Root hair curling is an early and essential morphological change required for the success of the symbiotic interaction between legumes and rhizobia. At this stage rhizobia grow as an infection thread within root hairs and are internalized into the plant cells by endocytosis, where the PI3K enzyme plays important roles. Previous observations show that stress conditions affect early stages of the symbiotic interaction, from 2 to 30 min post-inoculation, which we term as very early host responses, and affect symbiosis establishment. Herein, we demonstrated the relevance of the very early host responses for the symbiotic interaction. PI3K and the NADPH oxidase complex are found to have key roles in the microsymbiont recognition response, modulating the apoplastic and intracellular/endosomal ROS induction in root hairs. Interestingly, compared with soybean mutant plants that do not perceive the symbiont, we demonstrated that the very early symbiont perception under sublethal saline stress conditions induced root hair death. Together, these results highlight not only the importance of the very early host-responses on later stages of the symbiont interaction, but also suggest that they act as a mechanism for local control of nodulation capacity, prior to the abortion of the infection thread, preventing the allocation of resources/energy for nodule formation under unfavorable environmental conditions.

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Knowledge about the specific characteristics of floral transition process in oregano (Origanum vulgare L.) is scarce. Photoperiod is one of the environmental factors regulating oregano development. The aim of this study was to evaluate anatomical changes associated with the phenological transition from vegetative to floral stages of O. vulgare L. subsp. vulgare (Compacto ecotype) and O. vulgare L. subsp. hirtum (Link) Ietsw. (Criollo ecotype), in response to day length variations. These two traditional subspecies were grown under natural and artificially 6-h extended photoperiod. Phenology and internal floral transition did not follow a single evolution pattern, since these processes respond to complex geneenvironment interactions. In the oregano subspecies studied, transition from vegetative to reproductive stages presented an intermediate step, consisting of the formation of an inflorescence meristem. Thus, floral transition occurred gradually and acropetally along the floral axis. Within the floral meristem, the appearance of reproductive perianthic pieces also occurred acropetally. Extended photoperiod seemed to act as a signal accelerating the floral transition process of the meristems, which confirms that oregano is a long-day plant. In addition, phenological and anatomical analyses confirmed that Criollo ecotype was most sensitive to photoperiodic lengthening, as evidenced by an earlier bud development all along the floral branch.