Cytokinin functions as an asymmetric and anti-gravitropic signal in lateral roots

Abstract

31Directional organ growth allows the plant root system to strategically cover its 32 surroundings. Intercellular auxin transport is aligned with the gravity vector in 33 the primary root tips, facilitating downward organ bending at the lower root 34 flank. Here we show that cytokinin signaling functions as a lateral root specific 35 anti-gravitropic component, promoting the radial distribution of the root system. 36 We performed a genome-wide association study and revealed that signal peptide 37 processing of Cytokinin Oxidase 2 (CKX2) affects its enzymatic activity and, 38 thereby, determines the degradation of cytokinins in natural Arabidopsis 39 thaliana accessions. Cytokinin signaling interferes with growth at the upper 40 lateral root flank and thereby prevents downward bending. Our interdisciplinary 41 approach revealed that two phytohormonal cues at opposite organ flanks 42 counterbalance each other's negative impact on growth, suppressing organ 43 growth towards gravity and allow for radial expansion of the root system. 44 45 128 regulate angular growth of LRs, we initially transferred 7-day old seedlings of the 129 reference accession Col-0 to medium supplemented with CKs. We observed a 130 concentration-dependent increase in GSA values of LRs emerging in presence of 131 6active CKs, such as 6-Benzylaminopurin (BAP) ( Figure 2D), trans-zeatin (tZ) and 132 isopentenyladenine (iP) ( Figure S3F and G). Conversely, CK receptor mutants showed 133 accelerated bending of LRs and accordingly decreased GSA values ( Figure 2E). 134These data suggest that cytokinin signaling interferes with downward bending of 135 emerged LRs. Notably, emerging LRs of winter oilseed rape also displayed reduced 136 bending of LRs when treated with BAP ( Figure S3H), suggesting that the effect of CK 137 on directional LR growth is likely to be conserved. 138To further assess the importance of CKX2 in GSA establishment, we disrupted 139 CKX activity in the reference accession Col-0. Treatments with the CKX inhibitor 140 INCYDE (Zatloukal et al., 2008) phenocopied the ckx2 loss-of-function mutant, both 141 displaying more horizontal LRs when compared to its respective controls (Figure 2F, 142 G). On the other hand, CKX2 overexpressing (OX) plants showed accelerated bending 143 of LRs, phenocopying the CK receptor mutants (Figure 2G). These data suggest that 144 CK signaling defines directional lateral root growth by reducing LR bending after 145 emergence. 146 147 157 ARR4 (pARR3:GUS/ pARR4:GUS; Figure S4C)). Moreover, angular growth of LRs 158 was not altered in arr3 or arr4 mutants ( Figure S4D). On the other hand, we confirmed 159 expression of CRF2 and CRF3 in the early stages of LR development ( Figure 3A and 160 S4E). pCRF2:GFP-GUS was ubiquitously expressed in young LRs, while pCRF3:GFP-161 GUS was preferentially expressed in cortical and epidermal cell files ( Figure 3A and 162 S4E). Notably, compared to emerged laterals, the main root displayed much weaker 163 CRF2 and CRF3 expression ( Figure S4F), suggesting that CRF2 an...

Optimizing Drought Resilience in Okra (Abelmoschus esculentus) Through Synergistic Actions of Drought-Tolerant Rhizobacterial Strains and Brassinosteroids Hormone Application: A Jar Trial Study

“…Binenbaum et al (2018). observed that there are other hormones such as cytokinins and gibberellic acid, which affect the root shape by increasing its length and development while reducing ethylene levels (Waidmann et al 2019) Additionally, Plazas et al (2019) found that the combination of PGPRs and BR s can improve soil hydration rate by playing a crucial signal between ecological conditions and roots as it has been presented in (Mustafa et al 2023). Hence, when Brassinosteroid is used in conjunction with rhizobacterial strains, plants absorb nutrients and more water from the soil; greatly increasing root absorption area without negative behavior (Mustafa et al 2022).…”

Chromosome‐level genome assembly of Pedicularis kansuensis illuminates genome evolution of facultative parasitic plant

“…Cytokinin-activating enzymes are in charge of cytokinin activation (Kurakawa et al, 2007), and the repressed expression of these genes in roots may contribute to establishing connections to their host through a developed lateral root system in parasitic plants as a decrease in cytokinin levels in roots can enhance lateral root growth (Waidmann et al, 2019). Notably, this is not conserved among parasitic plants because the LTR-associated cytokinin-activating enzyme genes were not present in S. asiatica (Table S21).…”

“…Interestingly, 6 and 1 cytokinin-activating-enzyme-related genes with barely detectable expression were separately found in superset-type and overlap-type genes (Table S10). Cytokinin activating enzyme in rice has been reported to work in the final step of bioactive cytokinin synthesis (Kurakawa et al, 2007), and cytokinin in roots could inhibit root initiation though directly acting on lateral root founder cells (Waidmann et al, 2019). The repressed expression of these genes in the roots of P. kansuensis may enhance lateral root growth and contribute to the establishment of parasitic associations for improving the survival fitness of parasites.…”

Determinants of root system architecture for future‐ready, stress‐resilient crops

“…Mechanism III suggests that root exploration of the soil by LRs depends on CK concentration (Figure 1D). Waidmann et al, (2019) showed that CK signalling affects root gravitropic growth. In particular, asymmetric CK signalling reduces growth at the upper side of recently emerged LRs, but not in PRs.…”

“…CKX enzymes are responsible for the irreversible degradation of CK via the oxidative cleavage of their side chain, thus blocking CK accumulation. The inactive CKX2 variant shared by specific Arabidopsis accessions, besides allowing better acclimation to extreme winters (Xiao and Zhang, 2020), promotes a shallower RSA and seems to be a promising trait to cope with hypoxic stress (Waidmann et al, 2019). Indeed, CK signalling plays an important role in adaptation to hypoxia.…”