## 4 Modulation of Phytoalexin Accumulation through Engineering of Plant Hormones
Rishitin and lubimin production, twophotoalexins from Solanaceae, was also found to be diminished by ABA in potato tubers infected by _P. infestans_[125]. To understand the mechanisms of the regulation of phytoalexin expression under the control of ABA, capsidiol biosynthesis was assessed in two tobacco (_N. plumbaginifolia_) nonallelic mutants, _Npaba2_ and _Npaba1_ deficient in ABA _vs._ wild-type plants as a response to abiotic (induction with cellulose or arachidonic acid) or biotic stresses [126]. The mutant _Npaba2_ was impaired in zeaxanthin epoxidase (ZEP) and _Npaba1_ was deficient in abscisic aldehyde oxidase. Total capsidiol phytoalexin accumulation in the two mutants doubled the amount of capsidiol in wild-type plants, even in those inoculated with _B. cinerea_ which is known for its ability to produce ABA. This was the consequence of an enhanced expression of two genes acting on the capsidiol biosynthesis pathway, _5-epi-aristolochene synthase_ (_EAS_) and _5-epi-aristolochene hydroxylase_ (_EAH_). At the same time, the transcription activity of ZEP and 9-_cis_-epoxy-carotenoid dioxygenase which encode enzymes of the upstream pathway of ABA synthesis, decreased. ABA thus negatively regulates elicitor-induced phytoalexin production in tobacco. Induction of plant hormone catabolic pathways was also observed during capsidiol accumulation, being this catabolism not limited to ABA, but also affecting the gibberellins. In fact, both the expression of the gene encoding _ABA-8'-hydroxyalase_ (_ABAH_) and of that encoding _gibberellin-2-oxidase_ (_GA-2 Ox_) [127] were induced in parallel to the expression of _EAS_ and _EAH_ as well as the 2-fold increase in capsidiol. The expression pattern observed for _ABAH_ and _GA-2 Ox_ implied a regulatory role of abscisic acid and likely gibberellins catabolic pathways on capsidiol accumulation in tobacco [126]. This clearly illustrates plant hormone cross talk for phytoalexin regulation. There is little evidence of phytoalexin expression being regulated by gibberellins with the exception of the possible implication of gibberellin catabolism in the accumulation of capsidiol in tobacco (see above) [126]. Altered disease development was reported in rice plants overexpressing Elongated uppermost internode (_Eui_) gene encoding a gibberellin deactivating enzyme [128]. Conversely, knockout of this gene was found to compromise resistance to disease, indicating that gibberellins negatively regulate rice basal disease resistance. Although the induction of a PR1a pathogenesis-related protein was reported during pathogen resistance, no mention of a possible modulation of phytoalexins by gibberellins was made. Cytokinins (CKs) are phytohormones involved in various regulatory processes during plant development [129]. Reports of the role of CKs in plant disease resistance are still scarce. Elevated CK levels were associated to resistance against viruses [130,131] and suppression of the HR in tobacco [132]. Plant-derived CKs were also shown to promote resistance of _Arabidopsis_ to _Pseudomonas syringae_ pv. _tomato DC3000_ (_Pst_) via the salicylic acid response factor TAG3/Non-expressor of pathogenesis-related genes-1 (_NPR1_) [133]. Modulating CK levels or signaling activity in cytokine-oxydase- or isopentenyl transferase-overexpressing plants in _ahk2 ahk3_ mutants (double knockout of the CK receptor) correlated with altered resistance. Induction of phytoalexin synthesis upon resistance of _Arabidopsis_ to _P. syringae_ was not reported in this study. More recently, CK overexpression was shown to enhance resistance against _P. syringae_ pv _tabaci_ in tobacco (_Nicotiana tabacum_) [100]. The CK-mediated resistance in tobacco strongly correlated with up-regulated synthesis of two major phytoalexins from that plant, the diterpen capsidiol and the coumarin scopoletin. Since CKs are aminopurine-derived compounds containing an isopentenylmoiety, the role of this phytohormone in the pathosystem between tobacco and _P.