Tissue‐specific hormone profiles from woody poplar roots under bending stress

Abstract

Mechanical forces induced by bending are able to trigger an asymmetrical response in Populus nigra L. woody taproots. This response includes the recruitment of new lateral roots on the convex side and the deposition of reaction wood (RW) on the opposite concave side. Since these responses seem to be induced by asymmetric activity and differentiation of cambium cells, we investigated, in the present work, how mechanical forces could influence the activation of specific phytohormone signaling pathways on the two sides of the vascular cambium. Thus, distinctive tissues were isolated from convex and concave sides of bent poplar root using cryosectioning. Successively, the isolated tissues, represented by the cambial zone, and the developing phloem and xylem, were analyzed using liquid chromatography coupled to tandem mass spectrometry to profile auxins, abscisic acid (ABA), cytokinins (CKs) and their metabolites. The auxin gradient on the concave side, with the IAA maximum localized in the cambium and decreasing level toward the developing phloem and xylem, suggests a pivotal role of IAA in the control of cambial growth rate, xylem differentiation and RW production. The IAA differences between the two bent root sides could be at the basis of the strictly unidirectional RW production. The higher levels of ABA and all CKs metabolites on the concave side support their involvement in RW production, whereby ABA could mediate the adaptation to the deforming conditions generated by bending, while CKs could act in synergy with IAA in controlling cell differentiation and meristem size.

Dynamics of Auxin and Cytokinin Metabolism during Early Root and Hypocotyl Growth in Theobroma cacao

“…At 10 DAI, oxIAA levels were overall increased in all root segments and hypocotyls, with the highest concentration found in the Dz ( Figure 2 D). Worth highlighting is that oxIAA was a minor IAA catabolite compared to the conjugates IAAsp and IAGlu across our developmental segments and time points ( Figure 2 E), contrary to what has been traditionally observed in many other plant species [ 27 , 40 , 45 , 47 , 48 ]. This suggests that IAA conjugation to Asp and Glu is a major catabolic pathway regulating endogenous IAA levels in roots and hypocotyl during early cacao seedling development.…”

“…In growing seedlings, CKs modulate lateral root organogenesis [ 15 , 16 ], root meristem size [ 13 ], and elongation of the hypocotyl [ 23 ], among other processes. Levels of the 4 active CK free bases and their metabolites have been quantified in different tissues from different plant species [ 38 , 44 , 45 , 46 , 69 , 70 ]. These studies have found the four CK active bases to be differentially abundant in different plant tissues, suggesting that each CK type might play specific roles.…”

“…Quantitative changes in auxin and cytokinin and their metabolites in different plant tissues have been reported in several species but mostly in Arabidopsis [ 15 , 37 , 38 , 39 , 40 , 41 , 42 ], rice [ 43 ], maize [ 44 ], Populus [ 45 , 46 ], bryophytes [ 47 ], and algae [ 48 ]. As CK and auxin are closely interconnected and indispensable for plant growth and response to stresses, examining their fluxes and levels during development is very important, especially in agriculturally important crops.…”

Plant vascular development: mechanisms and environmental regulation

“…Additionally, ethylene was found to control cambial cellular proliferation both in normal conditions and during tension wood development in Populus [142]. During the process, no change in auxin levels was found in tension wood forming tissues [143], although the establishment of an auxin gradient -most likely led by auxin redistribution-seems to be necessary [144]. Furthermore, GA was shown to control all major aspects of tension wood formation when applied to Populus stems [145], and a role for it during the process cannot be excluded.…”

Table_1.doc

“…Another important hormone group, the CKs, could be a key signal to maintain appropriate levels of auxin biosynthesis (Jones and Ljung, 2011) but they have seldom been investigated in relation to RW formation (De Zio et al, 2019). FIGURE 7 | Model summarizing correlation among anatomical, phytohormonal and lignin dataset in bent stem and root.…”