Abstract
Nitrate (NO3-) is a key element for crop production but its levels in agricultural soils are limited. Plants have developed mechanisms to cope with these NO3- fluctuations based on sensing nitrate at the root apex. Particularly, the transition zone (TZ) of root apex has been suggested as a signaling-response zone. This study dissects cellular and molecular mechanisms underlying NO3- resupply effects on primary root (PR) growth in maize, confirming nitric oxide (NO) as a putative modulator. Nitrate restoration induced PR elongation within the first 2 h, corresponding to a stimulation of cell elongation at the basal border of the TZ. Xyloglucans (XGs) immunolocalization together with Brefeldin A applications demonstrated that nitrate resupply induces XG accumulation. This effect was blocked by cPTIO (NO scavenger). Transcriptional analysis of ZmXET1 confirmed the stimulatory effect of nitrate on XGs accumulation in cells of the TZ. Immunolocalization analyses revealed a positive effect of nitrate resupply on auxin and PIN1 accumulation, but a transcriptional regulation of auxin biosynthesis/transport/signaling genes was excluded. Short-term nitrate treatment repressed the transcription of genes involved in strigolactones (SLs) biosynthesis and transport, mainly in the TZ. Enhancement of carotenoid cleavage dioxygenases (CCDs) transcription in presence of cPTIO indicated endogenous NO as a negative modulator of CCDs activity. Finally, treatment with the SLs-biosynthesis inhibitor (TIS108) restored the root growth in the nitrate-starved seedlings. Present report suggests that the NO-mediated root apex responses to nitrate are accomplished in cells of the TZ via integrative actions of auxin, NO and SLs.
Highlights
Nitrogen (N) is one of the most important elements for plant life
These results suggest that high levels of nitric oxide (NO) caused by environmental stimuli or elicitors could positively regulate the exit of cells from the primary root (PR) meristem and the transition zone (TZ) into the elongation and differentiation zones, altering the PR growth
The results indicate that PR growth stimulation by short term nitrate provision could be attributed to a putative interference with the basipetal auxin flow and the XG deposition, affecting cell expansion in the TZ
Summary
Nitrogen (N) is one of the most important elements for plant life. In soil, N is present in different N-containing compounds. Fernández-Marcos et al (2011) reported inhibitory effects of NO on rootward polar transport, due to a PIN1 depletion and decreased numbers of dividing cells in the PR meristem They demonstrated that during early root development endogenous NO accumulates mainly in a zone situated between the apical meristem and the elongation zone, namely the TZ. Other studies demonstrated that endogenous NO application affects PR growth by reducing the pool of dividing cells in the root apical meristem, causing a reduction in cell-division rates and an increase in cell lengths of the meristem (Méndez-Bravo et al, 2010; Fernández-Marcos et al, 2011) These results suggest that high levels of NO caused by environmental stimuli or elicitors could positively regulate the exit of cells from the PR meristem and the TZ into the elongation and differentiation zones, altering the PR growth. Strigolactones (SLs) and NO seem to act upstream of this signaling, via regulating the balance between cell division and expansion
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