Some plants nurture soil bacteria that keep them healthy

THE bacteria which inhabit human beings, particularly the guts of those beings, have been found in recent years to be important for fending off disease. That something similar happens in other animal species is doubtless true as well. But work by Seon-Woo Lee at Dong-A University and Jihyun Kim at Yonsei University, both in South Korea, suggests that it is not only animals which benefit from such bacterial shielding. Their study, just published in Nature Biotechnology, shows that plants do, too. And that may have important implications for agriculture.

Crop plants of the nightshade family, such as potatoes and tomatoes, are susceptible to a soil bacterium called Ralstonia solanacearum. This enters their roots and spreads through their water-transport systems, causing them to wilt. Infection is usually lethal; the disease costs potato farmers alone $1bn a year. Some apparently suitable plants, though, seem exempt from R. solanacearum’s attentions. In particular, a variety of tomato called Hawaii 7996 does not suffer from such bacterial wilt. Dr Lee and Dr Kim wondered if the explanation for this exceptionalism lay with other bacteria in the soil.

To test that idea they grew crops of Hawaii 7996 and a second, wilt-vulnerable, tomato variety called Moneymaker. Once the plants were established, the researchers analysed bacteria in the soil around the plants’ roots and found systematic differences that depended on which tomato strain was growing. This observation made their hypothesis plausible.

They then transplanted some of their Moneymaker plants into soil that had previously supported Hawaii 7996s, and some of the Hawaiian plants into soil that had been home to Moneymakers. As controls, they similarly uprooted individuals of both varieties and replanted them in soil once inhabited by the same variety. That done, they exposed all of their plants to R. solanacearum and monitored them over the course of 14 days.

They found the disease progressed almost 30% more slowly in Moneymaker plants grown in “Hawaiian” soil than it did in those Moneymakers that had been replanted into their own soil. In contrast, it progressed rapidly in the normally resistant Hawaiian variety when this was transferred into Moneymaker soil.

Further study revealed that a single type of soil bacterium, called TRM1, appeared to be providing the protection. Dr Lee and Dr Kim therefore cultivated this bug in their laboratory and used it to treat soil into which Moneymaker plants were then planted. When these were infected with R. solanacearum they proved, though not completely resistant to it, certainly more resistant than others that had been planted into untreated soil as controls. More than 40% of them were still alive after 16 days. Only 12% of the control plants lasted that long.

These findings suggest to Dr Lee and Dr Kim that the roots of Hawaii 7996 are releasing compounds which encourage the growth of TRM1. What those compounds are has yet to be determined. But the two researchers’ work suggests at least three ways in which bacterial wilt might be tackled. One is to apply TRM1 itself to the soil, if it can be cultured in sufficient quantities. The second is to apply the stimulating chemicals to soil, once they have been identified. The third is to tweak the DNA of vulnerable crops to produce the stimulating chemicals directly.