Plant growth and productivity are orchestrated by a network of signaling cascades involved in balancing responses to perceived environmental changes with resource availability. Vascular plants are divided into the shoot, an aboveground organ where sugar is synthesized, and the underground located root. Continuous growth requires the generation of energy in the form of carbohydrates in the leaves upon photosynthesis and uptake of nutrients and water through root hairs. Root hair outgrowth depends on the overall condition of the plant and its energy level must be high enough to maintain root growth. TARGET OF RAPAMYCIN (TOR)-mediated signaling cascades serve as a hub to evaluate which resources are needed to respond to external stimuli and which are available to maintain proper plant adaptation. Root hair growth further requires appropriate distribution of the phytohormone auxin, which primes root hair cell fate and triggers root hair elongation. Auxin is transported in an active, directed manner by a plasma membrane located carrier. The auxin efflux carrier PIN-FORMED 2 is necessary to transport auxin to root hair cells, followed by subcellular rearrangements involved in root hair outgrowth. This review presents an overview of events upstream and downstream of PIN2 action, which are involved in root hair growth control.

• Keywords
• PIN-FORMED 2, ROP2, ROS, TOR signaling, auxin, plant adaptation, polar cell elongation, root growth, root hair growth,
• Publication type
• Journal Article MeSH
• Review MeSH

Plant roots are very plastic and can adjust their tissue organization and cell appearance during abiotic stress responses. Previous studies showed that direct root illumination and sugar supplementation mask root growth phenotypes and traits. Sugar and light signaling where further connected to changes in auxin biosynthesis and distribution along the root. Auxin signaling underpins almost all processes involved in the establishment of root traits, including total root length, gravitropic growth, root hair initiation and elongation. Root hair plasticity allows maximized nutrient uptake and therefore plant productivity, and root hair priming and elongation require proper auxin availability. In the presence of sucrose in the growth medium, root hair emergence is partially rescued, but the full potential of root hair elongation is lost. With our work we describe a combinatory study showing to which extent light and sucrose are antagonistically influencing root length, but additively affecting root hair emergence and elongation. Furthermore, we investigated the impact of the loss of PIN-FORMED2, an auxin efflux carrier mediating shootward auxin transporter, on the establishment of root traits in combination with all growth conditions.

• Keywords
• PIN-FORMED2, dark grown roots, gravitropic index, light grown roots, root growth, root hair, root hair elongation, shootward auxin transport, sucrose, sugar, total root length,
• Publication type
• Journal Article MeSH

BACKGROUND: Nitrogen starvation is known to cause drastic alterations in physiology and metabolism leading to the accumulation of lipid bodies in many microalgae, and it thus presents an important alternative for biofuel production. However, despite the importance of this process, the molecular mechanisms that mediate the metabolic remodeling induced by N starvation and especially by stress recovery are still poorly understood, and new candidates for bioengineering are needed to make this process useful for biofuel production. RESULTS: We have studied the molecular changes involved in the adaptive mechanisms to N starvation and full recovery of the vegetative cells in the microalga Chlamydomonas reinhardtii during a four-day time course. High throughput mass spectrometry was employed to integrate the proteome and the metabolome with physiological changes. N starvation led to an accumulation of oil bodies and reduced Fv/Fm.. Distinct enzymes potentially participating in the carbon-concentrating mechanism (CAH7, CAH8, PEPC1) are strongly accumulated. The membrane composition is changed, as indicated by quantitative lipid profiles. A reprogramming of protein biosynthesis was observed by increased levels of cytosolic ribosomes, while chloroplastidic were dramatically reduced. Readdition of N led to, the identification of early responsive proteins mediating stress recovery, indicating their key role in regaining and sustaining normal vegetative growth. Analysis of the data with multivariate correlation analysis, Granger causality, and sparse partial least square (sPLS) provided a functional network perspective of the molecular processes. Cell growth and N metabolism were clearly linked by the branched chain amino acids, suggesting an important role in this stress. Lipid accumulation was also tightly correlated to the COP II protein, involved in vesicle and lysosome coating, and a major lipid droplet protein. This protein, together with other key proteins mediating signal transduction and adaption (BRI1, snRKs), constitute a series of new metabolic and regulatory targets. CONCLUSIONS: This work not only provides new insights and corrects previous models by analyzing a complex dataset, but also increases our biochemical understanding of the adaptive mechanisms to N starvation in Chlamydomonas, pointing to new bioengineering targets for increased lipid accumulation, a key step for a sustainable and profitable microalgae-based biofuel production.

• Publication type
• Journal Article MeSH