A new normal: recovery lessons learned from symplastic “lockdown” of the root stem cell niche

Growth and development of most plant organs require the production of new cells via mitosis. New cells arise from populations of stem cells, which are maintained in an undifferentiated state. Stem cells located in the root apical meristem surround mitotically inactive cells, the quiescent center (QC; van den Berg et al., 1997). Signals from the QC are essential for maintaining the undifferentiated status of the surrounding stem cells (Aichinger et al., 2012; Liu et al., 2017). Collectively, the QC together with its neighboring stem cells are referred to as the root stem cell niche (SCN). When either the entire SCN is lost due to root tip removal or QC cells are specifically ablated using a laser, meristematic activity is restored quite rapidly by the formation of a new QC (van den Berg et al., 1995; van den Berg et al., 1997; Sena et al., 2009; Efroni et al., 2016). However, laser ablation of the QC leads to a temporary loss of stem cells. This effect is observed most clearly in columella stem cells (located directly distal to the QC), which stop dividing and accumulate starch granules as they begin to differentiate (van den Berg et al., 1997).

In this issue of Plant Physiology, Xu et al. (2021) further explore the process of SCN regeneration following disruption. Their previous work shows QC-mediated maintenance of the SCN critically depends on symplastic communication through plasmodesmata (Liu et al., 2017). Here they describe how the SCN regenerates following disruption of QC symplastic communication using plants that carry an inducible construct. Expression of this construct causes callose to accumulate in the QC cells (Figure 1A), inhibiting movement of signals through plasmodesmata to surrounding cells (Vatén et al., 2011; Liu et al., 2017). Inducing callose in these plants, similar to laser ablation of QC cells, leads to a loss of QC identity and differentiation of columella stem cells in the root SCN after 3 d of compromised QC symplastic communication, but these effects are reversed by transferring plants to noninducing medium (Figure 1B; van den Berg et al., 1997; Liu et al., 2017).

Figure 1.

Symplastic blockage between the QC and surrounding cells disrupts SCN maintenance and promotes reorganization of the anatomic structure of the root tip. Mutual antagonism between auxin and cytokinin helps to position the QC cells and confer specific cellular identities, suggesting the presence of an intrinsic developmental program activated during root regeneration. A and B, Aniline blue staining highlights the QC-specific callose deposition in inducible pWOX5:icals3m roots after estradiol induction (A) and removal during recovery (B). C and D, Temporal pattern of the interaction between auxin and cytokinin during SCN recovery showing the initial asymmetric, nonoverlapping domains. Asterisks (A and B) and dashed circles (D) mark the QC. Adapted from Figures 1D, 1E, and 7H in Xu et al. (2021).

In this study, the authors focused on the process of SCN regeneration following the removal of callose-induced symplastic blockage. Four days after transferring plants to noninducing medium, QC identity was re-established, thus demonstrating the effects of temporary symplastic blockage were reversed. Three days after QC reformation, dedifferentiated columella stem cells were visible. Early during the recovery process, a new group of undifferentiated cells was observed forming in the SCN. The signal from a QC-specific reporter, which was mostly absent in the QC following callose induction, was initially decentralized during recovery but later became focused on the newly formed QC cells. Collectively, these observations suggest that recovery involves an early-stage anatomic reorganization and a late-stage cell fate determination, with the former driven by mitotic divisions.

The authors next used cell cycle and tissue identity markers to spatiotemporally track division distribution. They found that the stele and ground tissues contribute to the early and late stages of recovery, respectively. Thus, time-course analysis enabled the authors to distinguish the immediate and late events during SCN re-establishment in roots that retain normal anatomy and positional information.

Auxin and cytokinin play important roles during SCN regeneration following root tip excision (Sena et al., 2009; Efroni et al., 2016). The eventual formation of complementary, nonoverlapping auxin and cytokinin response domains instructs cell identity acquisition and determines the position of the new SCN (Efroni et al., 2016). Xu et al. (2021) used auxin and cytokinin reporter lines to investigate the potential role of these hormones during SCN re-establishment in their system. Time-lapse observation of a single root expressing both reporters revealed auxin and cytokinin responses occur mainly on opposite sides of the root early during SCN recovery, with the response domains for these two hormones rarely overlapping (Figure 1C and D). These findings suggest that the formation of complementary auxin and cytokinin response domains might be important for positioning the QC and conferring specific cell identity during SCN re-establishment, similar to SCN regeneration following root tip excision (Sena et al., 2009; Efroni et al., 2016).

Notably, the findings by Xu et al. (2021) further suggest that SCN re-establishment does not always occur via precisely the same mechanism. Early after root tip excision, symmetric auxin and cytokinin responses overlap, and the two hormone responses only separate into unique domains later, thus mimicking embryogenesis in the hypophysis as the root SCN is established (Müller and Sheen, 2008; Efroni et al., 2016). By contrast, in this study, the auxin and cytokinin responses were initially asymmetric and nonoverlapping, suggesting that while in some cases re-establishment of the root SCN may be mediated by a developmental pathway similar to the one that orchestrates SCN establishment during embryogenesis, another distinct pathway likely exists through which the root SCN can regenerate.