# Regulation of the _Pseudomonas syringae_ Type III Secretion System by Host Environment Signals
## 2 Environmental Regulation of the _P. syringae_ T3 SS
### Induction of the P. syringae T3 SS by Specific Plant-Derived Signals  
Although _lrrp_/_hrc_ genes are induced by synthetic media conditions, multiple studies observed comparatively higher levels of _lrrp_/_hrc_ expression during plant infection [42; 53; 70; 75], suggesting the presence of additional T3 SS-inducing signals specific to the plant environment. In early studies, the induction of most _lrrp_/_hrc_ genes including _hrp_ occurred during _P. syringae_ infection of both resistant and susceptible cultivars of host plants [75], as well as during incompatible interactions with nonhost plants [42; 79], indicating that the T3 SS may be stimulated by signals common among plants rather than host-specific factors. In a study by [PERSON] et al., T3 SS gene expression was assessed in _P. syringae_ pv. tomato DC3000 cultured in a phosphate buffered-minimal medium supplemented with cell-free exudates from tomato cell suspension culture. A ten-fold increase in promoter activity of _lrrpA1_, encoding the T3 SS pilus protein, was observed in a minimal medium containing tomato exudates relative to phosphate buffer alone, suggesting that soluble plant signals were capable of eliciting T3 SS gene expression [70]. Similarly, cell-free exudates from Arabidopsis suspension cell cultures were later observed to induce the expression of _lrpL_ as well as the T3 SS effector _avrPto_[80; 81]. Co-culture of _P. syringae_ with Arabidopsis suspension cells provides a tractable model system for measuring the abundance of plant T3 SS-inducing metabolites and their effects on _P. syringae_ virulence. Using this \"infection-in-a-flask\" system, a recent study discovered that the growth of _P. syringae_ pv. tomato DC3000 was induced in the presence of Arabidopsis cells [80]. This effect was dependent on the production of a functional T3 SS by _P. syringae_, suggesting that plant-derived signals stimulated bacterial growth by enhancing T3 SS deployment [80]. Recent progress has been made in identifying specific metabolites in plant exudates that induce T3 SS during infection. In one study, plant exudates prepared by soaking Arabidopsis seedlings in water were found to strongly induce _hrpL_ expression and AvrPto accumulation in _P. syringae_ pv tomato DC3000 [82]. A metabolomics analysis of these seedling exudates, followed by functional testing of the individual compounds identified, revealed multiple organic acids and amino acids, including citric acid and aspartic acid, as the bioactive T3 SS-inducing compounds. Citric acid and aspartic acid are likely abundant in the tissues of all terrestrial plants, supporting previous observations that the _P. syringae_ T3 SS is induced by universal features of the plant environment, rather than host-specific signals [83; 84]. Citric acid was also recently identified as a bioactive T3 SS-inducing compound in Arabidopsis suspension cell exudates [80]. Although all of the bioactive compounds have carboxyl groups and are acidic, not all carboxyl-containing compounds present in exudates, for example, the amino acids leucine and valine are able to induce T3 SS, indicating some level of specificity in their detection by _P. syringae_. Additionally, as described in more detail below, the maximal bioactivity of the T3 SS-inducing acidic metabolites requires the presence of a simple sugar such as fructose, suggesting that _P. syringae_ may rely on multiple distinct chemical cues during infection. Simple sugars have also been shown to increase phytotoxin production by _P. syringae_ pv. _syngae_ B3 AR132 in response to plant phenolic glucosides [85]. Together, these findings indicate that simple sugars such as fructose may \"prime\" the _P. syringae_ response to other plant compounds that induce virulence-related phenotypes. The importance of specific plant-derived T3 SS-inducing signals in determining _P. syringae_ infection outcomes was demonstrated through studies of an Arabidopsis mutant plant lacking the immune regulator _MAPK PHOSPHATASE 1 (MKPI)_[82].