Nitrate acts at the Arabidopsis thaliana shoot apical meristem to regulate flowering time

Abstract

Summary Optimal timing of flowering, a major determinant for crop productivity, is controlled by environmental and endogenous cues. Nutrients are known to modify flowering time; however, our understanding of how nutrients interact with the known pathways, especially at the shoot apical meristem ( SAM ), is still incomplete. Given the negative side‐effects of nitrogen fertilization, it is essential to understand its mode of action for sustainable crop production. We investigated how a moderate restriction by nitrate is integrated into the flowering network at the SAM , to which plants can adapt without stress symptoms. This condition delays flowering by decreasing expression of SUPRESSOR OF OVEREXPRESSION OF CONSTANS 1 ( SOC 1 ) at the SAM . Measurements of nitrate and the responses of nitrate‐responsive genes suggest that nitrate functions as a signal at the SAM . The transcription factors NIN ‐ LIKE PROTEIN 7 ( NLP 7) and NLP 6, which act as master regulators of nitrate signaling by binding to nitrate‐responsive elements ( NRE s), are expressed at the SAM and flowering is delayed in single and double mutants. Two upstream regulators of SOC 1 ( SQUAMOSA PROMOTER BINDING PROTEIN ‐ LIKE 3 ( SPL 3 ) and SPL 5 ) contain functional NRE s in their promoters. Our results point at a tissue‐specific, nitrate‐mediated flowering time control in Arabidopsis thaliana .

Tissue-specificNIA1andNIA2expression inArabidopsis thaliana

“…NIA1 transcript is also present in cauline leaves of the inflorescence apex (Figure 1(e)). Moreover, as previously observed, 3 NIA1 is expressed only in the rib and peripheral zones of the SAM and axillary meristems, but is not present in the central region including the organizing center and stem cell niche of the vegetative (Figure 1(d)) and the inflorescence SAM (Figure 1(e)). NIA2 is strongly expressed in the center of the vegetative (Figure 2(d)), inflorescence and floral meristems (Figure 2(e)).…”

“…We concluded that nitrate controls the flowering time at the SAM through NLP6 and NLP7 by regulating SOC1 at least in part via SPL3 and SPL5. 3 Interestingly, we found that NIA1 and NIA2 expression patterns at the SAM were barely overlapping with each other, suggesting that they might be regulated independently and assimilate nitrate in a stage-and tissue-dependent manner. Since their transcriptional response rapidly and directly responds to nitrate and does not require de novo protein synthesis, 5 the analysis of NIA transcripts provides information on the status of nitrate signaling and assimilation in cells.…”

“…We made use of RNA in situ hybridizations, in order to investigate cell-and tissue-specific expression of NIA1 and NIA2 transcripts in planta. Using tissue-specific probes for NIA1 and NIA2, 3 we detected transcripts in various tissues of Arabidopsis wild-type plants. Representative images of different tissue sections are presented in Figures 1 and 2.…”

“…Our previous analyses showed that nitrate is present at the shoot apical meristem (SAM) of Arabidopsis plants, where it directly triggers flowering. 3 We demonstrated that a moderate, non-stressful restriction of nitrate availability delays flowering time, due to decreased expression of SUPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE3 (SPL3) and SPL5 at the SAM. Moreover, we found that the nitrate-inducible genes NIA1 and NIA2 and the master regulators of nitrate signaling, NIN-LIKE PROTEIN 6 (NLP6) and NLP7, 4 are present at the SAM.…”

Transgenic overexpression of endogenous FLOWERING LOCUS T-like gene MeFT1 produces early flowering in cassava

“…Foliar application of plant growth regulators [6] and grafting onto rootstocks obtained from high flowering varieties are also techniques employed to improve flowering and allow controlled breeding programs [4,7]. Temperature, day length (photoperiod), levels of sugar, nitrates and plant growth regulators have been identified as triggers for flowering in Arabidopsis thaliana and other plant species [8][9][10]. FLOWERING LOCUS T (FT) protein is synthesised in the leaf and is thought to be translocated via the phloem and through the graft unions to control flowering [11].…”