We postulated that a synergistic combination of two innate immune functions, pathogen surface recognition and lysis, in a protein chimera would lead to a robust class of engineered antimicrobial therapeutics for protection against pathogens. In support of our hypothesis, we have engineered such a chimera to protect against the Gram-negative Xylella fastidiosa (Xf), which causes diseases in multiple plants of economic importance. Here we report the design and delivery of this chimera to target the Xf subspecies fastidiosa (Xff), which causes Pierce disease in grapevines and poses a great threat to the wine-growing regions of California. One domain of this chimera is an elastase that recognizes and cleaves MopB, a conserved outer membrane protein of Xff. The second domain is a lytic peptide, cecropin B, which targets conserved lipid moieties and creates pores in the Xff outer membrane. A flexible linker joins the recognition and lysis domains, thereby ensuring correct folding of the individual domains and synergistic combination of their functions. The chimera transgene is fused with an amino-terminal signal sequence to facilitate delivery of the chimera to the plant xylem, the site of Xff colonization. We demonstrate that the protein chimera expressed in the xylem is able to directly target Xff, suppress its growth, and significantly decrease the leaf scorching and xylem clogging commonly associated with Pierce disease in grapevines. We believe that similar strategies involving protein chimeras can be developed to protect against many diseases caused by human and plant pathogens.

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SUMMARY Polygalacturonase-inhibiting proteins (PGIPs) are plant cell-wall proteins that specifically inhibit fungal endo-polygalacturonases (PGs) that contribute to the aggressive decomposition of susceptible plant tissues. The inhibition of fungal PGs by PGIPs suggests that PGIPs have a role in plant tolerance to fungal infections and this has been observed in transgenic plants expressing PGIPs. Xylella fastidiosa, the causal agent of Pierce's disease (PD) in grapevines, has genes that encode cell-wall-degrading enzymes, including a putative PG. Therefore, we hypothesized that PGIP expression could confer tolerance against this bacterium as well as against the fungal pathogen Botrytis cinerea. To test this hypothesis, Vitis vinifera cvs. 'Thompson Seedless' and 'Chardonnay' were transformed to express pear fruit PGIP-encoding gene (pPGIP) under the control of the CaMV 35S promoter. Substantial pear PGIP (pPGIP) activity was found in crude extracts from leaves and in xylem exudate of transgenic lines obtained from independent transformation events, but not in untransformed controls. pPGIP activity was detected in xylem exudate of untransformed scions grafted on to transgenic rootstocks expressing pPGIP. Leaves of transgenic plants infected with B. cinerea had reduced rates of lesion expansion. The development of PD was delayed in some transgenic lines with increased pPGIP activity. PD-tolerant transgenic lines had reduced leaf scorching, lower Xylella titres and better re-growth after pruning than the untransformed controls.

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A protocol for in vitro induction of tetraploids of the grape rootstock 101-14 Millardet et de Grasset (Vitis riparia 9 V. rupestris) (101-14 Mgt) (2n = 2x = 38) 9 Muscadinia rotundifolia cv. Trayshed (2n = 2x = 40) hybrids was established. Different explant materials were exposed to the antimitotic chemical agents colchicine and oryzalin, at three concentrations, for 24, 48 and 72 h. Treated in vitro shoot tips, anthers and pre-embryogenic cultures were regenerated, and their ploidy levels were confirmed by chromosome counts and flow cytometry. It is generally considered that anatomical characteristics of leaves, including leaf stomatal size and number of chloroplasts in guard cells, correlate with ploidy levels of grape plants. Chromosome counts and flow cytometry are labor intensive and difficult, while the number of chloroplasts in stomatal guard cells and stomatal size are easily measured and can be stable and reliable markers. Analysis of stomata revealed clear differences between regenerated diploid and tetraploid plants, and these measurements were used as a preliminary screen, followed by chromosome counts and flow cytometry for confirmation. This study presents the results obtained by various combinations of explants, antimitotic agents, exposure time, concentration and ploidy analysis to evaluate in vitro chromosome doubling. These results provide an easy and effective method for production of tetraploid, potentially fertile Vitis 9 Muscadinia hybrids for utilization in rootstock breeding programs.