant of both photosynthetic capacity and growth.