Maize ZmCCD8 knockout mutant is impaired in strigolactones biosynthesis and displays a different response to nitrogen availability.

Nitrogen (N) is among the most important macronutrients for plant growth and development, and the most widely used as fertilizer. In a world with a growing global population, one of the main challenges for modern agriculture is to improve food production, while protecting the environment. Therefore, understanding how plants sense, signal and respond to nitrogen starvation is essential to optimize the Nitrogen Use Efficiency (NUE) and reduce the use of chemical fertilizers, lightening agricultural costs and the excessive use of these non-renewable resources. Maize (Zea mays L.) is one of the major world crop productions and therefore it is expected to give an important contribution to human nutrition in the next few decades. The plant can absorb nitrogen mainly in two forms, nitrate (NO_3^-) or ammonium (〖NH_4〗^+), which in addition to being nutrients also act as signal molecules inside the plant. In particular, it appears that the perception of nitrate is very high in the transition zone (TZ) of maize root. The early stages of nitrate perception in the root epidermal TZ cells seems to be characterized by a sophisticated interplay of highly specific events, including NO and ROS homeostasis regulation, and hormonal (auxin and strigolactones) accumulation and signalling which in turn activate a transduction pathway to the whole root. Strigolactones (SLs) are carotenoid-derived phytohormones that regulate plant development in response to various environmental stimuli and in concert with many other regulators through their action as both endogenous and exogenous signalling molecules. In this work, the potential role of SLs in the regulation of the response of maize to N starvation was assessed. Physiological and transcriptional responses were analyzed in both seedlings of the wild type Zea mays B73 inbred and of a ZmCCD8 knockout mutant grown in hydroponics under three different N regimes. The mutant has a Ds transposon insertion in an essential SL biosynthetic gene that encode CAROTENOID CLEAVAGE DIOXYGENASE8 (CCD8). A N-free treatment, a treatment containing NO_3^- 0.1 mM and finally a treatment containing NO_3^- 1 mM were evaluated. To assess the physiological responses due to the different treatments, the leaf levels of chlorophyll and anthocyanins were analyzed through the DUALEX SCIENTIFIC+™. Leaf pigments were evaluated after three, four, six, seven and ten days of growth of the seedlings in the different nutrient solutions. From these analyzes it emerged that the wild type seedlings and the mutant ones has a very different phenotype especially those grown in nitrogen-free solutions. The chlorophyll levels in the first and second leaf of the wild type seedlings are much higher than in the mutants which, on the other hand, have a higher level of anthocyanins, especially in the second leaf. Furthermore, the fresh weights of roots and shoots and the length of the primary root of seedlings grown for two (T2) and seven (T7) days in the different treatments were evaluated. These analyses show that in N deficiency, the mutant has a higher fresh unit weight of roots and a shorter primary root than the wild type. For all the parameters considered it was noted that the differences between the two phenotypes decrease with increasing nitrogen availability in the various treatments. At the same time, the expression levels of several genes previously selected were measured through qPCR in root and shoot tissues sampled after two days of growth. Results obtained highlight the existence of shared and divergent pathways between the two genotypes, in terms of gene regulation in response to nitrate availability. Furthermore, the two genotypes grown in field showed very different phenotypes. The analysis of their leaf pigments with the DUALEX SCIENTIFIC+™ confirmed the net difference seen in the seedlings grown in hydroponics.