Why does a pepper rootstock confer tolerance to water stress? The case of NIBER

Drought incidence on plants triggers adverse effects on all plant levels: molecular, biochemical, physiological and morphological, leading to reduced crop yields. Vegetable grafting technique is based on the rootstock’s ability to overcome water stress and to modify scion stress perception. Our research team obtained and registered NIBER^® rootstock through a classic breeding program, which confers tolerance to water stress. In this paper, we summarize the main traits related to water stress mitigation on a scion cultivar (V) grafted plants using NIBER^® (N) as rootstock (V/N). The first evidence was the 1.8-fold higher marketable yield from V/N plants compared to ungrafted plants (V), associated with higher biomass and root volume. These long-term advantages using NIBER^® could be related to prompt strategies in the early phase of water stress (<48 h). To assess this hypothesis, the short-term modulation of water stress responses was evaluated with the analysis of gene expression, phytohormonal balance and metabolomic profiles. Constitutive differences in roots gene expression between NIBER^® and a water stress sensitive accession were observed mainly for ROS detoxification-related genes and sustained ABA induction by NCED gene, that could be advantageous to NIBER^® when the stress comes into play. NIBER^® was able to develop a quick response to water stress in roots regarding the increase on DREBs, MYC2, aquaporins and chaperones gene expression. Under short-term water stress, minor oxidative damage was observed in NIBER^® roots compared to sensitive rootstocks (lower GSSG) along with increased vitamin B6 synthesis in the scion leaves. In addition, NIBER^® stimulated JA and ABA synthesis that promote stomata closure, and raffinose and trehalose acting as osmoprotectant sugars. Moreover, NIBER^® increased protective metabolites related to suberin and cutin biosynthesis and anthocyanins. The results demonstrated that the drought tolerance observed in pepper grafted onto NIBER^® is a consequence of both constitutive traits and activated mechanisms to overcome water stress.