Shoot circumnutation and winding movements require gravisensing cells

Abstract

Circumnutation and winding in plants are universal growth movements that allow plants to survive despite their sessile nature. However, the detailed molecular mechanisms controlling these phenomena remain unclear. We previously found that a gravitropic mutant of Japanese morning glory (Pharbitis nil or Ipomoea nil), Shidare-asagao (weeping), is defective not only in circumnutation but also in the winding response. This phenotype is similar to that of the Arabidopsis SCARECROW (SCR) mutant. We therefore investigated whether morning glory SCR (PnSCR) is involved in the weeping phenotype. We found that one amino acid was inserted into the highly conserved VHIID motif in weeping-type PnSCR; this mutation caused abnormal endodermal differentiation. We introduced either the mutant or WT PnSCR into Arabidopsis scr mutants for complementation tests. PnSCR of the WT, but not of weeping, rescued the shoot gravitropism and circumnutation of scr. These results show that both the abnormal gravitropism and the circumnutation defect in weeping are attributable to a loss of PnSCR function. Thus, our data show that gravisensing endodermal cells are indispensable for shoot circumnutation and the winding response and that PnSCR is responsible for the abnormal phenotypes of weeping.

Fig. 1.

Growth of the morning glory cultivars Violet and weeping. (A) A typical plant of the WT morning glory cultivar Violet was grown for 2.5 weeks. (B) A typical plant of cultivar Shidare-asagao (weeping) was grown until its shoot displayed weeping growth. Plants were grown at 25°C under continuous white light. The diameter of the pot is 10 cm. Arrow g indicates the direction of gravitational force.

Fig. 2.

Comparison of the VHIID motif of Violet-type PnSCR and weeping-type PnSCR and F₂ linkage analysis between agravitropism and the PnSCR mutation. (A) A part of the VHIID motif of Violet-type PnSCR (PnSCR) and weeping-type PnSCR (PnSCRm). (B) Linkage between agravitropism and the mutation. Dark-grown F₂ seedlings were distinguished by their gravitropic responses. Then, PnSCR was amplified by PCR from genomic DNA isolated from each F₂ seedling. The PCR products were digested with Sau3AI and analyzed by agarose gel electrophoresis. M, molecular weight marker (φX174/HaeIII); WE, Violet; we, weeping. Data from eight F₂ individuals of each phenotype are shown.

Fig. 3.

Shoot gravitropism of WT, scr, and transformant Arabidopsis. (A) Construct of pBI101ΔGUS::pAtSCR::PnSCR used for complementation tests. (B) Gravitropism of WT Arabidopsis. (C) Gravitropism of sgr1-1/scr-3 with pBI101ΔGUS::pAtSCR. (D) Complementation of sgr1-1/scr-3 gravitropism with pAtSCR::PnSCR.(E) Complementation test with pAtSCR::PnSCRm. WT and transgenic plants were placed in a horizontal position at 23°C for 6 h in the dark. Arrow g indicates the direction of gravitational force. (Scale bars: 2 cm.) (F) Time courses for gravitropic responses of the excised inflorescence stems of WT, sgr1-1/scr-3, and transgenic plants: WT (○), scr/PnSCR (•), scr/PnSCRm (□), scr/Vector (▪), sgr1-1/scr-3 (▵). The excised inflorescence stems were gravistimulated by being placed in a horizontal position at 23°C in the dark. Data represent means ± SE.

Fig. 4.

Histological analysis of WT and transformant Arabidopsis stained with toluidine blue. Longitudinal sections of inflorescence stems of WT (A), scr/Vector (B), scr/pAtSCR::PnSCR (C), and scr/pAtSCR::PnSCRm (D) Arabidopsis. Arrowheads indicate the sediment amyloplasts. Arrow g indicates the direction of gravitational force. Ep, epidermis; Co, cortex; En, endodermis. (Scale bar: 20 μm.)

Fig. 5.

The circumnutation of inflorescence stems of WT, scr, and transformant Arabidopsis. Potted plants were placed in a dark box when their inflorescence lengths were ≈9-10 cm. Shoot movement was observed from above. (A) WT. (B) scr/pAtSCR::PnSCR. (C) scr/pAtSCR::PnSCRm. (E) sgr1-1/scr-3. (G) scr/Vector. D, F, and H show magnified images of C, E, and G, respectively. The start and the end of the movement are indicated by ⋄.