Seed-specific elevation of non-symbiotic hemoglobin AtHb1: beneficial effects and underlying molecular networks in Arabidopsis thaliana

Abstract

BackgroundSeed metabolism is dynamically adjusted to oxygen availability. Processes underlying this auto-regulatory mechanism control the metabolic efficiency under changing environmental conditions/stress and thus, are of relevance for biotechnology. Non-symbiotic hemoglobins have been shown to be involved in scavenging of nitric oxide (NO) molecules, which play a key role in oxygen sensing/balancing in plants and animals. Steady state levels of NO are suggested to act as an integrator of energy and carbon metabolism and subsequently, influence energy-demanding growth processes in plants.ResultsWe aimed to manipulate oxygen stress perception in Arabidopsis seeds by overexpression of the non-symbiotic hemoglobin AtHb1 under the control of the seed-specific LeB4 promoter. Seeds of transgenic AtHb1 plants did not accumulate NO under transient hypoxic stress treatment, showed higher respiratory activity and energy status compared to the wild type. Global transcript profiling of seeds/siliques from wild type and transgenic plants under transient hypoxic and standard conditions using Affymetrix ATH1 chips revealed a rearrangement of transcriptional networks by AtHb1 overexpression under non-stress conditions, which included the induction of transcripts related to ABA synthesis and signaling, receptor-like kinase- and MAP kinase-mediated signaling pathways, WRKY transcription factors and ROS metabolism. Overexpression of AtHb1 shifted seed metabolism to an energy-saving mode with the most prominent alterations occurring in cell wall metabolism. In combination with metabolite and physiological measurements, these data demonstrate that AtHb1 overexpression improves oxidative stress tolerance compared to the wild type where a strong transcriptional and metabolic reconfiguration was observed in the hypoxic response.ConclusionsAtHb1 overexpression mediates a pre-adaptation to hypoxic stress. Under transient stress conditions transgenic seeds were able to keep low levels of endogenous NO and to maintain a high energy status, in contrast to wild type. Higher weight of mature transgenic seeds demonstrated the beneficial effects of seed-specific overexpression of AtHb1.

A Tale of Two Sugars: Trehalose 6-Phosphate and Sucrose

“…Exposure of plants to low oxygen leads to a widespread reconfiguration of gene expression and metabolism (Bailey-Serres et al, 2012), including a decrease in Tre6P levels in developing Arabidopsis seeds (Thiel et al, 2011). Recently, a quantitative trait locus for anaerobic germination tolerance in rice (Oryza sativa) was found to be linked to expression of a functional OsTPP7 gene in the tolerant genotype, Khao Hlan On (KHO); the OsTPP7 gene is essentially missing from the intolerant IR64 genotype (Kretzschmar et al, 2015).…”

Plant Oxygen Sensing Is Mediated by the N-End Rule Pathway: A Milestone in Plant Anaerobiosis

“…Indirect sensing methods are suggested to involve detection of changes occurring because of variation in oxygen availability, such as the sensing of redox status (NADH/ NAD + ratios), reactive oxygen species (ROS) (H 2 O 2 , NO), or the energy status (ATP levels) of a plant cell. Nonsymbiotic hemoglobins have been suggested to be involved in the sensing of NO or other ROS (Sowa et al, 1998;Dordas et al, 2003a;Dordas et al, 2003b;Thiel et al, 2011). However, it has never been demonstrated how a possible signal from hemoglobin could be transferred to the nucleus.…”

“…The hypoxia-inducible, oxygen-binding plant hemoglobins have been suggested to be involved in oxygen sensing (Hunt et al, 2002;Thiel et al, 2011). Despite evidence for a role in hypoxia tolerance (Hunt et al, 2002), the low dissociation constant of hemoglobins for oxygen precludes their possible function as an oxygen sensor or carrier (Abbruzzetti et al, 2011).…”

Non-symbiotic hemoglobin and its relation with hypoxic stress

“…Since they were discovered, diverse biochemical and physiological functions have been attributed to plant hemoglobins, including the transport, storage, and sensing of oxygen. The nsHb have also been associated with the transport of ligands such as CO, NO, and fatty acids, as well as the function of an oxygen scavenger (ArredondoPeter et al, 1998;Jokipii-Lukkari et al, 2009;Thiel et al, 2011). Knock-out silencing studies in A. thaliana have demonstrated the fundamental role of nsHb during plant development, showing that at least one functional nsHb gene is essential for plant survival (Hebelstrup et al, 2006;Dordas, 2009).…”

“…Nonsymbiotic hemoglobins (nsHb), as their name indicates, are not involved in symbiotic N fixation; apparently this type of protein exists in the entire vegetable kingdom, including plants with symbiotic capacity. Expression has been found in seeds, roots and other organs, both in monocotyledons and dicotyledons (Hill, 1998;Thiel et al, 2011). The ubiquity of the nsHb, along with the fact that they are probably evolutionary predecessors of the symbiotic hemoglobins, suggests that they have an important role in plant metabolism (Hebelstrup et al, 2006).…”