Strigolactone signaling is required for auxin-dependent stimulation of secondary growth in plants

Abstract

Long distance cell-to-cell communication is critical for the development of multicellular organisms. In this respect, plants are especially demanding as they constantly integrate environmental inputs to adjust growth processes to different conditions. One example is thickening of shoots and roots, also designated as secondary growth. Secondary growth is mediated by the vascular cambium, a stem cell-like tissue whose cell-proliferating activity is regulated over a long distance by the plant hormone auxin. How auxin signaling is integrated at the level of cambium cells and how cambium activity is coordinated with other growth processes are largely unknown. Here, we provide physiological, genetic, and pharmacological evidence that strigolactones (SLs), a group of plant hormones recently described to be involved in the repression of shoot branching, positively regulate cambial activity and that this function is conserved among species. We show that SL signaling in the vascular cambium itself is sufficient for cambium stimulation and that it interacts strongly with the auxin signaling pathway. Our results provide a model of how auxin-based long-distance signaling is translated into cambium activity and suggest that SLs act as general modulators of plant growth forms linking the control of shoot branching with the thickening of stems and roots.

Fig. 1.

Genetic analysis of the role of SL signaling and biosynthesis in cambium regulation. (A–D) Cross-sections from immediately above the uppermost rosette leaf of wild-type (A and B) and max1-1 (C and D) stems. Note that B and D are higher magnification images for areas labeled in A and C, respectively. The extension of the IC-derived tissue (ICD) is indicated by braces (}). [Scale bar in C (100 μm) also applies to A, and scale bar in D (50 μm) also applies to B.] Asterisks indicate the position of primary vascular bundles. (E) Quantification of lateral ICD extension immediately above the uppermost rosette leaf as indicated in A and C. (F) Quantification of the longitudinal progression of IC initiation, as illustrated in D. (G) Scheme of longitudinal IC extension in the Arabidopsis stem (red) and the relative position of the treatment zone described in Fig. 2 and Fig. 4.

Fig. 2.

GR24 treatments stimulate secondary growth in Arabidopsis inflorescence stems. (A and B) Sections from GR24- (A) and mock-treated (B) wild-type plants. Extension of tissue produced in interfascicular regions is indicated by the brace (}) in A. (C) Quantification of GR24-induced tissue production in interfascicular regions of wild-type, max1-1, and max2-1. Note that in mock-treated plants no cell divisions were observed in any of the genetic backgrounds (shown here for wild type). (D and E) PXY:CFP activity in mock- (D) and GR24-treated plants (E). Reporter gene-derived signal in the fascicular cambium is indicated by arrows in D and in interfascicular regions in E. (F and G) APL:CFP activity in mock- (F) and GR24-treated plants (G). Reporter gene-derived signal in the phloem of vascular bundles is indicated by arrows in F and in interfascicular regions in G. (H) APL:GUS activity in GR24- (Left) and mock-treated (Right) plants. (G) PXY:GUS detection in GR24- (Left) and mock-treated (Right) plants. Asterisks indicate the position of vascular bundles. [Scale bar in B (100 μm) also applies to A; scale bar in D (100 μm) also applies to E–G; scale bar in I (5 mm) also applies to H.] Note that sections in D–G were counterstained by propidium iodide (red), which highlights cell walls.

Fig. 3.

Auxin levels and signaling are enhanced in max1 mutants. (A) In pin1-613 and pin3-5 mutants, the acropetal progression of IC initiation is diminished. Plants were analyzed when shoots were 2, 5, 15, and 30 cm tall. (B) Comparison of levels of free IAA in wild type and max1-1 at different positions along the inflorescence stem. The first elongated internode above the rosette was counted as the first internode. IN, internode. (C and D) Analysis of DR5:GUS activity in wild-type (C) and max1-1 inflorescence stems (D). Rosette leaves have been removed for clarity. (E–H) Analysis of DR5rev:GFP activity at different positions of the inflorescence stem. (E and F) DR5rev:GFP detection 1 cm above the rosette in wild type (E) and max1-1 (F). (G and H) DR5rev:GFP detection immediately above the uppermost rosette leaf of wild type (G) and max1-1 (H). [Scale bar in D (5 mm) also applies to C; scale bar in E (100 μm) also applies to F–H.]

Fig. 4.

Genetic interaction between axr1 and max mutants and the effect of tissue-specific SL signaling. (A) Quantification of NPA-induced tissue production in interfascicular regions of wild type, max1-1, max2-1, axr1-3, axr1-3 max1-1, and axr1-3 max2-1. (B) Quantification of GR24-induced tissue production in interfascicular regions of wild type, max1-1, max2-1, axr1-3, axr1-3 max1-1, and axr1-3 max2-1. Note that in mock-treated plants no interfascicular cell divisions were observed in any of the genetic backgrounds (shown here for wild type). (C) Quantification of lateral ICD extension immediately above the uppermost rosette leaf in wild-type, max2-1, and max2-1 plants carrying APL:MAX2, NST3:MAX2, SCR:MAX2, or WOX4:MAX2 transgenes. For each construct, two independent transgenic lines were analyzed. In all cases, n = 10.