TDIF overexpression in poplars retards internodal elongation and enhances leaf venation through interaction with other phytohormones

As a member of the CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-related (CLE) peptide family, tracheary element differentiation inhibitory factor (TDIF) plays crucial roles in vascular meristem maintenance by promoting cell proliferation and inhibiting xylem cell differentiation. In Populus trichocarpa, six TDIF-encoding genes are all expressed in vascular tissues, and in Arabidopsis PtTDIFpro:GUS lines, the expression driven by PtTDIF promoters were predominantly detected in stem vascular bundles, initiating leaves and leaf veins. Although exogenous application of two poplar TDIF peptides did not evidently affect the shoot growth in vitro, overexpression of PtTDIF genes in hybrid poplar severely retarded the internodal elongation by upregulating the expression of GA2ox and GA20ox genes and thus decreasing the level of endogenous gibberellins (GAs), which phenotypic defect could be rescued by exogenously applied GA3. In addition, TDIF overexpression unexpectedly induced a more complex venation pattern in poplar leaves, which was underpinned by the elevated expression of WOX4 and WOX13 genes. Upon TDIF treatment, the DR5:GUS poplar leaves revealed a higher GUS activity and in TDIF-overexpressing leaves, the transcript abundances of several PIN-FORMED (PIN) genes, especially that of PIN1, were increased, which implied an integration of TDIF and auxin in mediating this process. Collectively, data of this work presented novel activities of TDIF involved in internode elongation and leaf vein formation, thus revealing the divergent functions of TDIF in perennial tree species from those in annual herbaceous Arabidopsis.

Internodal elongation in the grapevine (Vitis vinifera L.) : leaf influences and role of the shoot apex

<p style="text-align: justify;">The respective influences of the leaves and shoot apex on stimulation of internodal elongation were studied in the grapevine. Various defoliation or decapitation treatments were applied to the shoot of rooted hardwood cuttings grown in a controlled environment chamber.</p><p style="text-align: justify;">Concerning the foliar influences, the role of three sorts of leaves, in relation to their nodal position relative to the internode was analysed ; its own leaves, the leaves located below it and the young leaves neoformed above. The effects produced by excision of these different leaves show that the two adjacent leaves (below and above) of the internode play an essential role in the stimulation of its elongation. Among the subjacent leaves, these only at a position at least 6 nodes below promote internode growth. On the other hand, the young leaves neoformed in the upper part of the shoot have not influence.</p><p style="text-align: justify;">The results of decapitation experiments indicate that terminal bud contributes also to the stimulation of internodal elongation. However this influence appears only if an increment to the size (« compensatory growth ») of the leaves below the level of decapitation is not induced by the removal of the shoot tip. The leaves which realize this « compensatory growth » have indeed the ability to compensate for the laking apical bud stimulus.</p><p style="text-align: justify;">The influences vary with the stage of internode length. The longitudinal growth of very young internode (5 - 10 mm long) is stimulated by its own leaves, the terminal bud and leaves situated at least 6 nodes below. When the internode has reached at least 50 p. cent of its adult size, its elongation depends only on its own leaves and the apex. At the end of grown, only its own leaves stimulate lengthening.</p><p style="text-align: justify;">Exogenous applications of IAA, NAA and gibberellic acid (GA3) at different concentrations were also tested. GA3 at 100 mg.l^-1 and 1 g.l^-1 (incorporated to lanolin paste) applied to the cut surface of the petiole of the two adjacent leaves of young internode permits a normal elongation providing that apical bud is maintained. If the shoot is decapited the same treatment has almost the same effect only by supplying NAA at 1 g.l^-1 to the shoot section.</p><p style="text-align: justify;">The functions of the three elements contributing to internodal elongation are discussed, taking in consideration both the experimental results of exogenous applications of IAA, NAA and GA3, and the possible trophic role of the leaves. It is proposed that the two adjacent leaves of the internode stimulate its longitudinal growth through a production of diffusible gibberellic substances and also, probably of nutrient elements. The role of subjacent leaves is certainly of trophic nature. Terminal bud being an auxin source, we can expect that its stimulating influence is due to the supply of this growth regulator.</p>