Sinais oxidativos e do desenvolvimento regulam o endociclo para diferenciação tecidual e promoção de crescimento vegetativo em pitaya vermelha (Hylocereus costaricensis F.A.C.Weber)

Abstract

The pitayas (Hylocereus ssp.) are no longer considered backyard plants and started to occupy an important commercial niche within the exotic fruit market. These Cactaceae have a high rate of vegetative growth even under adverse environmental conditions. This may be related to endoreduplication cycles (endocycles) and cell cycle control. Endoreduplication is a variation of the cell cycle, in which the genome is duplicated, and the mitosis phase is selectively aborted. This study aimed to evaluate the role of the red pitaya (Hylocereus costaricensis F.A.C.Weber) endocycle in tissue differentiation, regulation of vegetative growth, and its relationship with oxidative stimuli. In the first chapter entitled “Oxidative stimuli in seeds induce endoreduplication in seedlings of red pitaya (Hylocereus costaricensis F.A.C.Weber)” we list evidence that seeds cryopreservation in liquid nitrogen (LN) for 30 and 60 days induced increases in hydrogen peroxide (H2O2) levels and activity of antioxidant enzymes (CAT, SOD, and APX). Oxidative signaling possibly triggered stimuli to promote endoreduplication by increasing the proportion of octaploid nuclei in plants. Seed cryopreservation for 60 days also allowed increases over 130% in seedling biomass, and this is possibly related to increment in ploidy levels resulting from endoreduplication. It was also shown that cryopreservation of seeds with cryoprotective solutions (PVS2) increases the efficiency of antioxidant metabolism in eliminating H2O2, repressing oxidative signaling, and consequently endoreduplication, maintaining ploidy levels and stable growth patterns. In the second chapter: "The role of the endocycle in pitaya (Hylocereus costaricensis F.A.C. Weber): oxidative stimuli, tissue differentiation and vegetative growth" we compared the variation in ploidy levels, via endoreduplication, in a space-time perspective in tissues throughout the plant development of seedlings from seeds noncryopreserved (control) and cryopreserved in liquid nitrogen (LN) for 60 days. The cryopreservation of seeds resulted in seedlings with greater increments of dry and fresh mass at 120 days of cultivation in the semi-hydroponic system when compared to the control treatment. The endoreduplication induced by oxidative stimuli in the seeds (60 days of cryopreservation) possibly explains this vegetative behavior. However, over the course of development (240 days), the successive duplications of the genome allowed the matching of ploidy levels of seedlings from cryopreserved and non-cryopreserved seeds. The endocycle was essential for tissue differentiation throughout plant development, and the different levels of ploidy between tissues are directly related to the specifications of cell functions. We hypothesized a relationship between the increase in ploidy levels, via the endocycle, with increases in plant biomass during the juvenile stages (vegetative growth). However, upon reaching the reproductive adult stage, a reduction in the levels of ploidy of the tissues constituting the cladodes was observed, and this, in principle, would be related to the reductions in vegetative growth. These results suggest that the endocycle in H. costaricensis is a highly dynamic process, responsive to oxidative stimuli, in addition to modulating ploidy levels for tissue differentiation and plant growth.