Chitosan is a natural polymer with applications in agriculture, which causes plasma membrane permeabilisation and induction of intracellular reactive oxygen species (ROS) in plants. Chitosan has been mostly applied in the phylloplane to control plant diseases and to enhance plant defences, but has also been considered for controlling root pests. However, the effect of chitosan on roots is virtually unknown. In this work, we show that chitosan interfered with auxin homeostasis in Arabidopsis roots, promoting a 2–3 fold accumulation of indole acetic acid (IAA). We observed chitosan dose-dependent alterations of auxin synthesis, transport and signalling in Arabidopsis roots. As a consequence, high doses of chitosan reduce WOX5 expression in the root apical meristem and arrest root growth. Chitosan also propitiates accumulation of salicylic (SA) and jasmonic (JA) acids in Arabidopsis roots by induction of genes involved in their biosynthesis and signalling. In addition, high-dose chitosan irrigation of tomato and barley plants also arrests root development. Tomato root apices treated with chitosan showed isodiametric cells respect to rectangular cells in the controls. We found that chitosan causes strong alterations in root cell morphology. Our results highlight the importance of considering chitosan dose during agronomical applications to the rhizosphere.

The fungal parasite of nematode eggsPochonia chlamydosporiais also a root endophyte known to promote growth of some plants. In this study, we analysed the effect of nineP.chlamydosporiaisolates from worldwide origin on tomato growth. Experiments were performed at different scales (Petri dish, growth chamber and greenhouse conditions) and developmental stages (seedlings, plantlets and plants). SevenP.chlamydosporiaisolates significantly (P< 0.05) increased the number of secondary roots and six of those increased total weight of tomato seedlings. SixP.chlamydosporiaisolates also increased root weight of tomato plantlets. Root colonisation varied between different isolates of this fungus. AgainP.chlamydosporiasignificantly increased root growth of tomato plants under greenhouse conditions and reduced flowering and fruiting times (up to 5 and 12 days, respectively) versus uninoculated tomato plants.P. chlamydosporiaincreased mature fruit weight in tomato plants. The basis of the mechanisms for growth, flowering and yield promotion in tomato by the fungus are unknown. However, we found thatP. chlamydosporiacan produce Indole‐3‐acetic acid and solubilise mineral phosphate. These results suggest that plant hormones or nutrient ability could play an important role. Our results put forward the agronomic importance ofP. chlamydosporiaas biocontrol agent of plant parasitic nematodes with tomato growth promoting capabilities.

Tomato (Solanum lycopersicum L.) is one of the most important crops worldwide as per its production and the surface cultivated. The use of biostimulant products plays a fundamental role in mitigating the negative effects of climate change and reducing the use of conventional fertilizers. Many of these products are formulated with amino acids (AAs). This study was conducted to elucidate the effects of the foliar application of tyrosine (Tyr) (15 mM), lysine (Lys) (15 mM), methionine (Met) (15 mM), and a Tyr + Lys + Met (15 mM + 15 mM + 15 mM) mixture on the physiological and metabolic processes, vegetative growth, and nutritional state of Optima variety tomato plants. The results showed that application of the AAs, individually and combined, was beneficial for the growth of the aerial part, net assimilation of CO₂, and water use efficiency (WUE). Application of Tyr resulted in the best WUE. The metabolomics study revealed that AA treatments increased the concentration of proline, fructose, and glucose, whose role was to stimulate glycolysis and the Krebs cycle. Thus, the plants could have greater reduction power and energy, as well as more carbon molecules for their growth processes.