Gibberellin Promotes Polar Auxin Transport to Regulate Cambium Stem Cell Fate Decisions

The vascular cambium is critical for the lateral growth of plant roots and stems. The master regulator of the vascular cambium is auxin, and mutations in genes encoding auxin signaling lead to defects in cambium development. Auxin is not homogeneously distributed in the cambium, and its maximum is usually located on the xylem side of the vascular cambium, where cellular decisions of the vascular cambium also occurs. Other phytohormones such as gibberellins also affect cambium development. Recent studies have shown that auxin response factors ARF6 and ARF8 mediate auxin-dependent xylem development, a process downstream of gibberellin signaling. However, how gibberellin affects the localization of auxin and determines the fate of stem cells between xylem and phloem is unknown.

Recently, the research group of Ari Pekka Mähönen of the University of Helsinki in Finland published a research paper entitled "Gibberellins promote polar auxin transport to regulate stem cell fate decisions in cambium" in Nature Plants, and found that gibberellins promote the limited specialization of cambium stem cells to form daughter cells of xylem identity by promoting the polar auxin transport. This study provides a new mechanism for gibberellin to regulate the ratio of xylem and phloem cells.

Previous studies have shown that gibberellins regulate xylem formation in Arabidopsis hypocotyls during the flowering season. To unravel the role of gibberellin regulation in the dynamic determination of cambium growth, the authors first analyzed gibberellin effects on cell fate in Arabidopsis roots at single-cell resolution. Gibberellin treatment in roots promotes the increase of secondary xylem vessels and xylem parenchyma cells. However, the number of secondary xylem vessels and xylem parenchyma cells decreased after the introduction of gibberellin biosynthesis mutant, ga requiring1 (ga1).

To investigate the mechanism of this phenomenon, the authors used a heat shock-inducible CRE–lox-based lineage-tracing system that enables the analysis of single-cell clones derived from dividing cells including the cambium. Under normal growth conditions, stem cells differentiate into xylem cells and phloem cells in almost equal proportions. However, after gibberellin treatment, the distribution of clonal cell lineages appeared unevenly distributed, preferentially favoring the xylem side and extending towards the phloem in the background of ga1 mutants.

Further, the authors examined the regulation of gibberellin on the auxin response gradient. Therefore, the authors used a novel auxin response reporter, DR5v2. The authors observed auxin reporter expression in the cambium xylem. In wild-type plants, DR5v2 expression reaches the xylem-side stem cell daughter but rarely occurs in the phloem-side daughter. In the ga1 mutant, fewer stem cell daughters express DR5v2, and this ratio increased significantly after 48 hours of gibberellin treatment. This suggests that gibberellins regulate the position of the auxin signalling maximum within cambium.

The authors then wondered how gibberellins regulate auxin accumulation in the vascular cambium. Examination of expression of auxin efflux vectors in previous studies showed that PIN1-GFP is only present on the xylem side of the vascular cambium. Through gibberellin treatment, the authors found that gibberellin promotes the expression of PIN1 in stem cells and the expansion of the auxin reporter DR5v2. In addition, through EdU pulse experiments, the authors found that gibberellin treatment promoted xylem stem cells to become xylem cells.

Collectively, this study suggests that gibberellin regulates the distribution balance of vascular cambium between xylem and phloem by regulating the maximum distribution of auxin. Auxin accumulation in xylem is mainly regulated by PIN1-dependent polar auxin transport. This work provides a new model for cell fate decisions in xylem and phloem.

Reference:

Mäkilä, R., Wybouw, B., Smetana, O. et al. Gibberellins promote polar auxin transport to regulate stem cell fate decisions in cambium. Nat. Plants (2023). https://doi.org/10.1038/s41477-023-01360-w