Tissue-cultured seedlings of mountain laurel (Kalmia latifolia L.) were treated in flasks with strain AOK-30 of Streptomyces padanus. In a previous report, this treatment induced drought tolerance in the seedlings. Because structural modification of cell walls and enhancement of osmotic pressure in the cells were the likely cause, cell walls of AOK-30-treated and -untreated seedlings were further analyzed focusing on callose accumulation and lignification. About 2.5 times more callose accumulated in AOK-30-treated than -untreated seedlings. Sequence analysis of amino acids derived from cell wall proteins proved that cell walls of treated seedlings contained an enzyme with 100% homology to putative mitochondrial NAD-dependent malate dehydrogenase in potato. Malate dehydrogenase has been known to bind the apoplast and cell walls of other plants and is thought to be associated with lignification. Histochemical observations using phloroglucine-HCl revealed that cell walls of sieve cells were more intensely lignified in AOK-30-treated seedlings in contrast to the weak staining in untreated seedlings. Based on these results, the drought tolerance of K. latifolia seedlings induced by AOK-30 treatment was shown to be associated with accelerated callose accumulation and lignification in cell walls of sieve cells.