The eight genes of photorespiratory enzymes viz., phosphoglycolate phosphatase-chloroplastic (cpPGLP), glycolate oxidase-peroxisomal (pGOX), glutamate:glyoxylate aminotransferase-peroxisomal (pGGT), serine hydroxymethyltransferase-mitochondrial (mSHMT), glycine decarboxylase-mitochondrial (mGDC), glycerate kinase-chloroplastic (cpGLYK), serine:glyoxylate aminotransferase-peroxisomal (pSGT), and hydroxypyruvate reductase (HPR); two genes encoding for glutamine synthetase-chloroplastic (cpGS2), glutamate synthase (cpGOGAT), were studied and found to be significantly downregulated in the developing grains (ca. 3–5 days post-anthesis) than the leaves, in all the three rice genotypes (Supplemental File S1). Although it can be argued that the downregulated expression pattern in developing grains is an expected one when compared to leaves, to identify the biological significance, expression pattern for rbcS gene transcript was also studied (Supplemental File_S1, ‘ratio’ worksheet). Those genes for which the expression pattern ratio between leaf and developing grains are greater than the ratio of rbcS gene, they were identified to play proportionately equal or higher role as in the leaves.
However, downregulation of photorespiratory genes might lead to cell toxicity due to the accumulation of 2-PG and glyoxylate, notably when the plant is under abiotic stress 9,10. Conversion of these two metabolites into non-toxic compounds is primarily important to overcome the cellular toxicity, as well as to sustain the availability of ADP and NADP for accepting light energy 23. Diversion of the 2-PG to bypass the photorespiratory process is reported to improve the plant biomass 6–8. Alternatively, chloroplastic pyruvate dehydrogenase complex (plPDC) is reported to detoxify glyoxylate, producing CO2 in chloroplast 12, potential for a natural photorespiratory bypass to enrich the CO2 for rubisco carboxylation process. In addition to glyoxylate, glycolate also acts as a substrate for plPDC, and CO2 production from these metabolites are competitively inhibited in the presence of pyruvate 12.
So, to understand the expression pattern at transcriptional level, we compared the expression levels of plPdc (Supplemental File_S1) and found that the transcript levels of plPdc were significantly higher in the developing grains as compared to the leaves, in all the three genotypes. It gives an insight on the possible use of plPDC to establish a photorespiratory bypass (Fig. 1). This would potentially aid in developing an efficient photorespiratory bypass, in planta, through enhanced plPdc gene expression levels targeting for higher biomass or yield. Whether the upregulated plPdc driverts the photorespiratory process or vice-versa, is unknown yet. Plants accomplishing C2 photosynthesis have evolved for the preferential downregulation of mGDC (glycine decarboxylase, mitochondrial) or anatomical rearrangements (with more chloroplasts at the periphery) in mesophyll cells either reduce CO2 release or provides high resistance to CO2 efflux 7,24. This is commonly called ‘C2 shuttle’ and helps increase the plant productivity with high biomass or yield. With initiation of photorespiration through oxygenation reaction of Rubisco, conversion of the toxic metabolites 2-PG (through glycolate) and glyoxylate in chloroplast itself through plPDC to release CO2 will enrich the carbon flux for Rubisco’s carboxylation process would simulate a natural photorespiratory bypass. Present study gives an insight for the probable existence of certain group of plants that have evolved to recapture the CO2 released in the process of converting glycolate / glyoxylate to Acetyl-CoA through plPDC in chloroplast itself, possibly having the shortest C2 shuttle that also help enhance the plant productivity. Expression levels of Acetyl-CoA carboxylase (ACCase), the key enzyme that channelizes the carbon flux for fatty acid (FA) biosynthesis, is insignificant between leaf and grain tissues studied and suggesting for the expression of plPDC is not associated with FA biosynthesis. Alternatively, these Acetyl-CoA pools formed through the action of plPDC might possibly utilized for N-terminal acetylation through the action of plastidic N-terminal acetyltransferases (plNATs). This is in line with the reports suggesting ca. 30% of the plastid proteins are subjected to the action of NATs, especially the chlorophyll binding proteins and other enzymes of photosynthetic apparatus 25–29.
Overall, our results show that, the significant downregulation of photorespiratory genes in the developing grains of rice as compared with leaves exhibit biological significance; with the ratio (leaf to developing grain tissues) for photorespiratory genes being greater than rbcS gene (Supplemental File_S1). The significant upregulation of the chloroplastic pdc gene specifically in the developing grains, might convert glyoxylate / glycolate, to CO2 in chloroplasts for carbon fixation (Fig. 1), thereby preventing carbon loss 12,30. This finding provides an insight for possible development of an in planta photorespiratory bypass in the leaves of C3 plants to envision for higher biomass or yield.