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Zinc (Zn) is an essential micronutrient for plant growth, playing critical roles in enzyme activation, protein stabilization, and cellular processes. Zn deficiency severely affects plant development, yet the regulatory mechanisms underlying Zn homeostasis remain incompletely understood. In our study, we investigated how bZIP19 and bZIP23 function as transcription factors to regulate the expression of Zn transporters, thereby contributing to Zn homeostasis. Through this research, we also discovered that bZIP19 regulates the expression of defensin-like family protein (DEFL) genes, which are highly induced under Zn deficiency in Arabidopsis thaliana. These DEFLs contain signal peptides at the N-terminus, indicating that they are secreted peptides. A phylogenetic analysis of 284 DEFL sequences revealed that Zn-responsive DEFLs form a distinct cluster, separate from other known peptide families. Notably, Zn deficiency-induced DEFL genes contain ZDRE motifs in their promoter regions. bZIP19 and bZIP23 bind to the motif and regulate the expression of Zn transporters, playing a crucial role in Zn homeostasis. In fact, DEFL202 and DEFL203 expression was shown to be directly regulated by bZIP19. To examine DEFL function, we analyzed T-DNA insertion mutants. While single mutants exhibited no apparent phenotype, defl202 defl203 double mutants displayed significantly longer roots, a phenotype observed under all growth conditions. Zn concentration in roots and shoots remained unchanged, suggesting that DEFLs do not regulate Zn uptake. Further analysis revealed that defl202 defl203 mutants exhibited increased meristem size and higher cell numbers, suggesting a role in cell cycle regulation. In summary, our findings suggest that bZIP19 plays a broader role than previously thought. While it is known to regulate Zn homeostasis, we discovered that it also controls DEFL expression. Unlike Zn transporters, DEFLs appear to regulate root growth through cell cycle control rather than Zn homeostasis.

References

Inaba et al., Identification of putative target genes of bZIP19, a transcription factor essential for Arabidopsis adaptation to Zn deficiency in roots. Plant J., 2015

Kimura et al., Zinc deficiency-induced defensin-like proteins are involved in the inhibition of root growth in Arabidopsis. Plant J., 2023