γ-Aminobutyric Acid Participates in the Adult-Phase Adventitious Rooting Recalcitrance

Abstract

In many tree species especially the rooting-recalcitrant woody perennials, the adventitious root (AR) in the juvenile phase can be easily induced by exogenous auxin, but AR formation becomes recalcitrant in the adult phase. Also, it is reported that the γ-aminobutyric acid (GABA) inhibits primary root growth in Arabidopsis and the AR formation in poplar (Populus ssp). So far, how GABA affects or is affected by the ontogenetic phase or auxin remains unclear. Here, we used an apple rootstock, Malus xiaojinensis, and tobacco (Nicotiana benthamiana) to investigate this question. We first analyzed the content of GABA, the activity of GABA synthetic enzyme GAD, and the expression of the coding gene MxGADs, respectively, in leafy cuttings of juvenile and adult phase. Next, the effect of exogenous GABA on AR formation was examined in in vitro shoots of M. xiaojinensis and tobacco. Interestingly, significant and consistent increases in GABA concentration, GAD activity, and expression of MxGAD genes in response to exogenous indole butyric acid (IBA) were detected in adult-phase cuttings, but not in juvenile-phase cuttings. Exogenous GABA application inhibited the AR formation by delaying rooting time and reducing root number and the total root length in in vitro shoots of both M. xiaojinensis and tobacco. The expression of MxPIN members increased in response to IBA application, but these changes were restrained by the addition of GABA. These results indicate that both the loss of juvenility and IBA are required to trigger GABA accumulation. GABA may affect the AR formation as a co-actor by inhibiting polar auxin transport. Together, these findings facilitate the understanding of the regulatory network among GABA, juvenility, and auxin signaling on the AR formation.

DNA methylation variation is crucial to restore adventitious rooting ability during in vitro shoot culture‐induced rejuvenation in apple rootstock

“…GAD gene encodes glutamate decarboxylase, which can catalyze glutamate decarboxylate to form γ‐aminobutyric acid (GABA; Bown & Shelp, 1997). As shown in our previous study, GABA participates in the adult‐phase apple rootstock adventitious rooting recalcitrance (Li, Xu, et al., 2021). Previous studies found that significant increases in expression of GABA synthetic enzyme genes GAD , activity of GAD, and GABA concentration were detected in adult‐phase cuttings, but not in juvenile‐phase cuttings in apple rootstock (Li, Xu, et al., 2021).…”

“…As shown in our previous study, GABA participates in the adult‐phase apple rootstock adventitious rooting recalcitrance (Li, Xu, et al., 2021). Previous studies found that significant increases in expression of GABA synthetic enzyme genes GAD , activity of GAD, and GABA concentration were detected in adult‐phase cuttings, but not in juvenile‐phase cuttings in apple rootstock (Li, Xu, et al., 2021). We found that the promoter region of GAD5a gene exhibited significantly high methylation levels in ‘T337’_R compared with ‘T337’_A in CHH content (Figure 7a).…”

“…Therefore, it is reasonable to deduce that rejuvenation‐induced hypermethylation in apple rootstock is associated with the repression of the genes suppressing adventitious rooting. In our previous study, we found GABA inhibited the AR formation as a repressor, which participates in the adult‐phase adventitious rooting recalcitrance in apple rootstock (Li, Xu, et al., 2021). Consistently, the expression level of GAD5a and the content of GABA were significantly decreased in ‘T337’_R compared with ‘T337’_A (Figure 7b,c), and GABA treatment inhibited the adventitious rooting (Figure 7d–g).…”

“…Stem bark samples of T337_A and T337_R from 0.5‐ to 1.0‐cm basal sections were collected and frozen in liquid nitrogen without treatment for three biological replicates (each replicate containing 30 leafy cuttings). GABA content was measured according to our previous report by HPLC (Li, Xu, et al., 2020). The SD was calculated among three biological replicates and three technical replicates.…”

A bulked segregant analysis tool for out-crossing species (BSATOS) and QTL-based genomics-assisted prediction of complex traits in apple

“…In inbreeding plants, GEBV is estimated in many GS models such as rr-BLUP, Bayesian least absolute shrinkage and selection operator (LASSO) etc., which emphasizes additive effects [11] . In outbreeding species, however, non-additive effects contribute to a large proportion of the genetic variations due to the high level of heterozygosity and the clonal propagation of many outcrossing plants [2] , [72] . In the GAP models, the genetic effect includes both allelic additive and non-additive effects, the prediction accuracy was therefore higher than that obtained by rr-BLUP.…”