Características anatômicas e bioquímicas para reconhecimento dos tipos fotossintéticos de poáceas da subtribo arthropogoninae

Abstract

C4 pathway arose as a response to Oligocene CO2 decline. It is divided between two compartments, mesophyll (M) and bundle sheath (BS) cell, being able to reduce the high C3 photorespiratory rate, concentrating CO 2 around the Rubisco site. Paleoclimatic conditions as increasing of aridity and fire allowed the expansion of C4 biomes. Currently, C4 grasses are classified in the PACMAD clade and occupy tropical and subtropical areas, with high temperatures. In warm climate conditions, C3 crops, as rice, reduce the photosynthetic yield, being the C4 engineering an alternative to increase production. The C4 origins clustered in the subtribe Arthropogoninae / Homolepis/ Mesosetum clade (PACMAD clade) make this group important for studies using intermediate species as C3 proto-Kranz, C2, and e C4-like. The aim was evaluate key features of C3, intermediate, and C4 pathways in species of Arthropogoninae. Occurrence data were taken for species already collected in Brazil, to determine the environmental distribution related to altitude, temperature, fire, and drought. Carbon isotope composition and leaf anatomy were obtained from herbarium material. Also, leaf anatomy, ultrastructure, immunolocalization, and gas exchange were assessed from grown plants. Data were compared by one-way analysis of variance (ANOVA) and Tukey’s test (P<0,05). C4 and “non C4” genus showed overlapped environmental distribution, although C4 distribution is more related to low latitudes, high temperatures, and higher drought risk areas. The δ 13 C data were able to establish a pattern between “non C4” and C4, besides indicating the presence of intermediate species. The distance between veins and the number of mesophyll cells were reduced in a C3, intermediate, and C4 gradient. In the M cells, the C3 species showed the higher number of organelles, while in the BS cells the higher number of chloroplast was observed in the C4 species. The presence of GDC in the mitochondria of both cell compartments allowed classifying H. isocalycia as C3 proto-Kranz. In contrast H. aturensis is a C2, since the GDC is almost restricted located in the BS mitochondria. Those features promote a reduction in CO2 compensation point when compared to C3 H. glutinosa. In conclusion, C3 and intermediate species of Arthropogoninae may have preadaptations to enable the C4 evolution. The Homolepis/Mesosetum clade has different intermediate types and does not lack phylogenetic relation with a C4 genus, being an emergent model for C4 studies in grasses.