## 5 Phytoalexin Accumulation in the Upregulation of Defense-Related Marker and Elicitor Genes
Correspondingly, overexpression of _OsSBP_ led to a 20-25-fold increase in momilactone A, being phytoalexin levels in _OsSBP_ downregulated lines similar as those in the wild-type parental rice. Besides, there was also an induction in the PR gene expression as well as enhancement of H\({}_{2}\)O\({}_{2}\) accumulation. The fact that overexpressing _OsSBP_ rice plants also exhibited disease resistance to bacterial blight was consistent with the overall relevance of this protein in modulating the defense mechanisms of that plant. Calcium regulates various cellular processes in plants as an ubiquitous internal second messenger, transferring signals received at the cell surface to the inside of the cell _via_ spatial and temporal changes in its concentration. The information encoded within the Ca\({}^{2+}\) transients is decoded and transmitted by a toolkit of Ca\({}^{2+}\)-binding proteins that regulate transcription _via_ Ca\({}^{2+}\)-responsive promoter elements, regulating protein phosphorylation [156]. Several families of calcium sensors have been identified so far of which the best known are calmodulins (CaMs) and CaM-related proteins [157]. The second major class is the Ca\({}^{2+}\)-dependent protein kinases [158]. The third family consists of calcineurin B-like proteins (CBLs) acting as Ca\({}^{2+}\) sensors to activate specific protein kinases, CBL interacting protein kinases (CIPKs) [159]. Since Ca\({}^{2+}\) mobilization as a reponse to pathogen-associated molecular patterns plays a pivotal role in innate immunity in plants, the molecular links between this cation and downstream defense responses, namely phytoalexin production, were investigated using gain-of-function approaches and an RNAi strategy to address regulation of PAMP-induced defenses by two genes _OsCIPK14_ and _OSCIPK15_ encoding CIPKs in rice (_O. sativa_) cultured cells [160]. Thesetwo genes were rapidly induced in rice transgenic lines by microbe-associated molecular patterns, including chitol oligosaccharidesacarides and a _Trichoderma viride_/ethylene-inducing xylanase (TvX/EIX). _OsCIPK14/15_ overexpressing plants showed an increase in the two major phytoalexins of rice, momilactones and phytocassanes, as evidenced by LC-MS-MS analyses. This was the result of a slow and prolonged expression of genes encoding cyclase enzymes involved in the biosynthesis of these phytoalexins, namely, _ent_-copalyl diphosphate synthase 4 and _ent_-kaurene synthase-like 4 for momilactones and _ent_-copalyl diphosphate synthase 2 and _ent_-kaurene synthase -like 7 for phytocassanes, respectively. The TvX/EIX-induced phytoalexin biosynthesis was significantly suppressed in the _OsCIPK14/15-RNAi_ lines. Momilactones and phytocassanes are also regulated in rice by the _Oryza sativa_ TGA factor for phytoalexin production 1 named _OsTGAP1_[161]. Transformed rice plants constitutively expressing _OsTGAP1_ indeed showed enhanced accumulation of both phytoalexins. OstGAP1 regulates momilactone phytoalexin biosynthesis through positive control of the _OsKSL4_ gene expression, responsible for the first committed step in the route to momilactone. Specifically, clustered genes for momilactone biosynthesis were shown to be coordinately regulated by OsTGAP1, leading to a hyper-accumulation of this phytoplankton as a response to elicitor treatment. Camalexin biosynthesis in _Arabidopsis_ was also shown to be regulated by microbial virulence factors of the Nep1-like protein family (NLP) [162]. Beside their role as toxin-like virulence factors, Nep1-like proteins (NLPs) act as signals triggering the plant innate immune responses involving a signaling cascade which includes reactive oxygen species and ethylene production, and ending in callose deposition, programme cell death and phytoalexin production [163,164].