Delayed Senescence and Disease Tolerance of Tomato Plants Cultivated in Cover Crop Mulch Correlates with Accumulation of Specific Gene Products

Vegetable production is heavily dependent upon high off-farm inputs of polyethylene (plastic) mulch, nitrogen fertilizer and pesticides. Such a production practice potentially contributes to the unintentional environmental pollution with fertilizer and pesticides as well as water run off and soil erosion. This raises serious environmental concerns for human and animal health. The integration of on-farm biological inputs into vegetable production system is one potential means of reducing the dependence on off-farm inputs. In recent years, alternative agriculture practices have tested cover crops like hairy vetch (Vicia villosa) as on-farm biological inputs that have the potential to reduce both erosion and the use of agrochemicals without impacting the yield or quality of the produce. Field-grown, fresh market tomato (Lycopersicon esculentum L.) plants cultivated in hairy vetch mulch display reduced defoliation and tolerance to disease as compared to plants cultivated in the plastic mulch. We have initiated a molecular approach to test whether these beneficial attributes are linked to changes in the expression profiles of one or more specific gene products. The data indicated that vetch-grown tomato plants have increased accumulation of transcripts and proteins that are central to delayed senescence and disease suppression. INTRODUCTION US consumer demand for fresh market vegetables has increased considerably in recent years due to their contribution to human health. Vegetables and fruits are rich sources of vitamins, minerals and fiber. Conventional production methods have met these demands but compromised on the environmental and social issues because these production systems heavily rely on materials synthesized off the farm, such as polyethylene mulch, nitrogen fertilizer and pesticides. The current reliance on off-farm inputs will continue unless profitable, lower-input systems for fresh market production are developed. The integration of on-farm biological inputs into vegetable production systems is one potential means of reducing the current dependence on off-farm inputs. Cover crops represent on-farm biological inputs that have the potential to reduce the use of polyethylene mulch, fertilizer and pesticides (Abdul-Baki et al., 1996a, b; Creamer et al., 1996). The benefits of cover crops include improvements in soil preservation (Flach, 1990), soil health (McVay et al., 1989), weed suppression (Teasdale, 1993; Creamer et al., 1996), and the reduction of soilborne diseases (Rothrock et al., 1995; Candole and Rothrock, 1997). Hairy vetch has been used successfully as cover crop/organic mulch for growing processing tomato (Abdul-Baki et al., 1996a). In most cases, marketable yield of fresh market tomatoes was equivalent or greater in hairy vetch mulch as compared to black polyethylene mulch (Abdul-Baki et al., 1996a; Mills et al., 2002b). In addition, vetch decomposition provided nitrogen reducing the need for commercial nitrogen input (Abdul-Baki et al., 1997); nitrogen content of hairy vetch aerial biomass has been estimated at 126-169 kg ha. Fresh market tomato production in hairy vetch mulch is Proc. XXVI IHC – Sustainability of Horticultural Systems Eds. L. Bertschinger and J.D. Anderson Acta Hort. 638, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 498 economical as compared to production using black polyethylene mulch (Kelly et al., 1995). Cover cropping has been shown to suppress disease in some vegetable and fruit crops. The incidence of Phytophthora blight was reduced in peppers grown in a wheat cover crop compared to bare soil and polyethylene mulch (Ristaino et al., 1997). This reduction in disease incidence affected dispersal of propagules of Phytophthora capsici, the causal agent of Phytophthora blight (Ristaino et al., 1997). Sudangrass was shown to reduce the splash dispersal of Colletotrichum acuatum conidia and has the potential to manage anthracnose of strawberry (Ntahimpera et al., 1998). Compared to no soil cover, sand and plastic mulch, the incidence of leather rot disease of strawberry was reduced by using straw as a soil cover to reduce the splash dispersal of P. cactorum (Madden and Ellis, 1990). Cotton grown in hairy vetch (Vicia villosa Roth)-amended soil exhibited reduced incidence of black root rot, caused by Thielaviopsis basicola (Candole and Rothrock, 1997). The reduction of black root rot incidence correlated with increased ammonia levels in the soil (Candole and Rothrock, 1997). Compared to winter fallow, reduced soil levels of T. basicola were observed in a hairy vetch cover crop (Rothrock et al., 1995). Similarly, foliar disease was reduced in tomato plants grown in hairy vetch compared to bare soil (Mills, et al., 2002a). In addition, fruit produced in hairy vetch mulch appeared to be more suitable for fresh-cut slices than those grown in black plastic (Hong, et al., 2000). The advantages of cover crop cultivation seem impressive. However, not much is known about the cellular processes impacted, signaling factors involved or molecular mechanisms that are fundamental to vetch-induced longevity and disease-suppression. To help understand these mechanisms, we initiated studies to identify genetic components that are regulated in plants cultivated in vetch-mulch. This report presents our findings to date. HYPOTHESIS Our hypothesis is that controlled release of carbon and nitrogen metabolites during water-mediated decomposition of vetch results in a certain carbon/nitrogen (C/N) ratio that influences expression of genes associated with senescence, longevity and/or disease infestation. This hypothesis is based on the following observations in the literature: 1. Sugars impact photosynthesis, nitrogen metabolism, sucrose and starch metabolism, and defense mechanisms by regulating a few key genes (Sheen, 1994; Koch, 1996; Sheen and Jang, 1997). 2. Nitrate regulates carbon metabolism, resource allocation, root development and flowering (Koch, 1997; Wang et al., 2000). 3. Nitrate down-regulates genes involved in starch synthesis, facilitating the mobilization of carbon skeletons into organic acids (Koch, 1997, Wang et al, 2000). 4. Nitrate is readily reduced to ammonia and subsequently incorporated into amino acids (Koch, 1997; Stitt, 1999). 5. Carbon : nitrogen (C/N) ratio may regulate metabolic development (Smeekens, 2000; Hellmann et al., 2000; Vidmar et al., 2000., Coruzzi and Zhou, 2001). RESULTS AND DISCUSSION Severer Foliar Disease Levels and Leaf Defoliation Occur in Black Polyethylene than in Hairy Vetch Mulch The onset of foliar disease was significantly delayed in hairy vetch-grown plants compared to those cultivated on black polyethylene. For example, foliar disease in plastic beds exceeded 41% at 84 days after transplant compared to hairy vetch beds that showed remarkably little sign of disease onset during a field test in year 2000. Similarly, plants grown in black polyethylene beds lost ~33% of their foliage at 84 days after transplant whereas those grown in hairy vetch beds lost only ~11% of their foliage in the same time frame (Fig. 1).