Unravelling the contribution of mesophyll and bundle sheath chloroplasts to increased stress tolerance

DEMARCHI, MARIANA; ARCE ROCÍO; CAMPI, MABEL; CHAN, RAQUEL; CARRILLO, NÉSTOR; LODEYRO, ANABELLA F

Mendoza

Congreso; Congreso SAIB 2022; 2022

Maize is the crop with largest global production, and its products are important as food and energy supply, being a major source of economic development. However, it is extremely susceptible to stress, which ultimately limits the corn yield. This species was domesticated in the tropical regions of southern Mexico and has been spread to habitats with dramatically different environmental conditions, in many cases, adverse to its development. Given its economic relevance, improving stress tolerance in maize could represent a major achievement in agricultural terms. In maize, the CO2 fixation is divided into two cell types (C4 photosynthesis). The primary assimilation occurs in the mesophyll (M) cells, where the chloroplasts generate ATP and NADPH principally by the linear electron flow. On the other hand, the classic C3 photosynthesis takes place in the chloroplasts of the bundle sheath (V) cells, performing only cyclic electron transport that produces only ATP, nor NADPH, the latter being generated by alternate enzymatic ways. Previous research from our laboratory has demonstrated that the introduction of a constitutively expressed cyanobacterial flavodoxin (Fld) directed to chloroplasts resulted in the generation of tobacco plants with increased tolerance to multiple stress sources. The protective function of Fld has been associated to its interaction with the photosynthetic electron transport chain at photosystem I level, sharing redox properties with isofunctional ferredoxin. Application of Fld technology to a C4 species is still uncharted territory. In order to elucidate the contribution of each cell type to the stress tolerance and to determine if Fld can productively interact with cyclic and/or linear electron transport as it does in C3 chloroplasts, we generated transgenic maize plants expressing Fld specifically in the chloroplasts of V (Zmv-pfld, for Zea mays V plastidic Fld) or M cells (Zmm-pfld, for Zea mays M plastidic Fld).We had already set up the conditions for the isolation of maize fractions enriched in V and M chloroplasts, demonstrating the presence of Fld expression in V and M chloroplasts in Zmv-pfld and Zmm-pfld genotypes, respectively, confirming tissue-specific location. Besides, application of oxidative conditions by paraquat, which act as an alternative electron acceptor from photosystem I generating superoxide, showed lower electrolyte leakage for Zmm-pfld in comparison to control and Zmv-pfld genotypes. Furthermore, Zmm-pfld plants subjected to extreme drought in soil also exhibited an improved tolerance compared to Zmv-pfld and null segregants lines. To sum up, our results indicate that presence of Fld in M chloroplasts provides an advantage when facing adverse situations. We expect to determine if the lack of Fld effect when expressed in V chloroplasts is due to a null interaction with the cyclic electron transport, and in general to thoroughly establish the features of the conferred tolerance.