Tunable recurrent priming of lateral roots in Arabidopsis: More than just a clock?

Lateral root (LR) formation in Arabidopsis is a continuous, repetitive, post-embryonic process regulated by a series of coordinated events and tuned by the environment. It shapes the root system, enabling plants to efficiently explore soil resources and adapt to changing environmental conditions. Although the auxin-regulated modules responsible for LR morphogenesis and emergence are well documented, less is known about the initial priming. Priming is characterised by recurring peaks of auxin signalling, which, once memorised, earmark cells to form the new LR. We review the recent experimental and modelling approaches to understand the molecular processes underlying the recurring LR formation. We argue that the intermittent priming of LR results from interweaving the pattern of auxin flow and root growth together with an oscillatory auxin-modulated transcriptional mechanism and illustrate its long-range sugar-mediated tuning by light.

Microtubule-based perception of mechanical conflicts controls plant organ morphogenesis.

Precise coordination between cells and tissues is essential for differential growth in plants. During lateral root formation in Arabidopsis thaliana, the endodermis is actively remodeled to allow outgrowth of the new organ. Here, we show that microtubule arrays facing lateral root founder cells display a higher order compared to arrays on the opposite side of the same cell, and this asymmetry is required for endodermal remodeling and lateral root initiation. We identify that MICROTUBULE ASSOCIATED PROTEIN 70-5 (MAP70-5) is necessary for the establishment of this spatially defined microtubule organization and endodermis remodeling and thus contributes to lateral root morphogenesis. We propose that MAP70-5 and cortical microtubule arrays in the endodermis integrate the mechanical signals generated by lateral root outgrowth, facilitating the channeling of organogenesis.

Root stem cell niche maintenance and apical meristem activity critically depend on THREONINE SYNTHASE1.

Indeterminate root growth depends on the stem cell niche (SCN) and root apical meristem (RAM) maintenance whose regulation permits plasticity in root system formation. Using a forward genetics approach, we isolated the moots koom1 ('short root' in Mayan) mutant that shows complete primary RAM exhaustion and abolished SCN activity. We identified that this phenotype is caused by a point mutation in the METHIONINE OVERACCUMULATOR2 (MTO2) gene that encodes THREONINE SYNTHASE1 and renamed the mutant as mto2-2. The amino acid profile showed drastic changes, most notorious of which was accumulation of methionine. In non-allelic mto1-1 (Arabidopsis thaliana cystathionine gamma-synthetase1) and mto3-1 (S-adenosylmethionine synthetase) mutants, both with an increased methionine level, the RAM size was similar to that of the wild type, suggesting that methionine overaccumulation itself did not cause RAM exhaustion in mto2 mutants. When mto2-2 RAM is not yet completely exhausted, exogenous threonine induced de novo SCN establishment and root growth recovery. The threonine-dependent RAM re-establishment in mto2-2 suggests that threonine is a limiting factor for RAM maintenance. In the root, MTO2 was predominantly expressed in the RAM. The essential role of threonine in mouse embryonic stem cells and in RAM maintenance suggests that common regulatory mechanisms may operate in plant and animal SCN maintenance.

Nitrogen Fixation in Cereals.

Cereals such as maize, rice, wheat and sorghum are the most important crops for human nutrition. Like other plants, cereals associate with diverse bacteria (including nitrogen-fixing bacteria called diazotrophs) and fungi. As large amounts of chemical fertilizers are used in cereals, it has always been desirable to promote biological nitrogen fixation in such crops. The quest for nitrogen fixation in cereals started long ago with the isolation of nitrogen-fixing bacteria from different plants. The sources of diazotrophs in cereals may be seeds, soils, and even irrigation water and diazotrophs have been found on roots or as endophytes. Recently, culture-independent molecular approaches have revealed that some rhizobia are found in cereal plants and that bacterial nitrogenase genes are expressed in plants. Since the levels of nitrogen-fixation attained with nitrogen-fixing bacteria in cereals are not high enough to support the plant's needs and never as good as those obtained with chemical fertilizers or with rhizobium in symbiosis with legumes, it has been the aim of different studies to increase nitrogen-fixation in cereals. In many cases, these efforts have not been successful. However, new diazotroph mutants with enhanced capabilities to excrete ammonium are being successfully used to promote plant growth as commensal bacteria. In addition, there are ambitious projects supported by different funding agencies that are trying to genetically modify maize and other cereals to enhance diazotroph colonization or to fix nitrogen or to form nodules with nitrogen-fixing symbiotic rhizobia.

The root indeterminacy-to-determinacy developmental switch is operated through a folate-dependent pathway in Arabidopsis thaliana.

Roots have both indeterminate and determinate developmental programs. The latter is preceded by the former. It is not well understood how the indeterminacy-to-determinacy switch (IDS) is regulated. We isolated a moots koom2 (mko2; 'short root' in Mayan) Arabidopsis thaliana mutant with determinate primary root growth and analyzed the root apical meristem (RAM) behavior using various marker lines. Deep sequencing and genetic and pharmacological complementation permitted the identification of a point mutation in the FOLYLPOLYGLUTAMATE SYNTHETASE1 (FPGS1) gene responsible for the mko2 phenotype. Wild-type FPGS1 is required to maintain the IDS in the 'off' state. When FPGS1 function is compromised, the IDS is turned on and the RAM becomes completely consumed. The polyglutamate-dependent pathway of the IDS involves activation of the quiescent center independently of auxin gradients and regulatory modules participating in RAM maintenance (WUSCHEL-RELATED HOMEOBOX5 (WOX5), PLETHORA, and SCARECROW (SCR)). The mko2 mutation causes drastic changes in folate metabolism and also affects lateral root primordium morphogenesis but not initiation. We identified a metabolism-dependent pathway involved in the IDS in roots. We suggest that the root IDS represents a specific developmental pathway that regulates RAM behaviour and is a different level of regulation in addition to RAM maintenance.

Apical meristem exhaustion during determinate primary root growth in the moots koom 1 mutant of Arabidopsis thaliana.

An indeterminate developmental program allows plant organs to grow continuously by maintaining functional meristems over time. The molecular mechanisms involved in the maintenance of the root apical meristem are not completely understood. We have identified a new Arabidopsis thaliana mutant named moots koom 1 (mko1) that showed complete root apical meristem exhaustion of the primary root by 9 days post-germination. MKO1 is essential for maintenance of root cell proliferation. In the mutant, cell division is uncoupled from cell growth in the region corresponding to the root apical meristem. We established the sequence of cellular events that lead to meristem exhaustion in this mutant. Interestingly, the SCR and WOX5 promoters were active in the mko1 quiescent center at all developmental stages. However, during meristem exhaustion, the mutant root tip showed defects in starch accumulation in the columella and changes in auxin response pattern. Therefore, contrary to many described mutants, the determinate growth in mko1 seedlings does not appear to be a consequence of incorrect establishment or affected maintenance of the quiescent center but rather of cell proliferation defects both in stem cell niche and in the rest of the apical meristem. Our results support a model whereby the MKO1 gene plays an important role in the maintenance of the root apical meristem proliferative capacity and indeterminate root growth, which apparently acts independently of the SCR/SHR and WOX5 regulatory pathways.